Note: Descriptions are shown in the official language in which they were submitted.
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TITLE OF THE INVENTION:
SURFACE TREATED STEEL SHEET FOR WELDED CANS
FIELD OF THE INVENTION:
This invention relates to an eletrolytically
chromated steel sheet and, more particularly, to a surface
treated steel sheet which has a high degree of weldability
and presents a surface having an outstanding good appear-
ance when painted, and is, therefore, suitable for use
in making cans by high-speed resistance seam welding.
BACKGROUND OF THE INVENTION:
An electrolytically chromated steel sheet, which
is obtained by forming on a surface of a steel sheet a
film composed of an undercoatlng layer of metallic chromium
and an overcoating hydrated chromium oxide layer consisting -
mainly of chromium oxide, is widely used for making cans,
such as cans for beverage and food, pail cans , 18-liter
cans and oil cans, since it is excellent in paintability
and corrosion resistance, and is less expensive than a tin
plate. The film is usually composed of an undercoating
layer of metallic chromium having a thickness of, say,
0.005 to 0.02 micron and an overcoating hydrated chromium
oxide layer having a thickness of, say, 0.01 to 0.02 micron.
There are two methods for forming the film, i.e.
the one~step method and the two-step method. The one-step
method forms the metallic chromium and hydrated chromium
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oxide layers simultaneously by the cathode electrolytic
treatment of a steel sheet in an electrolyte consisting
mainly of chromium trioxide and containing one or two
additives selected from sulfates and fluorine compounds.
The two-step method repeats the one-step method to form
the metallic chromium and hydrated chromium oxide layers,
but further includes dissolving away the hydrated chromium
oxide layer and forming a new hydrated chromium oxide layer
by cathode electrolytic treatment in an electrolyte con-
sisting mainly of chromic acid.
The electrolytically chromated steel sheet has
hitherto been used for making a two-piece can, which is
made by drawing, or a three-piece can, which is made by
joining the seams with an adhesive, such as an organic
resin or a special cement. It has, however, not often
been used for making a seam welded can, since it is very
low in weldability.
The recent increase in demand for strong and highly
reliable welded cans has, however, been calling for the
supply of an electrolytically chromated steel sheet having
an improved weldability without grinding.
The electrolytically chromated steel sheet known
in the art has a low weldability for the reasons which
will hereunder be set forth. The overcoating hydrated
chromium oxide layer serving as a surface coating is of
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the nature not conducting electricity or heat. There-
fore, the hydrated chromium oxide layer acts as an
insulator and produces a very high contact (or static)
resistance when electric resistance seam welding is
carried out to form a welded seam extending longitudinally
of the body of a can.
The value of contact resistance can be used as a
measure for evaluation as to the possibility of a localized
flow of an excessive current in a welding job. If a high
value of contact resistance exists, a welding current is
allowed to flow only through so narrow a path that a
localized flow of an excessive current is likely to occur.
The electrolytically chromated steel sheet has a very high
value of contact resistance as compared with any other type
of surface treated steel sheet used for making welded cans.
Therefore, the welding current flows only in a small quan-
tity during the initial stage of a welding operation and
begins to flow in the desired quantity only after the
passage of a certain length of time. As a consequence,
the localized heating of the steel sheet is likely to occur
and result in a splashing, or the formation of a welded
joint having blowholes or other defects.
It has, therefore, been necessary to remove by e.g.
grinding the chromate film from that portion of the steel
sheet along which a welded seam is going to be formed. This
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has been a job which requires a great deal of time and
labor.
There is known a method proposed to overcome the
problems as hereinabove stated. This method is charac-
terized by forming hard granular crystals on the whole
surface of the metallic chromium layer, so that, when a
welding pressure is applied to the sheet, those crystals
may destroy the overlying insulating hydrated chromium
oxide layer and thereby lower the contact resistance of
the film to a level enabling welding. The electrolytically
chromated steel sheet product of this method including a
layer of metallic chromium having granular crystals formed
on its whole surface (hereinafter referred to as "granular
metallic chromium") can be said to be a material having
an improved seam weldability without grinding.
When this type of chromated steel sheet is used
to make a welded can, however, it exhibits different heat-
and cooling characteristics between the overlapping inner
and outer edge portions thereof to be welded together to
form a seam extending longitudinally of the body of a can.
More specifically, it is usually the case that, as an inner
electrode roll is smaller in diameter than an outer elec-
trode roll, the inner edge portion of the sheet is likely
to generate a greater amount of heat, and that the inner
edge portion is also slower in cooling than the outer edge
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portion. Therefore, the inner edge portion is likely
to cause a splash or flash of molten material from its
edge, and a nugget is formed closer to the inner surface
of the can than to its outer surface.
In view of -these problems, the applicants of this
application have proposed an improved method of producing
an electrolytically chromated steel sheet as disclosed in
their Japanese patent application laid open to the public
under No. 35797/1988. This method is characterized by
subjecting one surface of a steel sheet at least once to
anode electrolytic treatment during its cathodic electro-
lytic chromating treatment to form granular metallic
chromium on that surface of the sheet, while hardly any
granular metallic chromium is formed on the other surface
thereof. This method is based on the concept that, if
the formation of granular metallic chromium is restrained
on the other surface of the steel sheet, it is possible to
attain a low contact resistance on the surface of the sheet
defining the inner surface of a can relative to the surface
defining the outer surface of the can to thereby equalize
the amounts of heat generated in the inner and outer sur-
faces of the can being manufactured and prevent any splash
on its inner surface.
Further consideration by the inventors of this
invention has, however, indicated that the electrolytically
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chromated steel sheet produced by the proposed method
cannot necessarily be said to be satisfactory in weld-
ability, for the reasons which will be set forth below:
(a) As the formation of granular metallic chromium
is not satisfactorily restrained on the surface of the
sheet defining the outer surface of the can, it is impos-
sible to eliminate the difference between the amounts of
heat generated in the inner and outer surfaces of the can
being manufactured;
(b) As the contact resistance between the films on
the two surfaces of the sheet is lower than that between
each film and the corresponding electrode, the films fail
to generate therebetween a sufficiently large amount of
heat to ensure the continuous formation of nuggets. There-
fore, it is impossible to achieve a sufficiently wide
range of a permissible welding current to form a weld having
a large nugget pitch.
At any rate, it has been found necessary to reconsider
carefully the structures of the films to be formed on the
two surfaces of the steel sheet in order to enable it to
show an outstandingly improved weldability.
Moreover, it is usual practice to form a beautiful
pattern of lacquering or printing on the outer surface of
a can for various purposes including rustproofing, protec-
tion against scratching, and decoration. The good outlook
of the lacquered or printed surface of a can is a factor
which has recently come to be considered particularly
important, and has created a demand for a steel sheet
on which a printed pattern having a bright color tone
can be produced, and on which the pigment used for print-
ing is allowed to maintain its own color. There has also
arisen a demand for a steel sheet having a metallic white
luster on its surface, so that it may retain its metallic
luster when coated with a transparent paint. It has,
however, been found that the steel sheet which can be
produced by the method as hereinabove described is unsatis-
factory in that connection, too, since it is likely to
present a printed or lacquered surface having a dark and
easily changing color tone.
SUMMARY OF THE INVENTION:
In view of the drawbacks of the prior art as here-
inabove pointed out, it is an object of this invention to
provide an electrolytically chromated steel sheet for a
welded can which has an outstandingly good high-speed seam
weldability without grinding and also can form a lacquered
or printed surface having an outstandingly good outlook.
In connection with the high-speed seam welding of
an electrolytically chromated steel sheet without grinding,
we, the inventors of this invention, have made a detailed
study of the relation which may exist between the contact
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resistance of the films and the heating and cooling
characteristics of the steel sheet, and also of the
nature of the naggets which may be formed. As a result,
we have found that, in order to improve the weldability
of the sheet, it is important to achieve an optimum
balance of resistance heating at the interface between
the outer surface of a welded joint to be formed and the
electrode, at the interface between the contacting portions
of the sheet and at the interface between the inner surface
of the joint and the electrode, and also an optimum balance
of cooling by the electrodes on the inner and outer sur-
faces of the joint.
We have studied the film structure which may
realize the optimum balances of resistance heating and of
cooling, and have found the following:
(a) It is not sufficient to reduce the amount of
granular metallic chromium on one side of the steel sheet
which will form the outer surface of a can;
(b) It is necessary to define strictly the coating
weight of the film on each side of the steel sheet;
(c) Referring to the granular metallic chromium formed
on that side of the sheet which will form the inner surface
of the can, it is only the particles of a specified diameter or a
larger one that contribute effectively to achieving a lower
contact resistance upon application of pressure by the
electrodes. It is not sufficient to form granular
metallic chromium, but it is necessary to form suffi-
ciently large particles of granular metallic chromium
with a strictly defined density; and
(d) The same is true of that side of the sheet which
will form the outer surface of the can. It is necessary
-to define strictly the density of sufficiently large par-
ticles of granular metallic chromium formed on that side,
too.
