Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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331962
PROCESS FOR PRODUCING A MULTILAYER-COATED STEEL STRIP
HA~ING EXCELLENT CORROSION RESISTANCE AND
~ = _ ~
WELDABILITY AND USEFUL FOR CONTAINERS
~-","~:.
BACKGROUND OF THE INVENTION ~ -
1. Field of the Invention
The present invention relates to a process for
producing a multilayer-coated steel strip having an ~ ;
excellent corrosion resistance and weldability and
useful for producing containers. More particularly, the
present invention relates to a process for producing a -~
multilayex-coated steel strip having an excellent ~-
corrosion resistance and seam weldability, and thus is
useful as a steel material for forming cylindrical
portions of cans by a seam welding procedure.
2. Description of the Related Art
It is known that an electrolytic tin-plate
steel strip ~tir.plate), an electrolytic chromate-treated
steel strip (TFS-CT), and an electrolytic nickel-plated
steel strip (TFS-NT) are usable in the production of
three piece cans by soldering, bond-bonding, or seam
welding.
Formerly, tinplate was most widely used as a
steel material for producing cans, but conventional
tinplate is not always satisfactory in view of the
price thereof. Therefore, in order to reduce the
can-producing cost, attempts have been made to reduce
the thickness of the tin coating layer on the steel
25 strip, and to utilize a seam-welding method instead of
the conventional soldering method for the tinplate. It
has been found, however, that when the thickness of the
tin coating layer in the tinplate is reduced to a level
of 0.20 ~m or less, the resultant tinplate exhibits a
30 deteriorated paint corrosion resistance and a reduced
seam weldability.
; The conventional TFS-NT sometimes used as a
'
~ ~ 2 ~ ~ 3 3 ~
)~ ~teel mat~ri~l for produclng seam-welded can~ u~ually
exh~bits a ~ati~factory 8eam weldabillty, but th~s
weld~b$1ity is not ~lways sat~factory in prac~cal us~.
. Al~o, the conventional TFS-NT has a satlsfac~ory paint
S corrosion resistance in usual use, but the level of th~ :
paint corros~on resis~ance ~s not always sati~factory
when brough~ into con~ac~ with a corrosive material, for
ex~mple, ~rongly acidic food.
Accordingly, there i8 a strong demand for thé
10 prov~sion of a surface-coated steel ~trip wbich ~8 cheap ~::
and has an excellent paint corrosion resistance and seam
weldabil~ty, and thus ~s useful for the production o . ~:
cans and containers.
Japane~e Unexamined Paten~ Publication (~oka~
No. 60-75586 (Nippon Steel Corporation, published
7 April 27, 1985)d$sclose~ a proces~ for producing a coated
steel ~trip. ~n thi~ process, a ~teel 6trip is coated
with a small amount of nic~el, and the nickel-coated
steel strip ls then plated with tin. When the nickel
20 and t~n coated-steel str~p i~ heat treated, and ~he tin
coat~ng layer is converted to an Fe-Sn alloy layer,.the ~:
presenc~ of the small amount of nickel coat~ng layar : :
causes the structure of the Fe-Sn alloy layer to exhi~it
an enhanced density. There~ore, the xesultant coated
25 steQl strip exhibit~ an improved corrosion re~is~ance.
~180, the presence of the nickel coa~ing layer
~s effective for restxictinq the ~e-Sn alloy-forming
reaction ln the heat-treatment) and thus the resultant
coated steel exhibits an enhanced ~eam weldability~
30 Further, the inventors o~ the pxesent invention have
found that the propertie~, ~or example, seam weldability
and corrosion resis~ancet o the coated ~teel ~tr~p
usable ~8 a s~eel mater~al for 8eam weldea can6, vary
depending on the distriSution of metall~c tin coating
35 over the surface of a s~eel ~trip subs~rate. That i8,
it ha~ been ~ound that ~hQ propexties of the coated
steel atrlp over which the metallic tin layer i5 unevenly
distributed and having an uneven rough ~urface,.are
. . .
133~62
- 3 - ~- ~
- -:
better than those of a coated steel strip over which the
metallic tin layer is evenly distributed and having a
smooth uniform surface.
