Note: Descriptions are shown in the official language in which they were submitted.
L4~
This invention relates to a steel sheet having
a composite coating and particularly to a plated base
steel sheet provided thereon with multiple coatings
to improve corrosion resistance, paintability and
corrosion resistance after paint coating.
Chromate treatment has been commonly employed
as a rust preventive treatment for galvanized steel
sheets, zinc alloy plated steel sheets and aluminum
plated steel sheets. In most cases, such treatment is
intended merely as a temporary rust preventive measure
and the corrosion resistance thereby obtained is at a
low level such that white rust appears in 24 to 48
hours when the treated steel sheets are subjected to a
salt spray test. ~ccordingly, in a case where the
products are intended for use under a severe corrosion
environment for a long period of time, there has been
no practical way other than applying a thick paint
coating at a level of more than 10~ to prevent corrosion.
However, in recent years the price of paint has
increased rapidly, reflecting the increase in the
price of petroleum products, and it is strongly desired
to develop steel sheet products having good corrosion
resistance, which can be produced simply by a surface
treatment without using paint.
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. . ~ . , ~ .
~Z~4~
Having regard to these circumstances, there
has been developed in the art a chromate treatment.
A typical one is a coating type chromate treatment in
which various binders are added to the chromate treat-
ment solution, whereby products having improved cor-
rosion resistance are produced. Even in this case,
however, the corrosion resistance thereby obtainable
is at best at a level of 200 hours against formation
of white rust when subjected to a salt spray test.
Even when painting is required to provide an
aesthetic outer appearance, it is usual to attempt to
cut down the costs for the paint by choosing a low
grade paint or by minimizing the thickness of the paint
coating. In such a case, the steel sheets are required
to have not only high corrosion resistance but also
good paintability and corrosion resistance after
painting. It is necessary that these properties be
well balanced.
However, there have been no chromate treated
products which fully satisfy these requirements. Some
products which satisfy the requirement for high
corrosion resistance, tend to fail in providing
adequate paintability or corrosion resistance after
painting whereas those having good paintability
tend to fail in the requirement for high corrosion
resistance.
For example, so-called unichrome treatment is
known so as to provide high corrosion resistance with-
out painting by means of a reactive chromate.
~ILZ~L~4~6
According to this method , a plated steel sheet is dipped in a trea-tment
solution composed of chromic acid and a mineral acid for a long period
of time to form a chromate film coating having a thickness of from
about 500 to about 700 mg/m2 calculated as the amount of metal chromium.
However , such a treatment has drawbacks that the dipping process
requires a long period of time and the degradation or aging of the
treatment solution is rapid , and it is totally unsatisfactory as a
strip coating technique. Further , as the chromate fi~m is rather thick ,
it is susceptible to cracking and its paintability is not good. If the
thickness of the chromate film is decreased to a level of 100 mg/m2 ,
the film cracking may be avoided , but the corrosion resistance is
reduced to a level of 100 hours , thus losing the high corrosion resistant
char t r ti
ac e lS CS.
On the other hand , in coating type chrcnate treatment , a binder
is used to fix or trap a substantial amount of chromium therein and to
prevent the film cracking by the binder. However , it has the drawback
that the solution is susceptible to gelation, and even when it is not
susceptible to gelation , it tends to form a thick film which requires
a special treatment for d~ying.
In recent yeæs , it has been proposed , for the purpose of
avoiding the above mentioned difficulties , to form a double layer
coating by duplex plating treatment. Namely , a thin chromate -film is
formed as the first layer in a short period of time , and then ~n
inorganic or organic substance is coated thereon to form a film as the
seoond layer which protects the first layer of the chromate film.
Use of inorganic materials is
4~6
disclosed in Japanese Patent Application 60283 filed
May 31, 1973 published under No. 9545/75 on January 31,
197S, Saburo Ayusawa et al and Japanese Patent Publi-
cation No. 19981/78 published June 23, 1978 (Applic-
ation No. 50791, filed May 9, 1973) Saburo Ayusawa et
al, in which a chromate film is electrolytically
formed as the first layer and a treatment solution com-
prising a chromic acid and silica sol is applied and
then dried to-form a film as the second layer. High
10, corrosion resistance is obtainable by forming the
second layer sufficiently thick. However, the film
there~y obtained is apt to be peeled off because of the
thick silica layer and thus lacks in durability after
processing. Further, as the silica sol has poor
affinity ~o a paint, the treated surface is not satis-
factory as a substrate for paint coating.