The structures of the films formed on both sides
of the steel sheet as defined above not only enable the
satisfactory passage of a welding current and the preven-
tion of localized heating, as a result of the destruction
of hydrated chromium oxide by granular metallic chromium
on one side of the sheet, but also make it possible to:
(i) Eliminate substantially any difference between the
amounts of heat generated on the inner and outer surfaces
of the can being manufactured; and
(ii) Prevent the contact resistance at the interface
between the contacting portions of the sheet from becoming
too low as compared with that at the interface between each
film and the electrode, and achieve an optimum balance
therebetween, thereby enabling the sufficient heating of
the contacting portions to be welded, and facilitating the
continuous formation of nuggets.
We have also made a detailed study of the relation
which may exist between -the degree of granulation of a
metallic chromium layer on an electrolytically chromated
steel sheet and the outlook and color tone of a lacquered
or painted surface formed on the sheet. As a result, we
have found that the surface of the sheet is more likely to
scatter or absorb light having a short wavelength, as the
density of granular metallic chromium increases, and that
the lacquered or painted surface has, therefore, a dark
outlook and a color tone which is easily changeable
emphasizing a red or like color. It, therefore, follows
that the film on that side of the steel sheet which will
form the outer surface of a can may not contain any
granular metallic chromium, or that, if it contains any
granular metallic chromium, it may contain only an extremely
small proportion of relatively large particles. Thus,
the results of our study confirm that the electrolytically
chromated steel sheet as hereinabove defined presents a
lacquered or painted surface having an outstandingly good
outlook, as well as it has an improved weldability.
This invention is based on our findings as herein-
above described. The object of this invention as herein-
above stated is essentially attained by an electrolytically
chromated steel sheet carrying on one of the two pri~cipal sur-
faces thereof an electrolytic chromating film including a
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metallic chromium layer containing a high proportion of
granular metallic chromium having a large particle dia-
meter, while on the other of the two principal surfaces
thereof, it carries an electrolytic chromating film
including either a metallic chromium layer in a continuous
sheet form which is free of any granular metallic chromium,
or a metallic chromium layer containing only a very low
proportion of granular metallic chromium having a large
particle diameter. This invention may be reduced to
practice in a variety of modes as will hereunder be set
forth:
1. A surface treated steel sheet for a welded can
which carries on one of the two principal surfaces thereof
an electrolytic chromating film comprising 30 to 150 mg,
per square meter, of a metallic chromium layer consisting
of a mass of granular metallic chromium adhering to the
one surface of the sheet, the granular metallic chromium
containing at least 30 particles having a diameter of at
least 0.03 micron per square micron of the layer, and 3
to 15 mg, per square meter, of a hydrated chromium oxide
layer (in terms of metallic chromium) formed on the
metallic chromium layer, while on the other of the two
principal surfaces thereof, it carries an electrolytic
chromating film comprising 30 to 150 mg, per square meter,
of a metallic chromium layer consisting of metallic chromium
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adhering in sheet form to the other surface of the sheet
and granular metallic chromium laid on the said metallic
chromium in sheet form, the granular rnetallic chromium
containing less than 15 particles having a diameter of
at least 0.03 micron per square micron of the layer, and
3 to 30 mg, per square meter, of a hydrated chromium oxide
layer (in terms of metallic chromium) formed on the metal-
lic chromium layer.
2. A surface treated steel sheet for a welded can
which carries on one of two principal surfaces thereof
an electrolytic chromating film comprising 50 to 150 mg,
per square meter, of a metallic chromium layer consisting
of metallic chromium adhering in sheet form to the one
surface of the sheet and granular metallic chromium laidon the
said metallic chromium in sheet form, the granular metal-
lic chromium containing at least 50 to 300 particles having
a diameter of at least 0.03 micron per square micron of
the layer, and 3 to 15 mg, per square meter, of a hydrated
chromium oxide layer (in terms of metallic chromium) formed
on the metallic chromium layer, while on the other of the
two principal surfaces thereof, it carries an electrolytic
chromating film comprising 30 to 150 mg, per square meter,
of a metallic chromium layer consisting of metallic chromium
adhering in sheet form to the other surface of the sheet and
granular metallic chromium laid on the said metallic chromium
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in sheet form, the granular metallic chromium containing
less than 15 particles having a diameter of at least 0.03
micron per square micron of the layer, and 3 to 30 mg,
per square meter, of a hydrated chromium oxide layer
(in terms of me-tallic chromium) formed on the metallic
chromium layer.
3. A surface treated steel sheet for a welded can
which carries on one of two principal surfaces thereof
an electrolytic chromating film comprising 30 to 150 mg,
per square meter, of a metallic chromium layer consisting
of a mass of granular metallic chromium adhering to the
one surface of the sheet, the granular metallic chromium
containing at least 30 particles having a diameter of at
least 0.03 micron per square micron of the layer, and 3
to 15 mg, per square meter, of a hydrated chromium oxide
layer (in terms of metallic chromium) formed on the metal-
lic chromium layer, while on the other of the two principal
surfaces thereof, it carries an electrolytic chromating
film comprising 30 to 150 mg, per square meter, of a metal-
lic chromium layer consisting of a mass of granular metallic
chromium adhering to the other surface of the sheet, the
granular metallic chromium containing less than 15 particles
having a diameter of at least 0.03 micron per square micron
of the layer, and 3 to 30 mg, per square meter, of a
hydrated chromium oxide layer (in terms of metallic chromium)
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formed on the metallic chromium layer.
4. A surface treated steel sheet for a welded can
which carries on one of the two principal surfaces thereof
an electrolytic chromating film comprising 50 to 150 mg,
per square meter, of a metallic chromium layer consisting
of metallic chromium adhering in sheet form to the one
surface of the sheet and granular metallic chromium laidon the
said metallic chromium in sheet form, the granular metal-
lic chromium containing at least 50 to 300 particles having
a diameter of at least 0.03 micron per square micron of
the layer, and 3 to 15 mg, per square meter, of a hydrated
chromium oxide layer (in terms of metallic chromium) formed
on the metallic chromium layer, while on the other of the
two principal surfaces thereof, it carries an electrolytic
chromating film comprising 30 to 150 mg, per square meter,
of a metallic chromium layer consisting of a mass of
granular metallic chromium adhering to the other surface
of the sheet, the granular metallic chromium containing
less than 15 particles having a diameter of at least 0.03
micron per square micron of the layer, and 3 to 30 mg, per
square meter, of a hydrated chromium oxide layer (in terms
of metallic chromium) formed on the metallic chromium layer.
5. A surface treated steel sheet for a welded can which
carries on one of the two principal surfaces thereof
an electrolytic chromating film comprising 30 to 150 mg,
per square meter, of a metallic chromium layer consisting
of a mass of granular metallic chromium adhering to the
one surface of the sheet, the granular metallic chromium
containing at least 30 particles having a diameter of at
least 0.03 micron per square micron of the layer, and 3 -to
15 mg, per square meter, of a hydrated chromium oxide
layer (in terms of metallic chromium) formed on the metal-
lic chromium layer, while on the other of the two principal
surfaces thereof, it carries an electrolytic chromating
film comprising 30 to 150 mg, per square meter, of a metal-
lic chromium layer consisting of metallic chromium adhering
in sheet form to the other surface of the sheet, and 3 to
30 mg, per square meter, of a hydrated chromium oxide layer
(in terms of metallic chromium) formed on the metallic
chromium layer.
6. A surface treated steel sheet for a welded can
which carries on one of two principal surfaces thereof
an electrolytic chromating film comprising 50 to 150 mg,
per square meter, of a metallic chromium layer consisting
of metallic chromium adhering in sheet form to the one
surface of the sheet and yranular metallic chromium laidon the
said metallic chromium in sheet form, the granular metal-
lic chromium containing at least 50 to 300 particles having
a diameter of at least 0.03 micron per square micron of the
layer, and 3 to 15 mg, per square meter, of a hydrated
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chromium oxide layer (in terms of metallic chromium)
formed on the metallic chromium layer, while on the
other of the two principal surfaces bhereof, it carries
an electrolytic chromating film comprising 30 to 150 mg,
per square meter, of a metallic chromium layer consisting
of metallic chromium adhering in sheet form to the other
surface of the sheet, and 3 to 30 mg, per square meter,
of a hydrated chromium oxide layer (in terms of metallic
chromium) formed on the said metallic chromium layer.