Namely, the coated steel strip having an
uneven thin tin coating layer exhibits a better seam
weldability and corrosion resistance than those of a -
conventional coated steel strip having an even thin tin
coating layer. However, it is very difficult to control
the thickness of the unevenness of the thin tin coating
layer to a predetermined level, and to produce a coated
steel strip having predetermined levels of weldability~-~
and corrosion resistance with a stable reproductivity.
Fujimoto et al, "Iron and Steel~, vol. 72, No. 5,
page 39. 1986 discloses that, in order to provide a tin -~ -
15 coating layer having an uneven thickness with a stable --
reproductivity, it is effective to apply an anodic
electrolytic treatment to the steel strip in an alkaline
treating liquid before the nickel-plating step. Also,
it is known that, when a tin-coated steel strip is
subjected to a flux treatment, the unevenness in the
thickness of the tin coating iayer is greatly influenced
by the conditions of the flux treatment.
However, even if the anionic electrolytic
treatment or the flux treatment is utilized, the
resultant coated steel strip is unsatisfactory from the
viewpoint of corrosion resistance and weldability.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a
process for producing a multilayer-coated steel strip
having an excellent corrosion resistance and weldability
and useful for producing cans or containers with an
improved reproductivity.
The above-mentioned object can be attained by the
process of the present invention which comprises the
steps of (A) plating a substrate consisting of a steel
strip with metallic nickel or a nickel-based alloy to
form, on both the upper and lower surfaces of the
:
... ~ '
i$
~33~S2
substrate, nickel-~a~e~ coa~ing layer, each of which . ~:
layers $8 coated in ~n aver~ge amoun~ of 2 to 100 mg/m2
and ~8 provided with a number of convex and concave
portlons, and in which layer portions thereof having a
coa~ing ~hicXness of 0.001 ~m or more have a total area
corre~ponding ~o 10% to 95~ oP the entire area of the
surface~ of the sub~trate~ IB) ~oating the nickel-based -:
plated ~ubstrate with tin to ~orm tin coa~ng layer~ on
the nicXel-ba~ed coating layer~, each o~ which tin
coating layers i8 coated ~n an averaqe amount of 200 to
2000 mg/m , to provi~e ~ precur60ry ~oate~ steel
str~pS (C~ heating the precursory coated st~el strip at
a temperature equal to or higher than the melting point
of the tln ~oating layer, to c~u e th~ nic~el-~ased
coating layers and the tin coat~ng layer~ to be con~
verted to base ~oating layers, which ar~ ~ormed on ~oth
the upper and lower surface~ o~ ~hQ ~u~Btrat~ con- ~ :
8i8ting essentially of an Fe-Ni-Sn-based alloy and ~ :
having a n~mber of convox and concnv~ portions, a~d : :
intermediate coating layers, which arQ located on the
base coatinq l~yers, consisting essentlally of t~n And
having ~ number o~ convex ~nd concav~ portion~1 an~ tD) ~ :
applying an eleatrolytic ~hromate treatmen~ to th~
lntermediate tin coati~g layer~ to fonm surface coatlng
layers, consi8t~ng oi electxoly~ed chromate, on the
intermediate tin coating layer.
A~ther ob~ect o~ the invention is to provide a
multilaver-coated steel strip having excellent corrosion
resistance and weldability, comprising:
(a) a substrate consisting of a steel
strip; and -~
(b) a multi-layer coating covering both
the upper and lower surfaces of the substrate and
comprisinq:
~ 4a - 1 ~ 3 ~ 9 62
(i) a base coating layer deposited on the ~ ~ -
substrate surfaces, consisting essentially of a Fe-Ni-
Sn-based alloy, the layer having a plurality of convex
and concave portions thereof;
S (ii) an intermediate coating layer
deposited on the base coating layer to form a ~ ~ :
precursory coated steel strip, consisting essentially :- -
of tin deposited in an amount of 200 to 2000 mg/m~, the ~ ~
intermediate coating layer having a plurality of :
10 convex and concave portions thereo~ in contact with ~.
corresponding convex and concave portions of the base
coating layer; and :~.
(iii) a chromate coating layer deposited
by an electrolytic chromate treatment on the
15 intermediate coating layer in an amount of 3 to 30
mg/m2 in terms of metallic chromium, the base coating ~:~
layer (i) and the intermediate layer (ii) having been
formed by: ~ ;
(A) plating the substrate consisting of ~: .
