Note: Descriptions are shown in the official language in which they were submitted.
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TITLE OF THE INVENTION
Surface-Treated Steel Strip Having Improved
Weldability and Process for Making
BACKGROUND OF THE INVENTION
This invention relates to surface-treated steel strips
or sheets having improved weldability, particularly improved
seam weldability and sufficient corrosion resistance for use
as can forming stock, and a process for making such
surface-treated steel strips.
Among food can-forming materials there have been most
widely used tin-coated steel strips generally called tin
plates. In order to join the mating edges of a can body,
conventional soldering techniques were previously used.
Because of the toxicity of lead contained in conventional
solder, pure tin solder has recently become prevalent. The
pure tin solder, however, has a technical problem in making
a joint because of inferior wetability during the soldering
process and is so expensive as to create the economic
problem of increased manufacture cost.
On the other hand, in recent years, food containers have
enjoyed the development of inexpensive, competitive
materials such as polyethylene, aluminum, glass, processed
paper and the like. Despite their significantly improved
corrosion resistance among other advantages, tin plate cans
having expensive tin thickly coated thereon to a coating
weight of as great as 2.8 to ll.2 g/m2 require a relatively
high cost of manufacture and have encountered severe
competition.
In order to overcome the above-described drawbacks of
tinplate cans, electric resistance welding of can bodies has
recently replaced the conventional soldering technique and
become widespread. There is a continuing need for can-
forming steel compatible with electric resistance welding.
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In addition to the tinplate discussed above, tin-free
steel of chromium type is another typical example of
conventional can-forming steel. The tin-free steel is
prepared by carrying out an electrolytic chromate treatment
on steel to form metallic chromium and hydrated chromium
oxide layers on the surface. Since the relatively thick
hydrated chromium oxide layer at the top has a relatively
high electric resistance, the chromated steel is
ineffectively welded to form a weld of insufficient strength
and thus unsuitable as welded can-forming steel despite its
economic advantage.
Since other can-forming materials are also inadequate as
welded can-forming material, a variety of proposals have
been made. One example is nickel-plated steel, typically
"Nickel-Lite" announced by National Steel Corporation of the
U.S. which is prepared by plating a steel strip with nickel
to a thickness of about 0.5 g/m2 followed by a conventional
chromate treatment. Inferior adhesion of lacquer and
inferior weldability in high speed welding at 30 m/min. or
higher have limited the spread of this nickel-plated steel.
Another example is "Tin Alloy" announced by Jones &
Laughlin Steel Corporation of the V.S. This is prepared by
thinly coating a steel strip with tin to a thickness of
about 0.6 g/m2 and effecting tin reflow or flow melting
followed by a conventional chromate treatment.
~nfortunately, rust resistance, lacquer adhesion and
weldability are insufficient.
In general, can-forming steel sheets intended for
electric resistance welding are required to exhibit improved
weldability and corrosion resistance after lacquering.
These requirements will be explained in datail. There must
be an available welding electric current range within which
a weld zone having sufficient weld strength is provided at
the end of welding without any weld defects such as so-called
"splashes". Since welded cans are filled with foodstuffs
~ o 7~es 7lr u cJ~ f k
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after lacquer coating, the underlying steel must have
sufficient adhesion to lacquer to take full advantage of the
corrosion prevention of the lacquer film. Furthermore,
despite defects unavoidably occurring in a lacquer film, the
imp oved corrosion resistance of the underlying steel itself
must prevent corrosion from proceeding.
SUMMARY OF THE INVENTION
It is, therefore, an object of the present invention to
provide a novel and improved surface-treated steel strip
which can be seam welded into cans without the above-
mentioned drawbac~s and has improved weldability, corrosion
resistance after lacquering, and lacquer adhesion.
Another object of the present invention is to provide a
process for making such an improved can-forming stock in a
simple and inexpensive manner.