A typical example in which an organic mate-
rial is used for the second layer, is Japanesepatent
Publication ~o. 35620/77 published September 10, 1977
20 (Application ~o. 4646, filed January 22, 1969) Hiroshi
Uchida et al, in which a chromium hydrated oxide layer
is formed as the first layer and then a water soluble
organic resin layer is formed as the second layer.
However, no adequate effectiveness cannot be expected
from the organic resin layer disclosed therein, since
the functional groups present in the organic resin of
_ ~ _
, .
; ~,
4~6
the second. layer tend to attract water and therefore it
is impossible to obtain a hlgh level of corrosion
resistance. Even if the thickness of the second layer
is increased as much as 1~, a high level of corrosion
resistance will not be obtainable, and in such a case,
it will be difficult to completely dry the film by hot
air only and a special apparatus for drying will be
required.
Further, in ~apanese Patent Publications Nos.
10 36100/74 published Septe~ber 27, 1974 (Application
~o. 48605, filed July 19, 1960) Saburo Ayusawa et al,
~o. 18445/75, published June 28, 1975 (Application
117003, filed December 23, 1970) Kohi Yama~a ~t al,
~o. 4611/74 published February i, 1974 (Application
~o. 32177j filed April 15, 197G) Akimi Umezono et al
and ~o. 1986/74, published January 17, 1974 (Appli-
cation No. 23755, filed March 20, 1970) Akimi Umezono
et al, it is proposed to form a chromate film as the
first layer and an organic film as the second layer.
~owever, in each of these cases, the functional
groups in the organic resins in the second layer tend
to attract water and it is impossible to obtain a high
level of corrosion resistance. These products are pre-
sumably effective as a substrate for painting taking
advantage of the functional groups.
As menticned above, the products having a
second layer formed with an inorganic or organic mate-
rial are effective either for corrosion resistance
without painting or for paintability, but they can not
satisfy both of the required properties at the same
time. One possibility might be to form the second
layer as a mixture of an organic material and in inor-
ganic material. However, a mere mi~ture does not pro-
vide better results, and to the contrary, it is likely
in many cases that desirable properties of the
individual components will be impaired.
Thus, it has been difficult to produce chromate
treated steel sheets which are substrates for painting
and which at the same time have superior corrosion
~ resistance.
Accordingly, the present invention seeks to
overcome the above mentioned difficulties with con-
ventional surface treated steel sheets, and to provide
steel sheets having good corrosion resistance, paint-
ability and corrosion resistance after painting.
It has now been found that superior corrosion
resistance, paintability and corrosion resistance
after painting are obtainable by forming, on a plated
base steel sheet, for example, a galvaniæed steel
sheet, a zinc alloy plated steel sheet or an aluminum
steel sheet, a first layer of a chromate film by chro-
mate treatment of either reaction type or coating type
and then forming, on such first layer, a second layer
of a composite silicate resin film composed of a
silica sol and an organic resin. If either one of the
i,
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. ~
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~21~
first and second layers is omitted, the desired level
and balance of the properties are not obtainable.
Thus, the present invention provides a com-
posite coating steel sheet which comprises a plated
base steel sheet, a chromate film formed on the sur-
face o~ the plated steel sheet and a composite silicate
resin film formed on the chromate film and comprising
a colloidal silica, an organic resin and a silane com-
pound.
In another aspect of the invention there is
provided a process for forming the composite coated
steel sheet.
The invention is illustrated by reference to
the accompanying drawings in which:
Figs. 1 and 2 are graphs illustrating the
relationship between the coating amount of chromium
and the thickness of the composite silicate resin
film, and the corrosion resistance thereby obtained.