20 a steel strip, with metallic nickel or nickel-based ~ :
alloy consisting of at least 80% by weight of nickel : :
and 20% by weight or less of an additional metal :
element consisting of at least one member selected : :
from zinc, phosphorus, cobalt, copper and chromium, to
25 form a nickel-based coating layer OTI both the upper
and lower surfaces o~ the substrate, said nickel-based : ::
coating layers being in an average amount of 2 to 100
mg/m2 and provided with a number of convex portions and . :
concave portions thereof, and wherein said portions
30 thereof having a coating thickness of 0.001 ~m or more ~:
have a total area corresponding to from 10% to 95% of
the entire area of the surfaces of the substrate;
(B) coating the resulting nickel-based ;
plated substrate with tin to form tin coating layers
35 on the nickel-based coating layers, each tin coating~ ~:
~ :,
~33~62
- b -
layer being in an amount of 200 to 2000 mg/m2, to
provide a precursory coated steel strip;
and
(C) heating the precursory coated steel
S strip at a temperature equal to or higher than the
melting point of the tin coating layers, to cause the
nickel-based coating layers and the tin coating layers
to be converted to the base coating layer and the
intermediate coating layer.
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. 15 / : :
/
BRIEF DESCRIPTION 0~ THE DRAWINGS
Figure 1 is a schematic cross-sectional view of an
embodiment of the niokel coating layer formed on a steel
strip substrate in the first step of the proces~ of the
present inventions
Fig. 2 i~ a schema~ic cross-sectional view of
another embodiment of the nickel coating layer formed on
a steel strip ~ubstrate in the first ~tep o~ the process
of the pre~ent invent~on~ and, :
Fig. 3A to 3C are ~chematic cross-sectional v~ews ~:
35 of embodiment~ of the produc~s formed re~pectively in
~-" 133~2
the second, third, and fourth steps of the process of
the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
In the first step of the present invention, a
substrate consisting of a steel strip to be multilayer-
coated is plated with nickel or a nickel-based alloy to
form nickel-based coating layers on both the upper and
lower surfaces of the substrate to an extent such that -
the resultant nickel-based coating layers are coated in
a small average amount of from 2 to 100 mg/m2, preferably
from 5 to 100 mg/m2, per surEace of the substrate and
have an uneven thickness distribution, so as to provide
a number of convex and concave portions preferably
substantially evenly distributed in the layer.
That is, the uneven nickel based coating layer may
be, as shown in Fig. 2, in the form of a land having a
number of mountains and hills corresponding to the
convex portion~ and a number of lakes and valleys
corresponding to the concave portions, which mountains,
hills, lakes, and valleys are substantially evenly
distributed in the land. Some of the lakes and valleys
(concave portions) may have bottoms thereof formed by~ ~;
nickel or a nickel-based alloy plated on the substrate
surface~. Also, in the bottoms of other lakes and
valleyq ~concave portions), portions of the substrate
surfaces may be exposed to the outside. That is, the
nickel-based coating layer may incompletely cover the
surfaces of the substrate.
Alternatively, the uneven nickel-based coating
layer may be, as shown in Fig. 2, in the form of a
number o islands corresponding to the convex portions,
consisting of nickel or the nickel-based alloy and
preferably substantially evenly distributed in one or
more seas corresponding to the concave portions connected
to each other. Some of the island portions may be in
the above-mentioned form of a land having a number o
mountains, hills, lakes, and valleys. In the bottoms of
,. '.. '.' ~
~ .
',1'.. ~ : ,. ': .; ., ~
6 1~3~962
the sea portions of the nickel-based coating layer, the ',
corresponding portions of the substrate surfaces are
exposed to the outside~ ,
Referring to Fig. 1, a surface of a steel strip
substrate 1 is coated with an uneven nickel-based
coating layer 2 having convex portions 2a and concave
portions 2b.
In Fig. 2, a surface of a steel strip substrate 1
is coated with an islands-in-sea type nickel-based
coating la~er 2 consisting of a plurality of island~
formed nickel-based coating deposits 2c separated from
each other. Portions la of the surface of the substrate
1 are exposed to the outside but not coated with the
nickel-based coating deposit.