DETAILED DESCRIPTION OF THE INVENTION:
The electrolytically chromated steel sheet of
this invention is essentially characterized by carrying
on one of the two principal surfaces thereof an electrolytic
chromating film including a metallic chromium layer con-
taining a high proportion of granular metallic chromium
having a large particle diameter, while on the other of
the two principal surfaces thereof, it carries an electro-
lytic chromating film including either a metallic chromium
layer in sheet form which is free of any granular metallic
chromium, or a metallic chromium layer containing only a
very low proportion of granular metallic chromium having
a large particle diameter.
A variety of methods can be employed to form a
metallic chromium layer containing the desired granular
metallic chromium on the sheet surface to be treated. A
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few examples of the methods are:
(a) Anode electrolytic treatment in a plating bath
prior to chromium plating;
(b) Fine anode electrolytic treatment which is carried
out during chromium plating; and
(c) Discontinuous plating which is carried out by
providing a dipping time during chromium plating.
The anode electrolytic treatment of the steel sur-
face prior to electrolytic chromating is carried out in
a bath which is usually employed in the cathode electro-
lytic treatment for metallic chromium plating or hydrated
chromium oxide coating, whereby a very thin hydrated
chromium oxide film having a coating weight not exceeding
2 mg/m2 is deposited on the electrolytically treated sur-
face. This film has a multiplicity of fine discontinuous
portions which enable the subsequent electrolytic chromat-
ing treatment to form a metallic chromium layer consisting
of granular metallic chromium on the steel surface. This
method, therefore, makes it possible to form a film contain-
ing granular metallic chromium directly on the steel sur-
face.
The other two methods, i.e. anode electrolytic
treatment during chromium plating and discontinuous electro-
lytic treatment, are also carried out in a bath which is
usually employed in the cathode electrolytic treatment for
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chromium plating or hydrated chromium oxide coating,
whereby a hydrated chromium oxide film which facilitates
the formation of granular metallic chromium (i.e. which
has a low anion content and a very small thickness) is
formed on metallic chromium in sheet form adhering to
the steel surface. This film has fine discontinuous por-
tions containing anions locally which enable the subsequent
electrolytic chromating treatment to form granular metallic
chromium on the whole surface of the metallic chromium in
sheet form. Thus, it is possible to form by either method
a film comprising metallic chromium adhering in sheet form
to the steel surface and granular metallic chromium formed
thereon.
The metallic chromium layer which is formed on one
of the steel surfaces and contains a high proportion of
granular metallic chromium having a large particle diameter
consists either of a mass of granular metallic chromium
adhering to the steel surface, or of a combination of metal-
lic chromium adhering in sheet form to the steel surface
and granular metallic chromium formed thereon. The layer
having either of these two structures can be formed if an
appropriately selected method is employed as hereinabove
described.
If the metallic chromium layer consists of a mass
of granular metallic chromium, it is required to contain
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30 to 150 mg of metallic chromium per square meter.
If it contains only less than 30 mg of chromium per
square meter, the incomplete growth of granular metallic
chromium results in only an incomplete reduction of
contact resistance between the sheet surface forming the
inner surface of a can and the electrode, and also between
the contacting surfaces of the sheet. The incomplete
growth of chromium particles means also the incomplete
coating of the steel surface and therefore the low corrosion
resistance thereof. Any layer containing more than 150 mg
of chromium per square meter is uneconomical, though it
may satisfactorily achieve the intended result.
The particle diameter and density of gran~lar
metallic chromium have a critical bearing on the intended
result. It is necessary to ensure that granular metallic
chromium having a large particle diameter be formed in a
high density, or proportion. More specifically, it is
necessary to ensure that at least 30 particles having a
diameter of at least 0.03 micron be formed in an area of
square micron. The metallic chromium layer consisting of
a mass of granular metallic chromium usually contains
several hundred particles per square micron of its surface.
It is, however, relatively large particles that contribute
to achieving a lower contact resistance upon application
of pressure by the electrode. Hardly any such result can
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be expected from particles having a diameter which is
smaller than 0.03 micron. Even sufficiently large par-
ticles fail to produce any satisfactory result, unless
they are uniformly distributed. Therefore, it is neces-
sary for the layer to have a density of at least 30 par-
ticles per square micron.
If the metallic chromium layer consists of metal-
lic chromium adhering in sheet form to the steel surface
and granular metallic chromium formed thereon, it is
required to contain 50 to 150 mg of metallic chromium per
square meter. If it contains only less than S0 mg of
chromium per square meter, the incomplete growth of granular
metallic chromium results in only an incomplete reduction
of contact resistance between the sheet surface forming
the inner surface of a can and the electrode, and also
between the contacting surfaces of the sheet, though it may be satisfactory
in corrosion resistance. Any layer containing more than 150 mg of chromium
per square meter is uneconomical, though it may satisfactorily achieve the
intended result. Therefore 150 mg of chromium per square meter is set as
an upper limit.
The metallic chromium layer of this construction
is also required to contain a high density or proportion
of granular metallic chromium having a large particle dia-
meter. More specifically, it is required to contain 50
to 300 particles having a diameter of at least 0.03 micron
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per square micron. It is reIatively large particles
that contribute to achieving a lower contact resistance
upon application of pressure by the electrode, and hardly
any such result can be expected from particles having a
diameter which is smaller than 0.03 micron, as hereinabove
stated.
The deposition of granular metallic chromium on
metallic chromium in sheet form tends to be affected to
some extent by the crystal orientation of the underlying
chromium. In other words, granular metallic chromium is
distributed less uniformly than in the metallic chromium
layer consisting solely of granular chromium. Therefore,
the layer is required to contain at least 50 sufficiently
large particles per square micron to ensure that the granular
chromium show the expected result. This is a proportion
which is higher than the minimum proportion of such par-
ticles that the layer consisting solely of granular chromium
- is required to contain. The maximum proportion of 300
particles per square micron is a limit set to save the
amount of chromium, and does not mean that a higher propor-
tion will adversely affect the result expected from granular
chromium.
The steel surface which has been coated with
granular metallic chromium is electrolytically chromated,
whereby a hydrated chromium oxide layer is formed on the
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metallic chromium layer. The hydrated chromium oxide
layer is provided for ensuring the corrosion resistance
and paintability of the steel surface. The layer is
required to contain 3 to 15 mg of metallic chromium per
square meter. If it contains only less than 3 mg of
chromium per square meter, the steel surface is undesirably
low in corrosion resistance, and if it contains more than
15 mg of chromium per square meter, a satisfactorily low
contact resistance is difficult to achieve between the
steel surface forming the inner surface of a can and the
electrode.
The metallic chromium layer which is formed on
the other of the steel surfaces is a layer containing no
granular chromium (i.e. consisting solely of chromium in
sheet form), or a layer in which granular chromium having
a large particle diameter occupies a by far lower propor-
tion than in the layer on the one steel surface. If the
proportion of such granular chromium on the other steel
surface exceeds a certain limit, the contact resistance
at the interface between the contacting portions of the
steel sheet becomes too low, as compared with the contact
resistance at the interface between the film and the
electrode, to generate a sufficiently large amount of heat
in those contacting portions.
The metallic chromium layer on the other steel
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surface in which granular chromium having a large particle
diameter occupies a very low proportion, may consist of
either a mass of granular chromium adhering to the steel
surface, or a combination of chromium adhering in sheet
form to the steel surface and granular chromium formed on
it, as is the case with the layer on the one steel surface.
Either of these two layer structures can be formed by
employing an appropriate method as hereinabove described.
More specifically, a layer of the former construction is
usually formed by the anode electrolytic treatment which
is carried out in a plating bath prior to chromium plating,
while a layer of the latter construction is usually formed
fine
by the/anode electrolytic treatment which is carried out
after chromium plating, or the discontinuous plating which
is carried out by allowing a dipping time during chromium
plating. If the fine anode electrolytic treatment is
carried out on the other steel surface after chromium plat-
ing, however, no cathode electrolytic treatment is there-
after carried out, since the cathode electrolytic treatment
forms too large an amount of granular chromium having a
large particle diameter to be acceptable within the limits
as defined by this invention~
The other steel surface is required to contain only
a very low proportion of granular chromium having a large
particle diameter, as hereinabove stated. More specifically,
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the granular chromlum which is formed on the other steel
surface is required to contain only less than 15 particles
having a diameter of at least 0.03 micron per square
micron of the layer, irrespective of the structure of
the layer. It is relatively large particles having a
diameter of at least 0.03 micron that contribute to achiev-
ing a lower contact resistance upon application of pressure
by the electrode. If the layer contains 15 or more such
particles per square micron, it begins to show a lower
contact resistance, though locally, and disables the steel
sheet to exhibit the intended result. Moreover, it will
present only a printed or lacquered surface having an out-
look which is dark and does not have a good color tone.