According to a first aspect of the present invention,
there is provided a surface-treated steel strip having
improved weldability, comprising
a steel strip,
a metallic chromium layer deposited on the steel strip
to a coating weight of 50 to 150 mg/m2,
a metallic tin layer deposited on the chromium layer to
a coating weight of 50 to ~00 mg/m2, and
a chromate coating layer deposited on the tin layer and
consisting essentially of 3 to 15 mg/m2 of metallic chromium
and 3 to 15 mg/m2 of hydrated chromium oxide calculated as
elemental chromium.
According to a second aspect of the present invention,
there is provided a process for preparing a surface-treated
steel strip having improved weldability. The surface-
treated steel strip particularly suitable as can-forming
stock is prepared by
cleaning at least one surface of a steel strip,
,..... ,: ,.
subjecting the cleaned strip surface to chromium
electrodeposition to a coating weight of 50 to 150 mg/m2,
rinsing the chromium-deposited strip with water,
subjecting the strip to tin electrodeposition in an
acidic bath containing l.5 to l0 grams per liter of Sn ion
at a pH of lower than 0.5, the tin being deposited to a
coating weight of 50 to 400 mg/m2,
rinsing the tin-deposited strip with water, and
subjecting the strip to chromate treatment in a chromate
bath containing up to 50 grams per liter of CrO3 and at
least one member selected from the group consisting of.
sulfuric acid, sulfates, and fluorides at a current density
of at least l0 A/dm2, thereby forming a chromate coating
layer consisting essentially of 3 to 15 mg/m2 of metallic
chromium and 3 to 15 mg/m2 of hydrated chromium oxide
calculated as elemental chromium.
In one preferred embodiment of the present invention,
the acidic tin-plating bath contains hydrochloric acid
and/or sulfuric acid and an insoluble anode is used to carry
out the tin electrodeposition.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. l is a dlagram showing the tin content in an alloy
resulting from a baking treatment (210C, 20 min.) as a
function of the amount of underlying metallic chromium; and
FIG. 2 is a diagram showing the corrosion resistance
after lacquering as a function of the amount of metallic
chromium on the tin layer and the amount of hydrated
chromium oxide calculated as elemental chromium.
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DETAILED DESCRIPTION OF THE INVENTION
Makinq investigations on the weldability of various
can-forming stock materials by electric reslstance welding,
particularly wire seam welding as represented by the
5 Soudronic welding process which is now widely and rapidly
accepted as a can-forming welding process, we have found
-that the presence of metallic tin ensures satisfactory seam
welding performance.
More specifically, metallic tin which has a relatively
low melting point is readily melted during welding and
spread under the welding pressure to increase the contact
area between steel pieces and to facilitate mutual fusion of
the steel pieces. "Splashes" due to local concentration of
welding current are unlikely to occur. .Formation of a
strong welding joint provides a wide range of available
welding current. Conventional #25 tin plate has a wide
available welding current range because it contains about
2.2 g/m2 of metallic tin.
A continuing study about the relationship of weldability
and metallic tin amount has revealed that the presence of
metallic tin in amounts of at least 50 mg/m2, preferably at
least l00 mg/m2 provides a practically satisfactory wide
range of available welding current even in high speed
welding at 40 to 60 m/min.
Although it appears that applying at least 50 mg/m2 of
tin plating onto the steel surface results in satisfactory
weldability, a problem arises because actually such plated
strips are coated with lacquer before welding. A baking and
hardening treatment is done after lacquer coating and such
baking causes the tin to be alloyed with iron of the steel
substrate. The usual baking temperature is 170 to 220C and
an iron-tin alloy in the form of FeSn2 forms at the
temperature. Since the resultant FeSn2 alloy has a high
melting point, the weldability improving effect the metallic
tin inherently possesses is lost as a result of alloying.
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Then, to reserve satisfactory weldability, tin must be
plated in an extra amount by taking into account the loss of
tin by alloying durlng the baking treatment, resulting in an
economic disadvantage.
Making investigations to prevent alloying between tin
and substrate iron by baking treatment, we have found that
the formation of an iron-tin alloy can be substantially
controlled by interposing metallic chromium between tin and
substrate iron. The content of tin in an alloy formed by a
baking treatment at 210C for 20 minutes is plotted in FIG.