The coating thicknesses of the convex portions,
that is, the heights from the surface of the substrate
to the peaks of the convex portions, may be different.
Also, the coating thicknesses of the concave portions, '
that is, the thickness between the surface o the
substrate and the bottoms of the concave portions, may
be different.
In the formation of the uneven nickel-based coating
layers, the total area of portions o the layers having
a coating thickness of 0.001 ~m or more must be coated
to a level corresponding to 10% to 95~, preferably, 10
to 90%, o the entire area of the surfaces of the
substrate to be coated. Also, preferably the convex and
concave portions of the resultant nickel-based coating
layers satisfy the relationships (1), (2), and (3):
' hmax > 0.002 ~m (1)
, hmin > 0 (2)
and, hmin > 0, hmax > 2 hmin (31
wherein hmax represents a largest coating thickness of
the convex portions and hmin represents a smallest
coating thickness of the concave portions of the
nickel-based coating layer.
In the first step of the process of the present
:':
.~. : . .
~331~2
-- 7 --
invention, the steel strip substrate, which has been
degreased or surface cleaned by an ordinary method, is
subjected to a nickel-plating process. In the surface
cleaning procedures, the steel strip substrate may be
subjected to an anodic electrolytic treatment in a
pickling liquid, for example, a sulfuric acid aqueous
solution, or a degreasing liquid, for example, a sodium
hydroxide aqueous solution.
The surface cleaned steel strip substrate is
unevenly plated with nickel or a nickel-based alloy in
an amount of 2 to 100 mg/m2, preferably 5 to 100 mg/m2,
per surface of the substrate. The plating process can
be carried out in an ordinary nickel plating liquid, for
example, a sulfuric acid watt plating liquid. The -
composition of the plating liquid, plating current
density, plating temperature and time, and other plating
conditio~s are determined so that the resultant nickel~
based coating layers are in the above-mentioned specific
amount and have the above-mentioned uneven thickness
distribution. The plating method is not limited to a
specific method and may be an electric plating method or
a non-electrolytic plating method, as long as the
specific uneven nickel-based coating layers is obtained.
Also, ater the plating operation is completed, the
nickel-based pIated substrate may be additionally
subjected to an anodic electrolytic treatment. Alter-
natively, the nickel-based plated substrate may be ~ ~
subjected to a heat treatment at an elevated tempera- ~ ;
ture, to cause the plated nickel or nickel-based alloy
to diffuse into the steel strip substrate.
I the amount of the plated nickel or nickel-based
/ ¦ alloy is more than 100 mg/m2, the resultant coating
¦ ¦ layer will have a substantially even thickness and it
¦ ¦ will be difficult to provide a coated steel strip having `~
¦~5 a satisfactory corrosion resistance and weldability.
If the amount of the plated nickel or nickel-based
alloy is less than 2 mg/m2, it will be difficult to
~L331962
-- 8 --
provide a dense Fe-Ni-Sn based base layer having a
excellent effect for enhancing the corrosion resistance
of the resultant coated steel strip.
As stated above, the limitation in the amount of
the nickel-based coating layers to the range of from 2
to 100 mg/m2 per surface of the substrate is very
important when causing the resultant nickel-based -
coating layers to have an uneven coating thickness
distribution and to be provided with a number of convex
portions and concave portions thereof. This specific
form of the nickel based coating layers is essential
when providing a multilayer-coated steel strip having an
excellent corrosion resistance ana weldability and
useful as a steel matexial for producing cans or con-
tainers.
Also, if the total area of the portions of the/¦ nickel-based coating layers having a ~hickness of
¦¦ 0.001 ~m or more is more than 95% or less than 10% of
¦ the entire area of the surfaces of the substrate, the
I:20 unevenness in the coating thickness of the nickel-based
¦ coating layers will be unsatisfactory, and thus the
¦ resultant coated steel strip will exhibit an unsatis-
factory corrosion resistance and weldability.
The uneven nickel-based coating layer satisfying
the above-defined relationships (1), (2), and (3) is
very effective for further enhancing the corrasion
resistance and weldability of the resultant coated steel
strip.
The uneven distribution of the thickness of the
30- nickel~based coating layer can be observed by means of
an electron probe micro-analyser or an Auger electron
Spectroscopy.