The Japanese patent application laid open under No.
35797/1988 discloses a method of producing an electro-
lytically chromated steel sheet carrying granular metallic
chromium on one surface thereof, but hardly any such chromium
on the other surface thereof by subjecting the one surface
thereof at least once to anode electrolytic treatment during
cathode electrolytic chromating treatment. Although this
method may hardly form any granular chromium on the other
surface of the sheet, the amount of granular chromium which
it forms on that surface is considerably greater than the
maximum proportion defined for the sheet of this invention.
More specifically, the granular chromium which is formed on
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tne other surface of the sheet contains at least about
20 particles having a diameter of at least 0.03 micron
per square micron. This is a proportion which is too
high to be expected to produce the result of this inven-
tion.
~e have examined the reason why a certain amount
of granular chromium, which is undesirably large from the
standpoint of this invention, is formed on the other sur-
face of the sheet when the method of the Japanese patent
application as hereinabove referred to is employed, and
have found that it is due to the cathode electrolytic treat-
ment to which not only one, but also the other of the sheet
surfaces is subjected after one surface has been given
anode electrolytic treatment. It, therefore, follows that,
if the steel sheet of this invention is produced by the
method including the intermediate anode electrolytic treat- -
ment of one surface thereof during cathode electrolytic
treatment (i.e. the fine anode electrolytic treatment there-
of during chromium plating), it is essential that the sub~
sequent cathode electrolytic treatment be given only to
the one surface which has been subjected to the intermediate
anode electrolytic treatment.
The metallic chromium layer on the other surface
of the sheet is defined as containing 30 to 150 mg of
chrornium per square meter, irrespective of its structure.
- 25 -
2 ~
If it contains only less than 30 mg of chromium per
square meter, it fails to cover the sheet surface sufficiently to render
it fully resistant to corrosion. Any layer containing more than 150 mg
of chromium per square meter is uneconomical, though it may effectively
achieve the intended result. Therefore 150 mg of chromium per square
meter is set as an upper limit.
A hydrated chromium oxide layer is formed on the
other surface of the sheet, too, when it is electrolytically
chromated. This layer ensures the corrosion resistance
and paintability of the sheet, as hereinbefore stated. The
layer is required to contain 3 to 30 mg of chromium per
square meter. If it contains only less than 3 mg of
chromium per square meter, it fails to provide any satis-
factory corrosion resistance and is also likely to give
an undesirably low contact resistance. Any layer contain-
lng more than 30 mg of chromium per square meter is somewhat uneconomical,
though it may not present any particular problem from a
weldability standpoint. Moreover, the presence of too much hydrated
chromium oxide is likely to give the sheet a colored surface having an
uneven outlook due to the lack in uniformity of oxide distribution. There-
fore 30 mg per square meter is set as an upper limit.
EXAMPLES:
The invention will now be described more specifi-
cally with reference to a variety of examples.
- 26 -
,
- .
:
. , . ~ , :,
EXAMPLE l
Two cold rolled steel sheets having thicknesses
of 0.22 mm and 0.32 mm, respectively, were each treated
by a process comprising electrolytic degreasing in a
solution containing 30 g of sodium hydroxide per liter,
rinsing with water, electrolytic pickling in an aqueous
solution of sulfuric acid having a concentration of 5 g
per liter, rinsing with water, pretreatment on one surface
alone under the conditions listed below, electrolytic
chromating on both surfaces under the conditions listed
below, rinsing with water, and drying.
Conditions for pretreatment:
Solution: The solution used for the pretreatment
contained 100 g of chromium trioxide
and 1 g of sulfuric acid per liter;
Temperature: 25C;
Method: Anode electrolytic treatment;
Anode current density: 10 A/dm2;
Electrolyzing time: 0.3 sec.
Conditions for the electrolytic chromating:
Solution: The solution used for the treatment con-
tained 175 g of chromium trioxide, 5 g
of Na2SiF6 and 0.9 g of Na2SO4 per liter;
Temperature: 42C;
Method: Discontinuous cathode electrolytic treat-
ment-(cathodic on and off electrolysis);
- 27 -
2 ~
Cathode current density: 40 A/dm2;
Electrolyzing time: 0.3 sec.;
On and off cycle: Four cycles were repeated;
Dipping time: 0.3 sec.
EXAMPLE 2
Two cold rolled steel sheets having thicknesses
of 0.22 mm and 0.32 mm, respectively, were each treated
by a process comprising electrolytic degreasing in a solu-
tion of sodium hydroxide having a concentration of 30 g
per liter, rinsing with water, electrolytic pickling in an
aqueous solution of sulfuric acid having a concentration
of 5 g per liter, rinsing with water, electrolytic chromat-
ing on both surfaces under the conditions listed at (a)
below in a bath having the composition and temperature
shown below, intermediate anodic treatment on one surface
alone in the same bath under the conditions listed at (b)
below, electrolytic chromating under the conditions listed
at (a) below only on the surface given the intermediate
anodic treatment, rinsing with water, and drying.
Electrolytic bath:
Composition: The solution contained 175 g of
chromium trioxide, 5 g of Na2SiF6
and 0.9 g of Na2SO4 per liter;
Temperature: 40C.
(a) Cathodic on and off electrolysis:
Cathode current density: 40 A/dm2;
- 28 -
Electrolyzing time: 0.3 sec.;
On and off cycle: Two cycles were repeated;
Dipping time: 0.3 sec.
(b) Anodic electrolysis:
Anode current density: 4 A/dm ;
Electrolyzing time: 0.3 sec.
EXAMPLE 3
Two cold rolled steel sheets having thicknesses
of 0.22 mm and 0.32 mm, respectively, were each treated
by a process comprising electrolytic degreasing in a solu-
tion of sodium hydroxide having a concentration of 30 g
per liter, rinsing with water, electrolytic pickling in
an aqueous solution of sulfuric acid having a concentration
of 5 g per liter, rinsing with water, electrolytic chromat-
ing on both surfaces under the conditions listed at (A)
below, after 10 seconds of dipping, electrolytic chromating
on one surface alone under the conditions listed at (~)
below, rinsing with water, and drying.
(A) Conditions for electrolytic chromating:
Solution: The solution contained 175 g of chromium
trioxide, 5 g of Na2SiF6 and 0.9 g of
Na2SO4 per liter;
Temperature: 45C;
Method; Cathodic on and off electrolysis;
Cathode current density: 40 A/dm2;
- 29 -
.
Electrolyzing time: 0.3 sec.;
On and off cycle: Two cycles were repeated;
Dipping time: 0.3 sec.
(B) Conditions for electrolytic chromating:
Solution: The solution contained 175 g of chromium
trioxide, 5 g of Na2SiF6 and 0.9 g of
Na2SO4 per liter;
Temperature: 45C;
Method: Cathodic on and off electrolysis;
Cathode current density: 40 A/dm2;
Electrolyzing time: 0.3 sec.;
On and off cycle: Two cycles were repeated;
Dipping time: 0.3 sec.
EYA~IPLE 4
Two cold rolled steel sheets having thicknesses
of 0.22 mm and 0.32 mm, respectively, were each treated
by a process comprising electrolytic degreasing in a solu-
tion of sodium hydroxide having a concentration of 30 g
per liter, rinsing with water, electrolytic pickling in an
aqueous solution of sulfuric acid having a concentration
of 5 g per liter, rinsing with water, pretreatment on one
surface alone under the conditions listed below, electro-
lytic chromating on both surfaces under the conditions
listed below, rinsing with water, and drying.
Conditions for pretreatment:
Solution: The solution contained 100 g of chromium
trioxide and 1 g of sulfuric acid per
liter;
- 30 -
2~5~149
Temperature: 25C;
Method: Anodic electrolysis;
Anode current density: 10 A/dm2;
Electrolyzing time: 0.3 sec.
Conditions for electrolytic chromating:
Solution: ~he solution contained 175 g of chromium
trioxide, 5 g of Na2SiF6 and 0.9 g of
Na2So4 per liter;
Temperature: 42C;
Method: Cathodic on and off electrolysis;
Cathode current density: 80 A/dm2;
Electrolyzing time: 0.3 sec.;
On and off cycle: Two cycles were repeated;
Dipping time: 0.3 sec.
EX~PLE 5
Two cold rolled steel sheets having thicknesses
of 0.22 mm and 0.32 mm, respectively, were each treated
by a process comprising electrolytic degreasing in a solu-
tion of sodium hydroxide having a concentration of 30 g
per liter, rinsing with water, electrolytic pickling in an
aqueous solution of sulfuric acid having a concentration
of 5 g per liter, rinsing with water, pretreatment on one
surface alone under the conditions listed below, electro-
lytic chromating on both surfaces under the conditions listed
below, rinsing with water, and drying.