1 as a function of the amount of the underlying metallic
chromium. As apparent from data plotted in FIG. 1, the
intervening metallic chromium is significantly effective in
preventing an iron-tin alloy from forming during the baking.
The metallic chromium which itself is a highly corrosion
resistant metal has double functions of preventing formation
of an iron-tin alloy and improving corrosion resistance at
the same time.
By subjecting a steel strip to chromium plating and then
to tin plating, not only expensive tin can be effectively
utilized, but also corrosion resistance is improved.
Although forming metallic tin as an uppermost layer
provides good weldability, lacquer baking forms tin oxide at
the surface, resulting in insufficient lacqueradherence and
hence, poor corrosion resistance after lacquering. ~hen
cans are filled with such contents as foods containing
sulfur-bearing amino acids, so-called sulfide stain occurs,
that is, tin sulfide having a black color forms to blacken
the can inner surface. Prior art tinplate is cathodically
treated in an electrolytic solution of sodium dichromate to
form a coating of hydrated chromium oxides to mitigate the
sulfide stain. The hydrated chromium oxide coating,
however, does not perform well.
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Unexpectedly, we have found that by forming a coating of
double layers of metallic chromium and hydrated chromium
oxide, not only lacquer adherence and corrosion resistan
after lacquering are significantly improved, but sulfide
stain is substantially eliminated. Particularly, metallic
chromium is greatly effective in improving lacquer adherence
and sulfide staining resistance. A significant effect is
achieved with metallic chromium in as small amounts as 3
mg/m2 or more. Corrosion resistance after lacquering is
also improved.
- Thus, the surface-treated steel strip of the present
- invention should have as an uppermost layer a chromate
coating layer consisting essentially of 3 to 15 mg/m2 of
metallic chromium and 3 to 15 mg/m2 of hydrated chromium
oxide calculated as elemental chromium.
Since the steel strip of the present invention is
intended for use in forming food cans, it is always coated
with lacquer on one surface which becomes the inner surface
of a can made therefrom. Most important among the requisite
can properties are corrosion resistance after lacquering and
sulfide staining resistance. Metallic chromium on the tin
layer is essential to provide such resistances. Me-allic
chromium must be deposited in amounts of at least 3 mg/m2.
An excessive amount of metallic chromium can interfere with
weldability as it is oxidized to form chromium oxide at
elevated temperatures during welding. The upper limit of 15
mg/m2 is thus imposed to the metallic chromium amount.
The hydrated chromium oxide is effective in improving
lacquer adherence and corrosion resistance. Since hydrated
chromium oxide itself is a high electric resistance
material, it can interfere with weldability when present in
excessive amounts. For this reason, the hydrated chromium
oxide is limited to the range from 3 to 15 mg/m2 calculated
as elemental chromium.
The process for making a surface-treated steel strip
according to the present invention will now be described in
detail.
At the outset, a starting steel strip which may be one
commonly used as the starting steel strip for tinplate or
tin-free steel (TFS) strips is cleaned by any conventional
techniques, for example, electrolytic degreasing and
pickling.
The cleaned surface of steel strip is subjected to
electrodeposition of metallic chromium to a coating weight
of 50 to 150 mg/m2. The underlying metallic chromium is
plated for the purposes of preventing formation of an
iron-tin alloy during lacquer baking and improving corrosion
resistance. Metallic chromium is effective in preventing
iron-tin alloylng even in as small amounts as 5 mg/m2.
However, metallic chromium is desirably deposited in a
coating weight of at least 50 mg/m2 in order to effectively
prevent alloying of expensive tin. The larger the amount of
metallic chromium, the greater are the iron-tin alloying
prevention and the corrosion resistance. Metallic chromium
amounts in excess of 150 mg/m2 are uneconomic because the
iron-tin alloying prevention and the corrosion resistance
improvement reach plateau at such a level. Excessive
metallic chromium is also undesirable because such a thick
chromium plating is liable to cracks due to
electrodeposition stress.