The uneven nickel-based coating layer may consist
\ I ¦ of nickel or a nickel-based alloy consisting of at least
80% by weight of nickel and 20% by weight or less o~ an
¦¦ additional metal element consisting of at least one
member selected from zinc, phosphorus, cobalt, copper,
13319~2
g
.
and chromium. The additional metal element can be
alloyed with nickel by the heating treatment and is
effective for causing a portion of tin coating layer to
remain in the free tin state after the heat treatment.
The remaining free tin forms an intermediate tin coating
layer on the base coating layer after the heat-treatment
step.
In the second step of the process of the present
invention, the nickel-based plated substrate is coated
with tin in an average amount of 200 to 2000 mg/m2 per
surface of the substrate to provide a precursory coated -
steel strip. The tin coating procedures are not limited
to a specific method, and can be carried out by any
conventional tin plating method. However, the tin
coating is preferably carried out by an electric plating
method.
The average amount of the tin coating layers formed
on the nickel hased plated substrate i limited to a
specific range from 200 to 2000 mg/m2 per surface of
the substrate to provide a resultant coated steel strip
having an excellent corrosion resistance and weldability
at a low cost.
If the average amount of the tin coating layers is ~`
l more than 2000 mg/m2, the excess amount of tin over
¦25 2000 mg/m2 has no effect on the enhancing of the
¦ corro~ion reqistance and weldability of the resultant
coated steel strip, and undesirably increased the cost
of the resultant coated steel strip. Also, an average `~
amount o less than 200 mg/m of the tin coating layer
results in an unsatisfactory seam weldability and
corrosion resistance of the resultant coated steel
strip. -
After the tin coating step is completed, the coated -~
steel strip is usually washed with water and, if
necessary, is immersed in a flux comprising, as a
principal component, phenol sulfonic acid or ammonium
chloride, and finally, is dried. The flux may have a
.,
`` ~331962
-- 10 --
concentration corresponding to from 1/2 to 1/~ of that
in an ordinary flux for producing a usual tinplate. The
necessity for flux treatment and composition and con-
centration of the flux can be decided in consideration
of the type and constitution of the desired coated steel
strip.
In the third step in the process of the present
invention, the pxecursory coated steel strip is heat-
treated at a temperature equal to or higher than the
melting point of the tin coating layer. This heat
treatment may be carried out by, for example, an electric
resistance-heating method or high-frequency induction
heating method. Further, this heat treatment may be
effected in an atmosphere consisting of an inert gas,
for example, nitrogen or argon gas.
The heat treatment applied to the precursory coated
steel strip is effective-for converting the nickel-based
coating layers and tin coating layers to base coating
layers formed on the two surfaces of the substrate, and
consisting essentially of an Fe-Ni-Sn-based alloy and
having a number of convex and concave portions, and
intermediate coating layers f~rmed on the base coating
layers, consisting essentially of tin and having a
number of convex and concave portions. ~ ;
Preferably, the heat treatment is controlled to an
extent such that the content of tin in the resultant
base Fe-Ni-Sn-based alloy coating layers corresponds to
about 1/3, that is, from 30% to 35% of the entire weight
of the original tin-coating layers.
The heat treatment at a temperature equal to or
higher than the melting point of the original tin
coating layer results in the conversion of the nickel-
based coating layers and the tin coating layers to base
Fe-Ni-Sn based alloy coating layers and intermediate tin
coating layers, which are effective for imparting an
excellent corrosion resistance and weldability to the
resultant coated steel strip.
.
~ 33~2
-- 11 --
The above-mentioned conversion will be further
explained by referring to Figs. 3A to 3C.
Referring to Fig. 3A, a precursory coated steel
strip 10 which has been produced by the first and second -
steps of the process of the present invention, has a
steel strip substr ~ 11, an islands-in-sea type nickel-
based coating laye~ 1~ having a number of islands 12a,
wherein the--islands 12a are separated from each other,
and sea-shaped portions 12b between the islands 12a, and
, ' 10 a tin coating layer 13. When the precursory coated
' steel strip is heated at a temperature equal to or
higher than the melting poin~ of the tin coating layer,
the tin coating layer 13 is melted and the nickel-based
coating layer 12 is alloyed with a portion of iron in
the steel strip substrate 11 and a portion of tin in the
tin coating layer 13.