- 31 -
Conditions for pretreatment:
Solution: The solution contained 100 g of chromium
trioxide and 1 g of sulfuric acid per
liter;
Temperature: 25C;
Method: Anodic electrolysis;
Anode current density: 10 A/dm ;
Electrolyz,ing time: 0.3 sec.
Conditions for electrolytic chromating:
Solution: The solution contained 175 g of chromium
trioxide, 5 g of Na2SiF6 and 0.9 g of
Na2SO4 per liter;
Temperature: 45C;
Method: Cathodic on and off electrolysis;
Cathode current density: 80 A/dm2;
Electrolyzing time: 0.3 sec.;
On and off cycle: Two cycles were repeated;
Dipping time: 0.3 sec.
EXAMPLE 6
Two cold rolled steel sheets having thicknesses
of 0.22 mm and 0.32 mm, respectively, were each treated
by the process which had been employed in EXAMPLE 2, except
that cathodic on and off electrolysis was carried out under
the conditions shown at (a) below:
- 32 -
(a) Cathodic on and off electrolysis:
Cathode current density: 80 A/dm2;
Electrolyzing time: 0.2 sec.;
On and off cycle: Two cycles were repeated;
Dipping time: 0.5 sec.
EXAMPLE 7
Two cold rolled steel sheets having thicknesses
of 0.22 mm and 0.32 mm, respectively, were each treated
by repeating the process which had been employed in EXAMPLE
2, except that anodic electrolysis was carried out under
the conditions shown at (b) below:
(b) Anodic electrolysis:
Anode current density: 0.5 A/dm2;
Electrolyzing time : 0.3 sec.
EXAMPLE 8
EXAMPLE 2 was repeated, except that cathodic on
and off electrolysis was carried out at a cathode current
density of 30 A/dm2.
EXAMPLE 9
-
EXAMPLE 1 was repeated, except that pretreatment
was carried out on both surfaces under the conditions
employed in EXAMPLE 1, except for the following:
Anode current density: 10 A/dm2 for one surface, and
1 A/dm2 for the other surface;
Electrolyzing time : 0.3 sec. for each surface.
- 33 -
EX~IPLE _
EXAMPLE 9 was repeated, except that electrolytic
chromating (cathodic on and off electrolysis) was carried
out in a bath having a temperature of 45C.
EXAMPLE 11
EXAMPLE 1 was repeated, except that cathodic on
and off electrolysis was carried out by employing a cathode
current density of 30 A/dm and repeating the on and off
cycle twice.
EXAMPLE 12
Two cold rolled steel sheets having thicknesses
of 0.22 mm and 0.32 mm, respectively, were each treated
by a process comprising electrolytic degreasing in a solu-
tion of sodium hydroxide having a concentration of 30 g
per liter, rinsing with water, electrolytic pickling in
an aqueous solution of sulfuric acid having a concentration
of 5 g per liter, rinsing with water, pretreatment on one
surface alone under the conditions listed below, electro-
lytic chromating on both surfaces in a solution having the
composition shown below and under the conditions listed at
(a) below, intermediate anodic treatment on the other sur-
face in the same solution under the conditions listed at
(b) below, electrolytic chromating on the other surface
alone under the conditions listed at (a), rinsing with water,
and drying.
- 34 -
Conditions for pretreatment:
Solution: The solution contained 100 g of chromium
trioxide and 1 g of sulfuric acid per
liter;
Temperature: 25C;
Method: Anodic electrolysis;
Anode current density: 10 A/dm2;
Electrolyzing time: 0.3 sec.
Conditions for electrolytie chromating:
Solution: The solution eontained 175 g of chromium
trioxide, 5 g of Na2SiF6 and 0.9 g of
Na2SO4 per liter;
Temperature: 50C;
(a) Cathodie on and off eleetrolysis:
Cathode eurrent density: 40 A/dm ;
Electrolyzing time: 0.3 see.;
On and off eyele: Two eyeles were repeated;
Dipping time: 0.3 see.;
(b) Anodie eleetrolysis:
Anode eurrent density: 4 A/dm ;
Eleetrolyzing time: 0.3 see.
_ AMPLE 13
EXAMPLE 1 was repeated, exeept that after ehromat-
ing and before rinsing, posttreatment was earried out on
both surfaees under the eonditions listed below:
2 ~
Conditions for posttreatment:
Solution: The solution contained 50 g of chromium
trioxide per liter;
Temperature: 45C;
Method: Cathodic electrolysis;
Cathode current density: 10 A/dm2;
Electrolyzing time: 0.3 sec.
EXAMPLE 14
EXAMPLE 2 was repeated, except that after the final
ehromating and before rinsing, posttreatment was carried
out on both surfaces under the conditions employed in
EXAMPLE 13.
EXAMPLE 15
Two cold rolled steel sheets having thicknesses
of 0.22 mm and 0.32 mm, respectively, were each treated -
by a process eomprising eleetrolytie degreasing in a solu-
tion of sodium hydroxide having a eoncentration of 30 g
per liter, rinsing with water, electrolytic pickling in
an aqueous solution of sulfuric acid having a eoneentration
of 5 g per liter, rinsing with water, pretreatment on one
surfaee alone under the conditions listed below, eleetro-
lytie ehromating on both surfaees under the eonditions
listed below, rinsing with water, and drying.
Conditions for pretreatment:
Solution: The solution eontained 100 g of chromium
trioxide and 1 g of sulfuric acid per
liter;
- 36 -
2 ~
Temperature: 25C;
Method: Anodic electrolysis;
Anode current density: :L0 A/dm ;
Electrolyzing time: 0.3 sec.
Conditions for electrolytic chromating:
Solution: The solution contained 175 g of chromium
trioxide, 5 g of Na2SiF6 and 0.9 g of
Na2SO4 per liter;
Temperature: 40C;
Method: Continuous cathodic electrolysis;
Cathode current density: 160 A/dm2;
Electrolyzing time: 0.3 sec.
EXAMPLE 16
EXAMPLE 15 was repeated, except that pretreatment
was carried out at an anode current density of 5 A/dm2 by
employing a solution containing 175 g of chromium trioxide,
5 g of Na2SiF6 and 0.9 g of Na2SO4 per liter and having a
temperature of 40C, and that electrolytic chromating was
carried out by employing a solution having a temperature
of 45C.
EXAMPLE 17
Two cold rolled steel sheets having thicknesses
of 0.22 mm and 0.32 mm, respectively, were each treated
by a process comprising electrolytic degreasing in a solu-
tion of sodium hydroxide having a concentration of 30 g
per liter, rinsing with water, electrolytic pickling in
- 37 -
3~
an aqueous solution of sulfuric acia having a concentration
of 5 g per liter, rinsing with water, electrolytic
chromating on both surfaces under the conditions listed
at (a) below using a solution having the composition and
temperature shown below, intermediate anodic treatment on
one surface alone in the same solution under the conditions
listed at (b) below, electrolytic chromating on the one surface alone
given the said intermediate anodic treatment under the conditions listed
at (a), rinsing with water, and drying.
Solution: The solution contained 175 g of chromium
trioxide, 5 g of Na2SiF6 and 0.9 g of
Na2S04 per liter;
Temperature: 45C.
(a) Continuous cathodic electrolysis:
Cathode current density: 100 A/dm ;
Electrolyzing time : 0.3 sec.
(b) Anodic electrolysis:
Anode current density : 2 A/dm ;
Electrolyzing time : 0.3 sec. ~-
EXArlPLE 18
EXAMPLE 17 was repeated, except that after the
final chromating and before rinsing, posttreatment was
carried out under the conditions listed below:
Conditions for posttreatment:
Solution: The solution contained 50 g of chromium
trioxide and 0.5 g of NH4F per liter;
Temperature: 45C;
- 38 -
2~g~
Method: Cathodic electrolysis;
Cathode current density: 20 A/dm2;
Electrolyzing time: 0.5 sec.
EXA~IPLE 19
Two cold rolled steel sheets having thicknesses
of 0.22 mm and 0.32 mm, respectively, were each treated
by a process comprising electrolytic degreasing in a solu-
tion of sodium hydroxide having a concentration of 30 g
per liter, rinsing with water, electrolytic pickling in
an aqueous solution of sulfuric acid having a concentration
of 5 g per liter, rinsing with water, electrolytic chromat-
ing on both surfaces under the conditions listed at (A)
below, after 10 seconds of dipping, electrolytic chromating
on one surface alone under the conditions listed at (B)
below, rinsing with water, and drying.
(A) Conditions for electrolytic chromating:
Solution: The solution contained 175 g of chromium
trioxide, 5 g of Na2SiF6 and 0.9 g of
Na2SO4 per liter;
Temperature: 35C;
Method: Continuous cathodic electrolysis;
Cathode current density: 80 A/dm2;
Electrolyzing time: 0.3 sec.