Metallic chromium may be deposited on the steel strip by
any well-known methods, for example, by a well-known
electrodeposition method as by carrying out cathodic
electrolysis in an aqueous solution containing a major
proportion of chromic acid anh~dride and an additive amount
of sulfate (SO4 ) and fluoride (F ). Upon completion oE
chromium electrodeposition, the steel strip is rinsed with
water.
73~ja3~3
Next, tin plating is applied onto the metallic
chromium layer on the steel strip. ~hen tin
electrodeposition is performed on the electrodeposited
metallic chromium, a conventional tin electrodeposition
process fails to achieve satisfactory tin plating because
hydrated chromium oxide is present on the metallic chromium
surface. Illustratively, the ordinary industrial chromium
electrodeposition is to electrochemically reduce hexavalent
chromium ion (Cr6') to metallic chromium. Since Cr6' ion is
reduced to metallic Cr through the hydrated oxide of
trivalent chromium as is well known in the art, hydrated
chromium oxide is always present on the plating surface.
This hydrated chromium oxide interferes with tin
electrodeposition. When tin electrodeposition is effected
in the presence of hydrated chromium oxide on the metallic
chromium surface, there is obtained a deposit having poor
bond strength.
In order to effectively electrodeposit tin, the
hydrated chromium oxide must have been previously removed
from the electrodeposited chromium surface. Removal of
hydrated chromium oxide may be effected by a variety of
well-known methods, for example, by dissolving away in a
hot aqueous alkaline solution or by dissolving away in an
aqueous solution such as sodium hydroxide solution,
phosphate buffer solution and borate buffer solution
throu~h anodic electrolysis.
While hydrated chromium oxide dissolves away in hot
aqueous alkaline solution, a chromium oxide layer which is
insoluble in the alkaline solution is left to impede
satisfactory tin deposition. In the anodic electrolysis,
metallic chromium is preferentially dissolved and hydrated
chromium oxide is retained until the metallic chromium has
been dissolved away. Thus ~he anodic electrolysis cannot
be applied to the production of the Sn/Cr double layer
deposited steel strip according to the present invention.
3~3~3~
.
Also known as a special tin plating pretreatment is a
method for depositing various metals onto the surface of chromium
and chromium alloys as disclosed in Toyo Seikan Kabushiki Kaisha's
Japanese Patent Publication No. 33 1~55 published March 5, 1958.
The surface activating treatment with caustic allcali as disclosed
therein fails to provide a tin deposit having a practically
acceptable bond strength.
Another method is disclosed in Nippon Steel Corp.'s
Japanese Patent Publication No. 48-35136 published October 26,
10 1973 entitled "Chromium-tin double layer plating method". The
immersion in a chromium plating solution as disclosed therein
cannot completely remove the hydrated chromium oxide from the
electrodeposited chromium surEace, failing to achieve a
satisfactory bond strength or adherence. This method is difficult
to carry out in a commercial lnstallation because it requires a
striking treatment at a very high current density.
A further method is disclosed in Toyo Kohan Co., Ltd. 15
Japanese Patent Application Kokai No. 60-190597 published
September 28, 1985 entitled "Surface-treated steel strip for use
20 in welded cans and method for making". There are disclosed a
pretreatment ky cathodic electrolysis in an acid solution at pH
0.5 to 2 and tin electrodeposition from a tin plating bath having
a low tin ion concentration and pH 0.5 to 3. Removal of hydrated
chromium oxide is still insufficient to ensure sa~isEactory tin
deposition.
Making a series of experiments to search for effective
removal of the hydrated chromium oxide, we have found that the use
of a strongly acidic tin plating bath containing 1.5 to ln grams
--10--
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s
per liter of tin ion and having a pH of lower than 0.5 can
effectively remove the hydrated chromium oxide and conduct tin
deposition at the same time.
The process in such a specific bath according to the
present invention carries out -tin deposition and generates a great
volume of active H2 gas at the same time. Evolution of a great
volume of active hydrogen gas on the deposited chromium surface in
a strongly acidic solution can completely remove the hydrated
chromium oxide which could
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not otherwise completely be removed from the de~osited
chromium surface. Tin deposition proceeds concurrently. ~s
a result, there is deposited a tin layer having a high bond
strength to the chromium layer.