The alloying rate of nickel or nickel based alloy
with the iron and tin is proportional to the concen-
tration of nickel or nickel-based alloy in the alloying
system. Therefore, each of the nickel-based islands 12a
are rapidly converted to a corresponding alloy coating
while growing three-dimensionally. Namely, each alloy
coating becomes thicker than the corresponding nickel~
based islands and spreads on the substrate surface. The
spread alloy coatings are connected to each other and
form a continuous alloy coating layer which substantially
completely covers the surface of the substrate, as shown
in Fig. 3B.
Referring to Figs. 3A and 3B, the resultant alloy
coating layer 14 has a number of convex portions 14a
corresponding to the nickel-based islands 12a and a
number of concave portions 14b corresponding to the
sea-shaped portion 12b in the nickel-based coating
layer 12 in Fig. 3A.
The tin melt exhibits a larger wetting affinity and
a smaller free interface energy to the Fe-Ni-Sn-based
alloy layer surface than to the nickel based alloy layer
....
13319~2
- 12 -
surface and to the steel strip surface. Note, the
larger the thickness of the Fe-Ni-Sn-based alloy layer,
the greater the ~Jetting affinity of the tin melt thereto.
Accordingly, the thickness of the tin melt layer 15 on
the Fe-Ni-Sn-based alloy layer 14 corresponds to the
thickness of the Fe-Ni-Sn-based alloy layer 14 as shown
in Fig. 3B, when the heat-treatment is stopped and the
alloy coating layer and tin melt layer are cooled to
room temperature, the resultant tin coating layer 15 has
a number of convex portions 15a and concave portions 15b
thereof respectively corresponding to the convex portions
14a and the concave portions 14b of the alloy coating ~ -
layer 14.
If the nickel-based coating layer has an even ~
15 thickness, the conversion of the nickel-based coating ~ ;
layer progresses at an even converting rate throughout -
the layer, and the resultant alloy coating layer has a
substantially e~ven thickness. Accordingly, the even
base alloy coating layer causes the intermediate tin
coating layer to have a substantially even thickness.
The even tin coating layer sometimes can be con-
verted to an uneven tin coating layer as shown in
Fi~, 3B by a flux treatment under a certain condition.
However, the conversion by the flux treatment is not ~
25 always successful. Sometimes, the flux treatment fails `
to convert the even tin coating layer to an uneven tin
coating layer. Sometimes, the flux treated tin coating
layer contains uneven portions and even portions thereof.
In other ~ords, the flux treatment cannot stably convert
30 the even tin coating layer to an uneven tin coating
layer and, therefore, is not valuable for stably pro-
ducing the coated steel strip having an enhanced
corrosion resistance and weldability.
However, in the process of the present invention,
35 the uneven tin coating layers can be stably produced by
utilizing the uneven nickel-based coating l~yers formed
; on the steel strip substrate surfaces. The uneven tin
.1,```'~
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1331~62
- 13 -
coating layers are very effective for producing the
coated steel strip having an enhanced weldability and
corrosion resistance, and therefore, useful for cans and
containers.
Preferably, in the intermediate tin coating layers,
the convex portions are spaced 1 to 30 ~m apart, and
have a coating thickness of 0.20 ~m or more, the concave
portions have a coating thickness of 0 to 0.07 ~m, and
the average coating thickness of the entire intermediate -
tin coating layers is 0.17 ~m or less.
In the fourth step of the process of the present -~
invention, an electrolytic chromate treatment is applied, ~-~
as a final passive state-forming step, to the heat- ~ -
treated steel strip to form electrolysed chromate -
15 surface coating layers on the intermediate tin coating ~ -~
layers. The resultant surface coating layers have
substantially plain surfaces. That is, the thicknesses ~;
of portions of the surface coating layers formed on the
convex portions of the intermediate tin coating layers
is larger than that of portions of the surface coating
layer formed on the concave portions of the intermediate
tin coating layer. In other words, referring to Fig. 3C,
the surface coating layer 16 has a number of downward
convex portions 16a formed on the concave portions 15b
25 of the intermediate tin coating layer 15 and a number of ~;
~ard concave portions 16b formed on the convex portions
(15~ of the intermediate tin coating layer 15.