(B) Conditions for electrolytic chromating:
Solution: The solution contained 175 g of chromium
trioxide, 5 g of Na2SiF6 and 0.9 g of
Na2SO4 per liter;
- 39 -
2~581~9
Temperature: 45C;
Method: Continuous cathodic electrolysis;
Cathode current density: 80 A/dm2;
Electrolyzing time: 0.3 sec.
EXAMPLE 20
EXAMPLE 15 was repeated, except that electrolytic
chromating was carrled out in a solution having a tempera-
ture of 46C.
EXAMPLE 21
EXAMPLE 17 was repeated, except that anodic electro-
lysis was carried out at an anode current density of 0.5
A/dm .
EXAMPLE 22
EXAMPLE 15 was repeated, except that pretreatment
was carried out by employing a solution containing 175 g
of chromium trioxide, 5 g of Na2SiF6 and 0.9 g of Na2SO4
per liter and having a temperature of 40C, and an anode
current density of 5 A/dm , while electrolytic chromating
was carried out by using a solution temperature of 45C
and an electrolyzing time of 0.1 sec.
EXAMPLE 23
EXAMPLE 17 was repeated, except that cathodic
electrolysis was carried out by employing a cathode current
density of 150 A/dm and an electrolyzing time of 0.1 sec.
- 40 -
2 ~
EX~IPLE 24
EXAMPLE 16 was repeated, except that after chromat-
ing and before rinsing, posttreatment was carried out on
both surfaces under the conditions listed below:
Conditions for posttreatment:
Solution: The solution contained 50 g of chromium
trioxide per liter;
Temperature: 45C;
Method: Cathodic electrolysis;
Cathode current density: 10 A/dm2;
Electrolyzing time: 0.3 sec.
EXA~IPLE 25
EXAMPLE 17 was repeated, except that after the
final chromating and before rinsing, posttreatment was carried
out on both surfaces under the conditions listed below:
Conditions for posttreatment:
Solution: The solution contained 50 g of chromium
trioxide per liter;
Temperature: 45C;
Method: Cathodic electrolysis;
Cathode current density: 10 A/dm2;
Electrolyzing time: 0.3 sec.
- 41 -
- ~ ~
2 ~
COMPARATIVE EXAMPLE 1
Two cold rolled steel sheets having thicknesses
of 0.22 mm and 0.32 mm, respectively, were each treated
by a process comprising electrolytic degreasing in a solu-
tion of sodium hydroxide having a concentration of 30 g
per liter, rinsing with water, electrolytic pickling in
an aqueous solution of sulfuric acid having a concentration
of 5 g per liter, rinsing with water, pretreatment on both
surfaces under the conditions listed at (A) below, electro-
lytic chromating on both surfaces under the conditions
listed at (B) below, rinsing with water, and drying.
(A) Conditions for pretreatment:
Solution: A solution containing 100 g of chromium
trioxide and 1 g of sulfuric acid per
liter;
Temperature: 25C;
Method: Anodic electrolysis;
Anode current density: 10 A/dm2;
Electrolyzing time: 0.3 sec.
(B) Conditions for electrolytic chromating:
Solution: A solution containing 175 g of chromium
trioxide, 5 g of Na2SiF6 and 0.9 g of
Na2SO4 per liter;
Temperature: 40C;
Method: Continuous cathodic electrolysis;
2~8~
Cathode current density: 160 A/dm2;
Electrolyzing time : 0.3 sec.
CO~ARATIVE EXAMPLE 2
Two cold rolled steel sheets having thicknesses
of 0.22 mm and 0.32 mm, respectively, were each treated
by a process comprising electrolytic degreasing in a solu-
tion of sodium hydroxide having a concentration of 30 g
per liter, rinsing with water, electrolytic pickling in
an aqueous solution of sulfuric acid having a concentration
of 5 g per liter, rinsing with water, pretreatment of both
surfaces by dipping under the conditions listed below,
electrolytic chromating on both surfaces under the condi-
tions listed below, rinsing with water, and drying.
Conditions for pretreatment (dipping):
Solution: A solution containing 100 g of chromium
trioxide and 1 g of sulfuric acid per
liter;
Temperature: 25C.
Conditions for electrolytic chromating:
Solution: A solution containing 175 g of chromium
trioxide, 5 g of Na2SiF6 and 0.9 g of
Na2SO4 per liter;
Temperature: 42C;
Method: Cathodic on and off electrolysis;
Cathode current density: 40 A/dm2;
Electrolyzing time: 0.3 sec.; . .
- 43 -
20~81~3
On and off cycle: Four cycles were repeated;
Dipping time : 0.3 sec.
COMPARATIVE EXAMPLE 3
Two cold rolled steel sheets having thicknesses
of 0.22 mm and 0.32 mm, respectively, were each treated
by a process comprising electrolytic degreasing in a solu-
tion of sodium hydroxide having a concentration of 30 g
per liter, rinsing with water, electrolytic pic~ling in
an aqueous solution of sulfuric acid having a concentration
of 5 g per liter, rinsing with water, electrolytic chromat-
ing on both surfaces under the conditions listed below,
rinsing with water, and drying:
Solution: A solution containing 175 g of chromium
trioxide, 5 g of Na2SiF6 and 0.9 g of
Na2SO4 per liter;
Temperature: 45C;
Method: Continuous cathodic electrolysis;
Cathode current density: 160 A/dm2;
Electrolyzing time: 0.3 sec.
COMPARATIVE EXAMPLE 4
-
Two cold rolled steel sheets having thicknesses
of 0.22 mm and 0.32 mm, respectively, were each treated
by a process comprising electrolytic degreasing in a solu-
tion of sodium hydroxide having a concentration of 30 g
per liter, rinsing with water, electrolytic pickling in
- 44 -
~8~
an aqueous solution of sulfuric acid having a concentration
of 5 g per liter, rinsing with water, electrolytic chromat-
ing on both surfaces in a solution having the composition
and temperature shown below under the conditions listed at
(a) below, intermediate anodic treatment on both surfaces
in the same solution under the conditions listed at (b)
below, electrolytic chromating on both surfaces under the
conditions listed at (a), rinsing with water, and drying.
Solution: A solution containing 175 g of chromium
trioxide, 5 g of Na2SiF6 and 0.9 g of
Na2SO4 per liter;
Temperature: 40C.
(a) Cathodic on and off electrolysis:
Cathode current density: 40 A/dm2;
Electrolyzing time: 0.3 sec.;
On and off cycle: Two cycles were repeated;
Dipping time: 0.3 sec.
(b) Anodic electrolysis:
Anode current density: 4 A/dm2;
Electrolyzing time: 0.3 sec.
_MPARATIVE EXA~lPLE 5
Two cold rolled steel sheets having thicknesses
of 0.22 mm and 0.32 mm, respectively, were each treated
by a process comprising electrolytic degreasing in a solu-
tion of sodium hydroxide having a concentration of 30 g
per liter, rinsing with water, electrolytic pickling in
- 45 -
an aqueous solution of sulfuric acid having a concentration
of 5 g per liter, rinsing witll water, electrolytic
chromating on both surfaces in a solution having the
composition and temperature shown below under the condi-
tions listed below, dipping treatment of both surfaces in
the same solution, electrolytic chromating on both surfaces
under the conditions listed below, rinsing with water, and
drying:
Solution: A solution containing 175 g of chromium
trioxide, 5 g of Na2SiF6 and 0.9 g of
Na2SO4 per liter;
Temperature: 45C;
Method: Continuous cathodic electrolysis;
Cathode current density: 100 A/dm ;
Electrolyzing time: 0.3 sec.
COMPARATIVE EXAMPLE 6
Two cold rolled steel sheets having thicknesses
of 0.22 mm and 0.32 mm, respectively, were each treated
by a process comprising electrolytic degreasing in a solu-
tion of sodium hydroxide having a concentration of 30 g
per liter, rinsing with water, electrolytic pickling in
an aqueous solution of sulfuric acid having a concentration
of 5 g per liter, rinsing with water, electrolytic chromat-
ing on both surfaces under the conditions listed below,
posttreatment on both surfaces under the conditions listed
below, rinsing with water, and drying.
- 46 -
.
2~
Conditions for electrolytic chromating:
Solution: A solution containing 175 g of chromium
trioxide, 5 g of Na2SiF6 and 0.9 g of
Na2SO4 per liter;
Temperature: 45C;
Method: Continuous cathodic electrolysis;
Cathode current density: 100 A/dm2;
Electrolyzing time: 0.3 sec.