The tin electrodeposition may be carried out at a bath
temperature of 25 to 65C and a cathodic current density of
15 to 50 amperes per square decimeter (A/dm2).
The tin plating bath used herein contains tin ion at a
concentration of 1.5 to 10 grams per liter. Tin ion
concentrations of less than 1.5 g~l do not allow tin to
deposit on the chromium layer whereas high concentrations in
excess of 10 g/l result in a coarse deposit of powder or
dendrite structure having too poor appearance to meet
practical application.
The tin plating bath should have a hydrogen ion
concentration in excess of 0032 mols per liter, that is,
lower than 0.5 in pH. Baths having a hydrogen ion
concentration of 0.32 mol/l or lower, that is, at least pH
0.5 are insufficient to impart a strongly acidic environment
necessary to remove the hydrated chromium oxide and to
generate a great volume of active hydrogen gas, so that the
hydrated chromium oxide is not completely removed and a
satisfactory tin deposlt is not obtained. It is preferable
to add hydrochloric acid and/or sulfuric acid to the acidic
bath to provide a pH value of lower than 0.5.
When a soluble tin anode is used in the strongly acidic
tin plating bath according to the present invention, the tin
deposition efficiency at the cathode (steel strip) is ~i~e-r
than the tin dissolving efficiency at the anode (tin anode)
because of the hlgh acidlty. If tin electrodeposition is
continued under such conditions for an extended period of
time, the tin ion concentration of the bath is gradually
increased to eventually exceed the critial level of 10 g/l,
resultinq in a coarse deposit.
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The source for supplying tin ion to the tin plating bath
according to the present invention may be any tin compounds
such as tin sulfate~ tin chloride, tin borofluoride, tin
pyrophosphate, tin fluoride, and similar tin compounds. The
acid added to the tin plating bath may be any one or
mixtures of inorganic acids such as sulfuric acid,
hydrochloric acid, hydrofluoric acid, borofluoric acld, etc.
; and organic acids such as ~phonol~u~fonic acid,
cresolsulfonic acid, etc.
When a sulfuric acid-acidified tin plating bath is
prepared by using tin sulfate as the tin ion source and
sulfuric acid as the acid, an insoluble anode formed of Pb,
Pb alloys, Pt, etc. may conveniently be used to eliminate
the above-mentioned problem of rising tin ion concentration.
The tin ion may be made up simply by dissolving tin sulfate
or tin oxide or finely divided metallic tin into the plating
solution.
Since tin is electrodeposited for the purpose of
achieving excellent weldability, the amount of tin deposited
may be determined in conjunction with the underlying
metallic chromium amount such that at least 50 mg/m2,
preferably at least 100 mg/m2 of metallic tin is left at the
end of lacquer baking. Excessive amounts of tin deposited
have no detrimental effect, but the tin coating weight may
desirably be up to 400 mg/m2 for economic reasons. The
preferred amount of tin deposited thus ranges from 50 to 400
mg/m2 .
The tin plating bath used in the practice o~ the present
process may further contain a surface-active agent having an
oxyethylene chain -[CH2CH2O-]n-, for example, ethylene
glycol, polyethylene glycol, a polyethylene glycol alkyl
ether such as ethylene glycol monomethyl ether, etcO, an
aromatic ethylene oxide such as ethoxylated ~-naphthol,
etc., and a polyethylene glycol-fatty acid ester in amounts
of 0.1 to 10 grams per liter. The addition of such a
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surface-active agent results in a tin electrodeposit having
excellent appearance and tone on the chromium
electrodeposit. Concentrations of oxyethylene chain-bearing
surface-active agent of lower than 0.1 g/l are too low to
improve the appearance and tone whereas the effect is
saturated beyond the concentration of 10 g/l. The use of
` such an expensive surface-active agent in ~ unnecessarily
.; .
excessive amounts is, of course, not economical. Since the
oxyethylene chain-bearing surface-active agent is expensive,
it is desired to add it only when a particularly good
f ~ n - C ~ e ~
appearance and tone is required. The t~ e~sl-t-ed strip is
rinsed with water to be ready for subsequent treatment.