As the plated base steel sheet there may be
2Q used a galvanized steel sheet, a zinc-iron alloy plated
steel sheet, a zinc-nickel alloy plated steel sheet,
a zinc-manganese alloy plated steel sheet, or an
aluminum plated steel sheet. The steel sheet may also
be a multi-layer plated steel sheet, such sheets
having become popular in recent years, and which have
two or more such plated layers. In the case of the
zinc-iron alloy plated steel sheet, the iron content
in the plated layer is from 5 to 50% by weight, pre-
ferably from 10 to 30% by weight. If the iron content
is outside this range, the corrosion resistance and
paintability become poor. In the case where a zinc-
nickel plated steel sheet is employed, the nickel
content in the plated layer is from 5 to 20% by weight,
preferably from 12 to 13% by weight. If the nickel
~L2~ 6
content is less than 5%, the corrosion resistance be-
comes poor. On the other hand 9 if the nickel content
is more than 20%, it simply adds to the cost and such
is not economically practical.
CHROMATE FILM
A chromate film is formed onthe above mentioned
plated base steel sheet as the first layer.
The chromate treatment to form this first layer
of the chromate film may be conducted in accordance
with chromate treatment known per se in the art. The
amount of chromium deposited on the base steel sheet
must be from 10 to 150 mg/m2, preferably from 40 to
100 mg/m2. If the amount is less than 10 mg/m2, the
film tends to be uneven. On the other hand~ an
excessive amount over 150 mg/m2 is not desirable as it
facilitates degradation of the treatment solution and
adds to the costs. As a typical example, a reaction
type chromate treatment solution comprises from 1 to
100 g/l, as calculated as metal chromium, of a water
soluble chromium compound9 and from 0.2 to 20 g/l of
sulfuric acid, as major components, in which the tri~
valent chromium content in the total chromium is not
more than 50% by weight, preferably from 20 to 35% by
weight. Proper amounts of heavy metal ions, for
example, Zn2 , Co2 or FE3 , or other mineral acids,
~- for example, phosphoric acid or hydrofluoric acid, may
also be added.
With respect to the chromium compound of the
major components, if the amount, calculated as metal
chromium, is less than 1 g/l, it is difficult to
obtain the desired chromate film in a short period of
time. On the other hand, if the amount exceeds 100 g/l,
the stability of the treatment bath will be impaired
to a conside~able extent.
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. .
With respect to sulfuric acid, if the amount
is less than 0.2 g/l the desired chromate film cannot
easily be obtained in a short period of time, and uni-
formity in the treatment becomes poor. On the other
hand, if the amount exceeds 20 g/l, the etching rate
of zinc tends to be too fast to be desirable.
If the proportion of CR 3 in the total chromium
exceeds 50%, the stability of the bath will be dis-
turbed, whereupon the bath tends to undergo gelation,
and the corrosion resistance of the steel sheet before
painting will be poor so that even when the coating
amount of the chromate film is at the predetermined
level, the desired high level of corrosion resistance
is not obtainable.
The function of the heavy metal ions, for
example, Zn2+, which may be added as well as the afore-
mentioned major components, is to improve the treating
efficiency of the treatment solution, and their amounts
may suitably be determined depending upon the pro-
portions of the major components.
A~ a typical example of a coating type chromate
treatment solution, there may be mentioned a treatment
solution prepared by adding to the above-mentioned
reaction type chromate treatment solution, an organic
polymer resin containing in its molecule a substantial
number o~ carboxyl groups and which is water soluble
and compatible with the above-mentioned reaction-type
chromate treatment solution and adjusting the pH to
from 2.0 to 3.5. The organic polymer preferably has
an average molecular weight of from 1,000 to 500,000.
The amount of the organic polymer is selected within
a range of from 0.02 to 30 gtl calculated as the resin.
If the amount is less than 0.02 g/l, it is not possible
to completely trap chromium in the film when the film
is formed. On the other hand, if the amount exceeds
g _ .
.
`~
30 g~l, the stability of the bath tends to be poor. If
the pH is less than 2.0, the bath tends to have
characteristics similar to the reaction-type chromate
treatment solution, and if the pH exceeds 3.5, the
stability of the bath tends to be impaired.
In any event, the chromium amount in the first
layer of the chromate film should be within a range of
from 10 to 150 mg/m2. It is important that when the
treatment solution for the second layer is applied,
there should not be elution of chromium from the first
layer. If an eluted component from the first layer
enters the composite silicate resin treatment solution
during the treatment of the second layer, the balance
of the resin treatment solution will be disturbed, and
in an extreme case, gelation of the treatment solution
will be led. Accordingly, it is necessary to conduct
forcible drying and washing with water after the
treatment of the first layer so as to prevent elution
of the components from the first layer.