Conditions for posttreatment:
Solution: A solution containing 50 g of chromiurn
trioxide and 0.5 g of NH4F per liter;
Temperature: 45C;
Method: Cathodic electrolysis;
Cathode current density: 20 A/dm ;
Electrolyzing time: 0.5 sec.
_ MPARATIVE EXAMPLE 7
Two cold rolled steel sheets having thicknesses
of 0.22 mm and 0.32 mm, respectively, were each treated
by a process comprising electrolytic degreasing in a solu-
tion of sodium hydroxide having a concentration of 30 g
per liter, rinsing with water, electrolytic pickling in
an aqueous solution of sulfuric acid having a concentration
of 5 g per liter, rinsing with water, electrolytic chromat-
ing on both surfaces under the conditions listed at (A)
below, after 10 seconds of dipping, electrolytic chromating
on both surfaces under the conditions listed at (B) below,
- 47 -
2 ~3
rinsing with water, and drying.
(A) Conditions for electrolytic chromating:
Solution: A solution containing 175 g of chromium
trioxide, 5 g of Na2SiF6 and 0.9 g of
Na2SO4 per liter;
Temperature: 45C;
Method: Cathodic on and off electrolysis;
Cathode current density: 40 A/dm2;
Electrolyzing time: 0.3 sec.;
On and off cycle: Two cycles were repeated;
Dipping time: 0.3 sec.
(B) Conditions for electrolytic chromating:
Solution: A solution containing 175 g of chromium
trioxide, 5 g of Na2SiF6 and 0.9 g of
Na2SO4 per liter;
Temperature: 45C;
Method: Cathodic on and off electrolysis;
Cathode current density: 40 A/dm2;
Electrolyzing time: 0.3 sec.;
On and off cycle: Two cycles were repeated;
Dipping time: 0.3 sec.
COMPARATIVE EXAMPLE 8
Two cold rolled steel sheets having thicknesses
of 0.22 mm and 0.32 mm, respectively, were each treated
by a process comprising electrolytic degreasing in a solu-
tion of sodium hydroxide having a concentration of 30 g
- 48 -
2 ~
per liter, rinsing with water, electrolytic pickling in
an aqueous solution of sulfuric acid having a concentra-
tion of 5 g per liter, rinsing with water, electrolytic
chromating on both surfaces under the conditions shown at
(A) below, electrolytic chromating on one surface alone
under the conditions shown at (B) below, rinsing with water,
and drying.
(A) Conditions for electrolytic chromating:
Solution: A solution containing 175 g of chromiùm
trioxide, 5 g of Na2SiF6 and 0.9 g of
Na2SO4 per liter;
Temperature: 35C;
Method: Continuous cathodic electrolysis;
Cathode current density: 80 A/dm2;
Electrolyzing time: 0.3 sec.
(B) Conditions for electrolytic chromating:
Solution: A solution containing 175 g of chromium
trioxide, 5 g of Na2SiF6 and 0.9 g of
Na2SO4 per liter;
Temperature: 45C;
Method: Continuous cathodic electrolysis;
Cathode current density: 80 A/dm2;
Electrolyzing time: 0.3 sec.
COMPARATIVE EXAMPLE 9
Two cold rolled steel sheets having thicknesses
of 0.22 mm and 0.32 mm, respectively, were each treated
- 49 -
2~8~
by a process comprising electrolytic degreasing in a
solution of sodium hydroxide having a concentration of
30 g per liter, rinsing with water, electrolytic pickling
in an aqueous solution of sulfuric acid having a concentra-
tion of 5 g per liter, rinsing with water, pretreatment
on one surface alone under the conditions listed below,
electrolytic chromating on bo-th surfaces under the condi-
tions listed below, rinsing with water, and drying.
Conditions for pretreatment:
Solution: A solution containing 100 g of chromium
trioxide and 1 g of sulfuric acid per
liter;
Temperature: 25C;
Method: Anodic electrolysis;
Anode current density: 10 A/dm2;
Electrolyzing time: 0.3 sec.
Conditions for electrolytic chromating:
Solution: A solution containing 175 g of chromium
trioxide, 5 g of Na2SiF6 and 0.9 g of
Na2SO4 per liter;
Temperature: 42C;
Method: Cathodic on and off electrolysis;
Cathode current density: 120 A/dm2;
Electrolyzing time: 0.2 sec.;
On and off cycle: Two cycles were repeated;
Dipping time: 0.3 sec.
- 50 -
2 ~
CO~PARATIVE EX _PLE 10
COMPARATIVE EXAMPLE 4 was repeated, except that
intermediate anodic electrolysis was carried out on one
surface alone by employing an anode current density of
0.2 A/dm2, and that the subsequent chromating was carried
out on that surface alone.
COMPARATIVE EXAMPLE 11
COMPARATIVE EXAMPLE 4 was repeated, except that
intermediate anodic electrolysis was carried out on one
surface alone by employing an anode current density of
0.5 A/dm .
COMPARATIVE EXA~IPLE 12
COMPARATIVE EXAMPLE 4 was repeated, except that
a cathode current density of 20 A/dm2 was employed instead of 40 A/dm , and
that intermediate anodic electrolysis and the subsequent
cathodic electrolysis were carried out on one and the same
surface alone.
COMPARATIVE EXAMPLE 13
COMPARATIVE EXAMPLE 9 was repeated, except that
pretreatment was carried out on both surfaces by employing
an anode current density of 10 A/dm2 for one surface and
2 A/dm2 for the other surface and an electrolyzing time of
0.3 sec. for each surface, and that for the electrolytic
chromating cathodic electrolysis was carried out by employing a
cathode current density of 40 A/dm and an electrolyzing time of 0.3
sec., and repeating the on and off cycle four times.
:' '. , - ' : ..... . . ' .': ' :' ' .: '
: , ' . ~ - ~ ,
:'
COIlPARATIVE EXAMPLE 14
COMPARATIVE EXAMPLE 9 was repeated, except that
for the electrolytic chromating cathodic electrolysis was
carried out by emplying a cathode current densi-ty of 40 A/dm and
an electrolyzing time of 0.3 sec., and repeating the on and off
cycle four times, and that posttreatment was carried ou-t on both
surfaces under the conditions listed below:
Conditions for posttreatment:
Solution: A solution containing 50 g of chromium
trioxide per liter;
Temperature: 45C;
Method: Cathodic electrolysis;
Cathode current density: 30 A/dm2;
Electrolyzing time: 0.3 sec.
COMPARATIVE EXAMPLE 15
COMPARATIVE EXAMPLE 4 was repeated, except that
intermediate anodic electrolysis was carried out on one
surface alone, and the subsequent chromating on that sur-
face alone, too, and that posttreatment was carried out on
both surfaces under the conditions listed below:
Conditions for posttreatment:
Solution: A solution containing 50 g of chromium
trioxide per liter;
Temperature: 45C;
Method: Cathodic electrolysis;
Cathode current density: 30 A/dm2;
Electrolyzing time: 0.3 sec.
- 52 -
- . :
'
:
2 ~ 9
COMPARATIVE EXAMPLE 16
COMPARATIVE EXAMPLE 14 was repeated, except that
an electrolyzing time of 0.5 sec. was employed for the
posttreatment.
COMPARATIVE EXAMPLE 17
COMPARATIVE EXAMPLE 15 was repeated, except that
an electrolyzing time of 0.5 sec. was employed for the
posttreatment.
COMPARATIVE EXAMPLE 18
COMPARATIVE EXAMPLE 1 was repeated, except that
pretreatment was earried out on one surfaee alone, and that
ehromating was carried out in a solution having a tempera-
ture of 50C.
COMPARATIVE EXAMPLE 19
Two cold rolled steel sheets having thicknesses
of 0.22 mm and 0.32 mm, respectively, were each treated
by a process eomprising eleetrolytie degreasing in a
solution of sodium hydroxide having a eoneentration of
30 g per liter, rinsing with water, eleetrolytie piekling
in an aqueous solution of sulfurie aeid having a concentra-
tion of 5 g per liter, rinsing with water, electrolytic
chromating on both surfaces in a solution having the eom-
position and temperature shown below under the conditions
listed at (a) below, intermediate anodic treatment on one
surface alone in the same solution under the eonditions
listed at (b) below, electrolytic chromating on that surface
:
alone under the conditions listed at (a), rinsing with
water, and drying:
Solution: A solution containing 175 g of chromium
trioxide, 5 g of Na2SiF6 and 0.9 g of
Na2SO4 per liter;
Temperature: 45C.
~a) Continuous cathodic electrolysis:
Cathode current density: 100 A/dm2;
Electrolyzing time : 0.3 sec.
(b) Anodic electrolysis:
Anode current density : 0.3 A/dm2;
Electrolyzing time : 0.3 sec.