According to the present invention, a chromate coating
layer consisting essentially of metallic chromium and
hydrated chromium oxide is further deposited on the tin
layer for the purpose of improving lacquer adherence,
corrosion resistance after lacquering, and sulfide staining
resistance. The amount of metallic chromium should be at
least 3 mg/m2 to achieve significant improvements in lacquer
adherence, corrosion resistance after lacquering, and
sulfide staining resistance. Generally, the larger the
amount of metallic chromium, the ~t-eE are improved the
lacquer adherence, corrosion resistancea after lacquering,
and sulfide staining resistance. Too large amounts of
metallic chromium in excess of 15 mg/m2 tend to be oxldized
at elevated temperatures during subsequent welding into
chromium oxide to detract from weldability, and are thus
undesirable.
To provide sufficient lacquer adherence and corrosion
resistancea after lacquering, the amount of hydrated
chromium oxide should be at least 3 mg/m2 calculated as
elemental chromium. Since the hydrated chromium oxide
itself is a high electric resistance material, it impedes
weldability when present in large amounts. The amount of
hydrated chromium oxide should preferably be limited ~@ to
15 mg/m2 calculated as elemental Cr.
-14-
The metallic chromium and hydrated chromium oxide may
be formed by carrying out cathodic electrolysis in a bath
con~aining at least 50 grams per liter of CrO3 and an
effecti~e amount of one member selected from sulfuric acid,
sulfate salts, fluorides, and mixtures thereo~ at a current
densi~y of at least 10 A/dm2. The metallic Cr and hydrated
Cr oxide may be formed in any desired combination by
properly selecting cathodic electrolysis condltions
including current density, bath temperature, bath
concentration, and the like. The metallic Cr and hydrated
Cr oxide formation must remain within the previously
described limits.
Chromate baths containing CrO3 in concentrations of
higher than 50 g/1 undesirably attack and etch the tin
layer whereas low current densities of lower than 10 A/dm2
are difficult to produce metallic chromium.
EXAMPLES
Example 1
A steel stock strip generally intended for
conventional tinplate or TFS was electrolytically
degreased, pickled, and then subjected to chromium
electrodeposition in a bath containing 200 g/1 of CrO3 and 2
g/1 of H2S04 at a temperature of 50C and a current density
of 40 A/dm2. The Cr deposited strip was rinsed with water
and then subjected to tin electrodeposition in a tin
plating bath containing 4 g/1 of SnCl2.2H2O and adjusted to
pH 0.3 with HCl at a temperature of 50C and a current
density of 20 A/dm2. The Sn deposited strip was rinsed with
water and then subjected to chromate treatment in a bath
containing 20 g/1 of CrO3 and 0.2 g/l of H2S04 at a
temperature o~ 50C and a current density of 15 A/dm2,
thereby forming metallic chromium and hydrated chromium
oxide as a topcoat.
-15-
Example 2
A steel stock strip generally intended for conventional
tinplate or TFS was electrolytically degreased, pickled, and
then subjected to chromium electrodeposition in a bath
containing 180 g/l of CrO3, 6 g/l of Na2SiF6, and 0.75 g/l
of H2SO~ at a temperature of 50C and a current density of
50 A/dm . The Cr deposited strip was rinsed with water and
then subjected to tin electrodeposition in a tin plating
bath containing 6 g/l of SnSO4 and adjusted to pH 0.48 with
H2SO~ at a temperature of 40C and a current density of 30
A/dm . The Sn deposited strip was rinsed with water and
then subjected to chromate treatment in a bath containing 15
g/l of CrO3 and 1.5 g/l of NH4F at a temperature of 45C and
a current density of 15 A/dm2, thereby forming metallic
chromium and hydrated chromium oxide as a topcoat.