COMPOSITE SILICATE RESIN FILM
The composite silicate resin film is formed on
the above mentioned first layer of the chromate film.
The composike silicate resin is composed of a
resin or a mixture of resins, which are prepared by
reacting a water dispersible silica with a water
soluble or water dispersible organic polymer resin
having in its molecule a hydroxyl group (for example,
polyvinyl alcohol, hydroxyethyl cellulose, starch,
polyester, alkyd, epoxy ester or acrylic copolymer) in
the presence of a silane compound. As the silica, so-
called colloidal silica having a particle size of 7 to
100 nm, pref`erably from 10 to 50 nm, is used. The
resin to be used may be any resin so long as it can
react and bond with the silica. Further, ultraviolet
or electron beam curing type functional groups may be
introduced into the structure of the resin.
1 0 -
~lZ~
The role of the silane compound is to act as a
catalyst during the composite forming reaction of the
silica with the organic resin and to perform an
important function as a cross-linking agent ~or the
reactants and as a cross-linking agent to establish
a firm bond between the second layer and the first
layer. Suitably, silane compounds include commercially
available products.
The ratio of the water dispersible silica to
the water soluble or water dispersible organic resin
in the composite silicate resin composition, must be
from 5:95 to 95:5, based on the weight ratio of the
solids content. If the ratio is less than the lower
limit, a high level of corrosion resistance is not
obtainable even when applied onto the first layer of
the chromate film. On the other hand, i~ the ratio
exceeds the upper limit, good paintability is not
obtainable. The amount of the silane compound is
suitably fromO.5 to 15% by weight, preferably from 1
to 10% by weight, based on the weight of the total
solids content of the silica and organic resin. If
the amount is less than 0.5% by weight, adequate
cross-linking with the chromate film cannot be
expected. On the other hand, if the amount exceeds
15% by weight, no further improvement of the effect-
iveness is observed.
Further, by an addition of an alkoxide chelate
compound to the above mentioned composite silicate
resin treatment solution, the corrosion resistance can
be improved. The alkoxide chelate compound suitably
has the structure R2MR2, Rm3', MR4', or R3MR', where R
is an allyl group or an aryl group, which may have an
amino group or a mercapto group on its side chain, M
is titanium, zirconium or aluminum, and R' is a
radical selected from an alkoxy group having ~rom 1 to
8 carbon atoms or an alkoxyalkoxy group having from 2
-- 11 --
~LZ~ 6
- to 10 carbon atoms, which is condensed with a di-
carboxylic acid, a hydroxycarboxylic acid, a di-
ketone, an ester or an alkanolamine, as the ligand.
The alkoxide chelate compound is added to the
composite silicate resin in a solids content weight
ratio of the resin: the alkoxide chelate compound
being from 97:3 to 80:20. If the amount of the alkoxide
chelate compound is less than the lower limit, free
hydrcxyl groups left in the cured film becomes sub-
stantial and it is impossible to improve the cor-
rosion resistance or water repelling property to an
adequate degree. On the other hand, if the amount is
in excess of the upper limit, the condensation of
the alkoxide chelate compound itself occurs preferentially,
and it becomes difficult to form a uniform film. Further,
the alkoxide chelate compound tends to undergo self-
polymerization and becomes viscous as time passes thus
leading to thickening of the composite silicate resin
treatment solution whereby the treatment solution
becomes difficulty applicable after several days from its
preparation. In order to avoid, such a difficulty, at
least one of the additives selected from the group
consisting of oxy acids of molybdenum, tungsten,
vanadium, tin, boron and silicon and salts of such
oxy acids may be added to the composite silicate resin
treatment solution in place of the alkoxide chelate
compound.
The amount of such additives should be not more
than 10% by weight, preferably from 0.3 to 5% by weight,
based on the solids content weight of the composite
silicate resin treatment solution. If the amount
exceeds 10% by weight, there is the possibility of
deterioration in the stability of the treatment solution,
which is undesirable. ~
.
- 12 -
Further, one or more such additives may be
added together with the above mentioned alkoxide
chelate compound. In such a case, the total amount
of such additives and the alkoxide chelate compound
should be not more than 10% by weight, preferably,
from 0.3 to 5% by weight, based on the solids content
weight of the composite silicate resin, and the ratio
of the alkoxide chelate compound to such additives, should
be from 95:5 to 5:95, preferably from 80:20 to 20:80.