" ~ ~ The upward concave portions 16b of the surface
/ coating layers having a small coating thickness exhibit
/ 30 an excellent weldability. Also, the downward convex
portions 16a of the surface coating layers having a
large coating thickness exhibit a superior corrosion
resistance. Therefore, as a whole, the coated steel
strip of the present invention exhibits an enhanced
weldability and corrosion resistance and is useful for
cans and containers. When the coated steel strip having
the above-mentioned uneven surface coating layer is
.. . .
- 14 - ~33~9~'2
subjected to a seam welding procedure, the concave
portions of the uneven surface coating layers having a
- small coating thickness serve to stabilize the flow of
the electric current, and thus to improve the seam
weldability of the coated steel strip. Also, the thick
convex portions of the surface coating layers are
effective for enhancing the corrosion resistance of the
coated steel strip.
The uneven surface coating layers consisting
essentially of electrolysed chromate can be produced by
a conventional electrolytic chromate-treating method
usable for TFS-CT. Usually, the electrolytic chromate
treatment is carried out in accordance with a cathodic
reduction method in an aqueous solution of chromic -~
anhydride in the presence or absence of anions, for
example, sulfuric anions or fluoride anions. Also, any
known means for reducing co-depositing anions in the
electrolysed chromate layer can be applied to the - ;~
electrolytic chromate treatment. ~ ! ;
The electrolysed chromate surface coating layer may
consist essentially of chromium oxide hydrate alone.
The surface coating layer is preferably in an average
amount, in terms of metallic chromium, of 3 to 30 mg/m2
/ I per surface of the substrate. If the average amount is
¦ 1 25 less than 3 mg/m , the resultant coating steel strip
¦ ¦ sometimes exhlbits an unsatisfactory corrosion resis-
¦ ¦ tance and a poor bonding property to paint. Also, if
¦ ¦ the average amount of the surface coating layers is more
than 30 mg/m , the resultant coating steel strip
1 30 sometimes exhibits an unsatisfactory weldability.
I The electrolysed chromate surface coating layer may
comprise hot alkali-soluble chromium fractions and hot
alkali-insoluble chromium fractions.
In the surface coating layers, the proportion in
35 weight of the hot alkali-soluble fractions to the hot
alkali-insoluble ~ractions is not limited to a specific
level. However, in the concave portions of the surface ;~
, ,.:; '
- 133~962
- 15 -
coating layers, preferably the proportion of the hot
alkali-insoluble fractions is larger than that of the
hot alkali-insoluble fractions.
The present invention will be further explained by
5 way of specific examples, which, however, are merely ~ ;
representative and do not restrict the scope of the ~-
present invention in any way.
In the example, the following tests were carried
out. -~
IA) Seam Welding Tes~
A specimen, that is, a piece of a multilayer
coated steel strip, was formed into a peripheral portion
of a can in which edge portions of the specimen were
overlapped to a width of 0.4 mm. The overlapped portion
15 of the specimen was seam welded under a pressure of ~d~
45 hgf at a can-forming rate of 45 mpm. The value of
the second order welding current was varied to determine
a range of values of the second order welding current,
in which range an optimum seam welding was obtained.
The lower limit of the optimum range of the
second order welding current corresponded to a second
order welding current value at which the resultant
welded portion exhibited a lowest value of satisfactory
welding strength. Also, the upper limit of the optimum
second order welding current value range corresponded to
an upper limit of the second order welding current value
range in which the seam welding procedure can be carried
out without the generation of an undesirable splash
phenomenon.
The welding strength of the welded portion was
determined by an impact test and a peeling test in which
a V-shaped notch was formed in the welded portion of the
specimen and the welded two ends of the specimen were
peeled from each other by a pair of pincers.
The appearance of the seam welded portion of
the specimen was evaluated by naked eye observation in
which the generation and intensity of expulsion and
- 16 - 1331~62
surface flash on the welded portion were observed.
The specimen to be subjected to the seam
welding test was preliminarily heated at a temperature
of 210C for 20 minutes in an electric air oven.