COMPARATIVE EXAMPLE 20
Two cold rolled steel sheets having thicknesses
of 0.22 mm and 0.32 mm, respectively, were each treated
by a process comprising electrolytic degreasing in a
solution of sodium hydroxide having a concentration of
30 g per liter, rinsing with water, electrolytic pickling
in an aqueous solution of sulfuric acid having a concentra-
tion of 5 g per liter, rinsing with water, pretreatment on
one surface alone under the conditions listed below,
electrolytic chromating on both surfaces under the condi-
tions listed below, rinsing with water, and drying.
Conditions for pretreatment:
Solution: A solution containing 175 g of chromium
trioxide, 5 g of Na2SiF6 and 0.9 g of
Na2SO4 per liter;
- 54 -
,
~ .
2 ~
Temperature: 40C;
Method: Anodic electrolysis;
Anode current density: 5 A/dm2;
Electrolyzing time: 0.3 sec.
Conditions for electrolytic chromating:
Solution: A solution containing 175 g of chromium
trioxide, 5 g of Na2SiF6 and 0.9 g of
Na2SO4 per liter;
Temperature: 45C;
Method: Continuous cathodic electrolysis;
Cathode current density: 100 A/dm2;
Electrolyzing time: 0.1 sec.
COMPARATIVE EXAMPLE 21
COMPARATIVE EXA~IPLE 20 was repeated, except that
electrolytic chromating was carried out by employing a
cathode current density of 30 A/dm2 and an electrolyzing
time of 0.3 sec.
COMPARATIVE EXAMPLE 22
COMPARATIVE EXAMPLE 19 was repeated, except that
cathodic electrolysis was carried out by employing an
electrolyzing time of 0.1 sec., and anodic electrolysis
at an anode current density of 2 A/dm2.
COMPARATIVE EXAMPLE 23
COMPARATIVE EXAMPLE 20 was repeated, except that
electrolytic chromating was carried out by employing a
cathode current density of 160 A/dm2 and an electrolyzing
time of 0.3 sec., and that posttreatment was thereafter
carried out under the conditions shown below:
Conditions for pos-ttreatment:
Solution: A solution containing 50 g of chromium
trioxide per liter;
Temperature: 45C;
Method: Cathodic electrolysis;
- Cathode current density: 30 A/dm2;
Electrolyzing time: 0.3 sec.
COMPARATIVE EXAMPLE 24
COMPARATIVE EXAMPLE 19 was repeated, except that
anodic electrolysis was carried out at an anode current
density of 2 A/dm2, and that after the final chromating
and before rinsing, posttreatment was carried out on both
surfaces under the conditions shown below:
Conditions for posttreatment:
Solution: A solution containing 50 g of chromium
trioxide per liter;
Temperature: 45C;
Method: Cathodic electrolysis;
Cathode current density: 30 A/dm2;
Electrolyzing time: 0.3 sec.
COMPARATIVE EXAMPLE 25
COMPARATIVE EXAMPLE 23 was repeated, except that
an electrolyzing time of 0.5 sec. was employed for the post-
treatment.
- 56 -
_MPARATIVL _XAI~IPLE 26
COMPARATIVE EXA~lPLE 24 was repeated, except that
an electrolyzing time of 0.5 sec. was employed for the
posttreatment.
* * *
Measurements were made of the densities of metallic
chromium and hydrated chromium oxide in the film on each
side of each of the electrolytically chromated steel sheets
whlch had been produced in EXAMPLES 1 to 25 and COMPARATIVE
EXAMPLES 1 to 26 as hereinabove described. A replica was
prepared from the film and the number of chromium particles
having a diameter of at least 0.03 micron as observed
through an electron microscope was counted as a measure of
its granular metallic chromium density. The results are
shown in TABLES 1 to 5 below. The results of COMPARATIVE
EXAMPLE 11 appearing in TABLE 4 are those obtained from
the products of the method disclosed in the Japanese patent
application laid open under No. 35797/1988.
The sheets were also evaluated for weldability,
outlook after lacquering, and corrosion resistance (fili-
form corrosion resistance, corrosion resistance after
lacquering, and corrosion resistance) by the methods as
will hereinafter be described. The results are shown in
TABLES 6 to 10. TABLES 6 to 10 confirm the presence of
outstanding distinctions between the products of the EXAMPLES
embodying this invention and those of the COMPARATIVE
- 57 -
EXAMPLES in welding property and outlook after lacquer-
ing, and the grea-t advantages of this invention. The
results of the evaluation confirm also the good corrosion
resistance of all of the products embodying this invention.
The following is a description of the methods
which were employed for the evaluation:
High Speed Welding:
Each sheet having the thickness of 0.22 mm was
slit after printing, and the slit piece thereof was welded
to make a size 202 can by a Soudronic wire mushroom welding
machine employing a welding current set in a number of
ways and in accordance with the conditions set forth below:
Inverter power source frequency: 500 Hz
Welding rate : 50 m/min.
Electrode pressure : 60 kgf
Overlapping width : 0.4 mm
Nugget pitch : 0.75 mm
The range of a welding current within which a weld strength
satisfying a tearing test could be obtained without causing
any splashing was found from the graduations marked on a
current setting device. The following is a definition of
the symbols used in TABLES 6 to 10 to express the results
of evaluation:
~mbol Meaning
~ The sheet permitted the use of the range of
a welding current as marked at 5 or above,
and exhibited very good welding properties;
- 58 -
.~ ,
2~8~
o The sheet permittecl the use of the range
of a welding current as marked at 3 or 4,
and exhibited good welding properties;
~ The sheet permitted only the use of the
range of a welding current as marked at 1
or 2, and was found difficult to use for
any practical application, though its weld-
ing was not impossible;
x The sheet did not permit the use of any
welding current available on the current
setting device, and could not be welded.
5G Welding:
Each sheet having the thickness of 0.32 mm was
slit after printing, and an attempt was made to weld the
slit piece into a 5-gallon can by a Soudronic wire mushroom
welding machine employing a welding current set in a number
of ways and in accordance with the conditions set forth
below:
Inverter power source frequency: 180 llz
Welding rate : 22 m/min.
Electrode pressure : 65 kgf
Overlapping width : 0.8 mm
Nugget pitch : 1.2 mm
The range of a welding current within which a weld strength
satisfying a tearing test could be obtained without causing
any splashing was found from the graduations marked on a
- 59 -
current setting device. The meanings of the symbols
are as defined above.
Color Tone of Surface:
The outer surface of each welded can was coated
with a layer of a transparent lacquer having a dry coat-
ing weight of 60 mg/m2, and was visually checked for any
change in color tone. The symbols used to express the
results have the meanings as defined below:
Symbol Meaning
o Good. No change in color tone was found;
x Bad. A change in color tone was found.
Color Tone of Printed Surface:
The outer surface of each welded can was printed
with a wine-colored metallic paint, and the color tone of
the printed surface was visually compared with that of the
paint itself. The symbols used to express the results
have the meanings as defined below:
Symbol Meaning
o Good. No difference in color tone was
found;
x Bad. A difference in color tone was found.
Filiform Corrosion Resistance:
The outer surface of each 5G welded can was
visually inspected for any product of filiform corrosion
that might have been formed in any defective portion of
its coating during 12 months of its storage in a warehouse.
- 60 -
2 ~
The symbols used to express the results have the mean-
ings as defined below:
Symbol Meaning
o No product of filiform corrosion was
found;
x ~ product of filiform corrosion was found.
Corrosion Resistance of a Coa-ted Surface:
One side of each sheet, which would be used to
form the inner surface of a welded can, was coated with
a layer of an epoxy-phenol resin paint having a coating
weight of 50 mg/m2, and after the paint had been baked,
a cruciform cut was made in the coating layer by a sharp
cutter knife so as to reach the steel surface, and a force
was applied by an Erichsen tester to the other side of
the sheet (used to form the outer surface of the can) to
prepare an extruded test specimen having an extruded depth
of~5 mm from the center of the cruciform cut. The test
specimen was dipped in an aqueous solution containing
1.5~ of NaCl and 1.5~ of citric acid and having a tempera-
ture of 38C, and after 96 hours, measurement was made of
the width of the corroded portion of the steel surface in
the cruciform cut. The symbols used to express the results
have the meanings as defined below:
Symbol Meaning
o The corroded width was less than 1 mm;
x The corroded width was 1 mm or more.
- 61 -
Corrosion Resistance:
A stack of 100 sheets to be tested was placed
between a pair of plates, and bound together tightly
with steel wires passed around them in a cruciforrn way.
After one month of storage in a place having a tempera-
ture of 25C and a humidity of 85%, each sheet was examined
for corrosion. The syrnbols used to express the results
have the meanings as defined below:
~_bol Meaning
o No corrosion was found;
x Corrosion was found.
- 62 -
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-- 72 --