Example 3
A steel stock strip generally intended for conventional
tinplate or TFS was electrolytically degreased, pickled, and
then subjected to chromium electrodeposition in a bath
containing 150 g/l of CrO3 and 7.5 g/l of NaF at a
temperature of 50C and a current denslty of 35 A/dm2. The
Cr deposited strip was rinsed with water and then subjected
to tin electrodeposition in a tin plating bath containing 8
g/l of SnSO4 and adjusted to pH 0.2 with ~2SO4 at a
temperature of 45 C and a current density of 25 A/dm using
an insoluble anode of Pb-Sn alloy. The Sn deposited strip
was rinsed with water and then subjected to chromate
treatment in a bath containing 15 g/l of CrO3 and 0.12 g/l
of H2SO~ at a temperature of 50C and a current density of
20 A/dm , thereby forming metallic chromium and hydrated
chromium oxide as a topcoat.
~ 7~ `3
_xample 4
A steel stock strip generally intended for conventional
tinplate or TFS was electrolytically degreased, pickled, and
then subjected to chromium electrodeposition in a bath
containing 250 g/l of CrO3, 5 g/l of Na2SiF6, and l.5 g/l of
H2SO~ at a temperature of 55C and a current density of 50
A/dm . The Cr deposited strip was rinsed with water and
then subjected to tin electrodeposition in a tin plating
bath containing 4 g/l of SnSO4 and adjusted to pH 0.40 with
H2SO~ at a temperature of 30C and a current density of 20
A/dm using an insoluble anode of platinum. The Sn
deposited strip was rinsed with water and then subjected to
chromate treatment in a bath containing 30 g/l of CrO3 and
0.27 g/l of H2SO4 at a temperature of 40 C and a current
density of 25 A/dm2, thereby forming metallic chromium and
hydrated chromium oxide as a topcoat.
Exam~le 5
A steel stock strip generally intended for conventional
tinplate or TFS was electrolytically degreased, pickled, and
then subjected to chromium electrodeposition in a bath
containing 200 g/l of CrO3, 5.5 g/l of Na2SiF6, and 1.0 g/l
of H2SO~ at a temperature of 40C and a current density of
55 A/dm . The Cr deposited strip was rinsed with water and
then subjected to tin electrodeposition in a tin plating
bath containing 5 g/l of SnSO4 and adjusted to pH 0.45 with
H2SO~ at a temperature of 35C and a current density of 40
A/dm . The Sn deposited strip was rinsed with water and
then subjected to chromate treatment in a bath containing 20
g/l of CrO3 and 3 g/l of NH4F at a temperature of 45 C and a
current density of 30 A/dm2, thereby forming metallic
chromium and hydrated chromium oxide as a topcoat.
3~i~13
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Example 6
A steel stock strip generally intended for conventional
tinplate or TFS was electrolytically degreased, pickled, and
then subjected to chromium electrodeposition ln a bath
containing 180 g/1 of CrO3, 6 g/l of Na2SiF6, and 0.75 g/l
of H2SO~ at a temperature of 55C and a current density of
50 A/dm . The Cr deposited strip was rinsed with water and
then subjected to tin electrodeposition in a tin plating
bath containing 6 g/l of SnSO4 and adjusted to pH 0.46 with
H2SO~ at a temperature of 35C and a current density of 35
A/dm . T~e Sn deposited strip was rinsed with water and
then subjected to chromate treatment in a bath containing 24
g/l of CrO3 and 2 g/l of ~a at a temperature of 40C and a
current density of 35 A/dm , thereby forming metallic
chromium and hydrated chromium oxide as a topcoat.
Comparative Example 1
Cr electrodeposition and Sn electrodeposition were
carried out under the same conditions as in Example 1. The
Sn deposited strip was rinsed with water and then only
hydrated chromium oxide was formed thereon in a bath
containing 30 g/l of Na2Cr2O7.2H2O at a temperature of 40C
and a current density of 5 A/dm2.
Comparative Example 2
Cr electrodeposition and Sn electrodeposition were
carried out under the same conditions as in Example 2. The
Sn deposited strip was rinsed with water and then subjected
to chromate treatment in a bath containing 40 g/l of CrO3
and 0.1 g/l of H2SO4 at a temperature of 50C and a current
density of 3 A/dm to form hydrated chromium oxide
containing a minor amount of metallic chromium.