Effectiveness is obtainable within the above ranges,
and if the total amount of additive exceeds the upper
limit, there is the possibility that the stability of
the treatment solution is disturbed.
In the case where the composite silicate resin
is composed of a reaction product of polyvinyl alcohol
and a silica sol, it is possible to improve the drying
characteristic of the film by incorporating, together
with the alkoxide chelate compound, one or more
additives selected from the group consisting of water
soluble salts of copper, zinc, aluminum, zirconium,
chromium, cobalt, and nickel (for instance, zinc
chloride), and coordination compounds of such
elements (for instance, ethylenediamine tetraacetic
acid zinc complex salt). The amount of such additives
is preferably fromO.3 to 5% by weight, based on the
solids content weight of the composite silicate resin.
If the amount exceeds this upper limit, there is a
possibility that the stability of the composition is
disturbed. Further9 the ratio of the composite
silicate resin plus such additives to the alkoxide
chelate compound is from 97:3 to 80:20, based on the
solids content weight.
- 13 -
Further, in the case where an ultraviolet or
electron beam curing type resin is employed, it is
possible to facilitate the curing of the composite
silicate resin film by incorporating a photo-
sensitizer, for example, zinc oxide, titanium oxide
(anatage type) or titanic acid, and an oxy acid of
molybdenum, tungsten or vanadium (for instance,
vanadium trioxide) or its salt (for instance, lithium
orthovanadate). This is attributable to the fact
that an oxidation reduction reaction occurs among the
three components, whereby the functional groups in
the resin and a cation formed by the-
reaction,form a salt bond or a coordination hond.
The amount of the oxy acid or its salt is from
0.1 to 6% by weight, based on the solids content
weight of the composite silicate resin, and the
amount of the photosensitizer is from 30 to 200%
by weight, based on the amount of the oxy acid or
its salt.
RELATION BETWEEN THE THICKNESS OF THE CHROMATE FILM AND
THE THICKNESS OF THE COMPOSITE SILICATE RESIN FILM
Now, the relation between the first layer of the
chromate film and the second layer of the composite
silicate resin film will be described. An interrelation
exists, as shown in Fig. 1, between the coating amount
~ of chromium for the first layer and the film thickness
; of the second layer. For a coating amount of chromium
from 10 to 150mg/m2, the film thickness of the second
layer must be from 0~01 to 4~. Therefore, in order to
obtain a predetermined level of corrosion resistance,
it is necessary to select the coating amount of chromium
for the first layer and the film thickness of the second
layer based on this relationship. For instance, in
;
I - 13a -
,
~2~ 6
order to obtain corrosion resistance of 500 hours, if
the coating amount of chromium in the first layer is
40mg/m2, the film thickness of the second layer must be
at least 1.5~, and if the coating amount of chromium is
150mg/m2, the film thickness of the second layer must
be at least 0.4~. Generally, for the purpose of
practical application, it is desirable that for a coat-
ing amount of chromium from 10 to 150mg/m2, the film
thickness of the second layer is set to be from 0.4 to
4~. The above ranges are desirable from the standpoint
of the manufacture of the products, and as is apparent
from the graph of Fig. 1, if the film thickness of the
second layer is set to be less than 0.4~, it is
necessary to increase the coating amount of chromium,
which tends to lead to degradation of the chromate
treatment solution. On the other hand, if the film
thickness of the second layer is set to be greater than
4~, the costs will increase to an uneconomical level
and the products become difficult to weld under commonly
employed welding conditions, although the degra~dation of
the chromate treatment solution will be reduced.
However9 in a case where such a high level of
corrosion resistance is not required, the film thickness
of the second layer will be less than 0.4~ and at least
0.01~ for the coating amount of chromium in the first
layer being from 10 to 150mg/m2. In this case, the
conditions for both layers to provide the desired
corrosion resistance may be presented in a more
obvious manner by modifying the graph as shown in Fig.