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Com~arative Exam~le 3
Cr electrodeposition and Sn electrodeposition were
carried out under the same conditions as in Example 5. The
Sn deposited strip was rinsed with water and then subjected
to chromate treatment in a bath containing 60 g/1 of CrO3
and 3 g/1 of NaF at a temperature of 45C and a current
density of 40 A/dmZ to form metallic chromium and hydrated
chromium oxide.
The thus treated steel strip samples were evaluated
for various properties by the following proceduresO
Weldability
Sample pieces overlapping a distance of 0.~ mm were
seam welded by means of an electric resistance seam welding
machine using a copper wire as the electrode at a welding
speed of 40 m/minO and a welding force or can body joining
force of ~00 N. to determine the available welding current
range (ACR) within which a sound weld having a sufficient
strength could be accomplished without generating a splash.
Before the welding, the sample was subjected to a heat
treatment at 210C for 20 minutes which was the expected
lacquer baking.
I,acquer adherence
Two sample pieces (5mm x lOOmm) were coated with an
epoxy-phenol lacquer to a coating weight of 50 mg/dm2,
overlapped a distance of 90 mm from the edge, and heat
bonded to each other with a nylon adhesive. The unjoined
portions were outwardly bent over an angle of 90 to form a
T shape and oppositely pulled at a pulling speed of
200 mm/min. to determine tensile strength. The tensile
strength at which the pieces were peeled off ~as evaluated
as lac~uer adherence and called T-peel strength as
expressed in kg/5mm.
:,
....
3~
--19--
_orrosion resistance after lacquerinq_
A sample piece (40mm x 80mm) was coated with an
epoxy-phenol lacquer to a coating weight of 50 mg/dm2,
inserted into boiled tomato juice in a glass container such
that the lower half of the sample was immersed, and held in
the closed container at 55C for 1~ days. The sample was
taken out of the container and observed for blister
occurrence. The criterion of evaluation is shown below.
o: No blister occurred.
~: A few blisters occurred.
X: Numerous blisters occurred.
Sulfide staininq resistanc_
A sample piece was coated with an epoxy-phenol lacquer
to a coating weight of 50 mg/dm2, drawn by a distance of 5
mm by means of an Erichsen machine, immersed in a test
solution containing 1% Na2S and adjusted to pH 7 with lactic
acid (pH 3.5), and heat treated at 110C for 60 minutes.
The processed and unprocessed (flat) portions of the sample
were observed for sulfide staining. The criterion of
evaluation is shown below.
0: No staining.
1: Slight staining in only the processed portion.
2: Moderate staining in both the processed and flat
portions.
3: Severe staining in both the processed and flat
portions.
All the samples of Examples and Comparative Examples
were evaluated for the above properties. The results are
shown in Table 1. The data in Table 1 shows that the steel
strips treated according to the present invention are
significantly improved in weldability, lacquer adherence,
corrosion resistance after lacquering, and sulfide staining
resistance.
-20-
o o C~ O~:0
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-21-
FIG. 1 graphically illustrates the tin content in an
alloy resulting from a baking treatment (210C, 20 min.) as
a function of the amount of underlying metallic chromium.
FIG. 2 graphically illustrates the corrosion resistance
after lacquering as a function of the amount of metallic
chromium on the tin layer and the amount of hydrated
chromium oxide calculated as elemental chromium.
BENEFITS OF THE INVENTION
The surface-treated steel strip of the present
10 invention which has a surface coating consisting
essentially of a Cr layer, Sn layer, metallic Cr layer, and
hydrated Cr layer in specific coating weights displays
improved weldability, lacquer adherence, corrosion
resistance after lacquering, and sulfide staining
15 resistance.
The present process makes possible surface treatments
of a steel strip whereby a Cr layer, Sn layer, metallic Cr
layer, and hydrated Cr oxide layer are formed to the
specific coating weights, and thus ensures the efficient
2~ production of surface-treated steel strips meeting the
requirements for can-forming stock material.