2, in which the vertical axis is the film thickness of
the second layer and the horizontal axis is the coating
amount of chromium of the first layer, and the corrosion
resistance represented by the spraying time of a salt
spray test is presented as a parameter. From Fig. 2,
- 14 -
it will be understood that in order to obtain corrosion
resistance of 200 hours, for instance, the film thick-
ness of the second layer may be set to be O.l~ when the
coating amount of chromium of the first layer is
80mg/m2, and 0.02~ when the coating amount of chromium
is 120mg/m2.
In a particular embodiment the chromate film has
a thickness of from 70 to lO0 mg/m2 calculated as metal
chromium and the composite silicate resin film has a
thickness of from 0.5 to O.9~m.
Having thus described the relationship between
the coating amount of the first layer and the film thick-
ness of the second layer, it should be added that it is
essential to provide a double layer structure composed
of the chromate film and the composite silicate resin
film. Without this double layer structure, a high
level of corrosion resistance is not obtainabIe.
Now, the invention will be described with
reference to the Examples.
-: ~lZ1~6
EXAMPLE~
Tests for white rust forma-tion. paintability and
corrosion resistance af-ter paint coating were conducted with
respect to the steel sheets No. 1 to No. 37 of the present in-
vention which had various coating amounts of chromium in the
respective first layers and various film thicknesses of the
respective second layers, as indicated in the Table below. The
results thereby obtained are shown in comparison with comparative
steel sheets No. 38 to No. 53. As -the chromate treatment solu-
tion for the first layer, the following three compositions were
used as representatives:
( 03. H2~04. ~3~04) = (10 g/l, 2 g/l, 2 g/l)
,,,,~.,"i ,. ~
B: (CrO3. H2~04. crJ , Zn~ ) = (10 g/l, 2 g/l, 2 g/l. 3 g/l)
C: To the composition ~, 2 g/l of a polyacrylic acid having
a molecular weight of about 100,000 was added, and then the pH
was adjusted to 3 with aqueous ammonia.
Further, as the composite silicate resin treatment
solution, the following three compositions were used as repre-
sentatives:
(a): As the organic resin, an acrylic copolymer and an epoxy
resin were mixed in a solid content ratio of 70:30, and the
mixture was reacted and bonded with silica sol in a solid content
ratio of 60:40.
(b): To the treatment solution (a), trifunctional dibutyl
titanate prepared by reacting butyl titanate with triethanol
c~ Iqft: '
amine was added as the alkoxide ch~te compound in a solid content
weight ratio of 90:10.
(c)s To the treatment solution (a), ammonium metavanadate was
- 16 _
added in a solid conten-t weight ratio of 100:1.
However, it should be unders-tood that the treatment
solutions are not limited to those men-tioned above.
As the plated base steel sheets, there were used, an
electro-galvanized steel sheet, an zinc-nickel alloy plated steel
sheet, and an electrolytically formed zinc-iron alloy plated steel
sheet. As the plated base sheet, other zinc alloy plated steel
:sheets or aluminum plated steel sheets may also be used.
~he steps o~ the treatments were as follows: ~
Weak alkaline degreasing _ washing with water --~ drawing _~ i
reaction type chromate treatment (A, B) --~ drawing --~ washing
with water _ drying __~ application of the composite silicate
resin ___ drying.
Weak alkaline degreasing _ washing with water ~ drawing _~
coating type chromate treatment (C) __~ forcible drying
application of the composite silicate resin ~ drying.
It will be seen from the Table below, that the steel
sheets No. 1 to No. 37 of the present invention have distinctly
superior corrosion resistance to the conventional comparative
steel sheets r~O. 38 to ~3. It will also be seen that they are
superior to the conventional phosphate treated steel sheet
tlei~
. (comparative steel sheet No. 44~ in ~ paintability.
rhe comparative steel sheet No. 45 iB a steel sheet
on which the composite silicate resin film was formed without
the chromate treatment. ~rhe comparative steel sheet No. 46 is a
steel sheet on which the composite silicate resin film was formed
O.o~par~ ;v~
after phosphate treatment. The ompar&~e~ steel sheet No. 45
has good paintability comparable to the steel sheets of the
- 17 -
`` ~2~il 4~E~
present invention, but it is inferior in corrosion
resistance and in corrosion resistance after painting.
The comparative steel sheet No. 46 does not have
adequate corrosion resistance and paintability and it
is inferior in corrosion resistance after painting.
- 18 -
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