(B) Underpaint Rust Resistance Test
Two surfaces of a specimen were coated with an
ordinary epoxy-phenol coating material for cans, in an
amount of 55 mg/dm2 per surface of the specimen, by a
roll coating method and the resultant coating layers
were heated at a temperature of 205C for 10 minutes and
then further heated at a temperature of 190C for 10
minutes. The resultant paint layers were scratched with -
a cutting knife and then subjected to an Ericksen "!''~'i.'
process at a height of 5 mm by using an Ericksen testing
15 m~chine.
The resultant testing specimen was subjected
to a salt water spraying test for one hour, by spraying
an aqueous solution of 5% by weight of NaCl. Then the
specimen was left in a thermo-hydrostat at a temperature
20 of 25C at a relative humidity of 85% for 14 days. The
generation of rust in the scratched portions in the ~ -
specimen was observed by the naked eye.
In each example, all of the procedures were
repeated twice. The seam welding test and the rust
25 resistance test were applied to both the first product
and the second product of each example.
Examples 1 to 5 and Comparative Examples 1 to 3
In each o Examples 1 to 5 and Comparative
Examples 1 to 3, two surfaces of a substrate consisting
30 of a steel strip, which had been surface cleaned by an
ordinary cleaning method, were plated with nickel in a ~
plating aqueous solution containing 200 g/l of ~ ;
NiSO4-7H2O, 60 g/l of NiC12 6H2O, and 50 g/l of
~3PO3 at the temperature of 50C at the pH selected
35 from the range of from 1.8 to 4.0 and at the cathodic
current density selected from the range of from 5 to ~ i
50 A/dm2 as shown in Table 1. The resultant nickel ~ ;;
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~ 133196~ :
- 17 -
coating layers consisted of the plated nickel in an -
amount in the range of from 2 to 120 mg/m2 per surface
of the substrate, as shown in Table l. The resultant ;
nickel coating layers were in the form as indicated in
Table l and had the largest coating thickness (hmax) and
the percentage RA of the total area portions of the
nickel coating layers having a coating thickness f -
0.001 ~m or more based on the entire area of the surfaces
of the su~strate, as shown in Table l.
The form and thickness of the nickel coating layers
were determined by AES and EPMA analyses.
In Examples l to 5 in accordance with the process
of the present invention, the largest thickness (hmax)
of the nickel coating layers was 0.002 ~m or more and
the percentage RA of the portions of the nickel coating
layers having a coating thickness of 0.001 ~m or more
was in the range of from 10% to 95%.
The nickel-coated steel strip was plated with tin
in a tin plating aqueous liquid containing 25 g/l of tin
sulfate, 30 ~/l of phenol sulfonic acid, and 2 g/l of
ethoxylated a-naphthol sulfonic acid at a temperature in
the range of from 40 to 50C at a cathodic current
density of 20 A/dm2. The average amount of the
resultant tin coating layers was in the range of from
800 to 1000 mg/m per surface of the substrate, as
shown in Table l.
The resultant precursory coated steel strip was
immersed in an aqueous flux solution containing 1 to
2 g/l of phenol sulfonic acid at a temperature of 45C~
and then dried.
The flux-treated precursory coated steel strip was
heat-treated by an electric resistance heating method at
a temperature of from 240C to 280C for 2 seconds to
6 seconds in the air atmosphere. The heating tempera~
ture and time were decided so that the resultant
Fe-Ni-Sn alloy base layer contained tin in an amount
corresponding to about 1/3 of the entire amount of tin
1 3 3 1 .9 ~ 2
- 18 -
plated on the substrate.
The heat-treated steel strip was subjected to an
electrolytic chromate treatment in an aqueous treating
solution containing 2 to 100 g/l of CrO3 , 0.1 to
1.0 g/l of H2SO4 and 0 to 3 g/l of Na2SiF6 at a
temperature of from 40C to 60C at a cathodic current
density in the range of from 5 to 90 A/dm2 so as to
form electrolysed chromate surface coating layers in an
average amount of 12 to 17 mg/m2, in terms of metallic
chromium, per surface of the substrate.
The distribution of the electrolysed chromate in
the surface coating layers was determined from the
characteristic X-ray intensity of chromium measured by
EPMA analysis.
In Table 1, the term "even distribution" refers to
a distribution of thickness of the intermediate tin
coating layers in such a manner that the ratio of the
average thickn~.ss TV of the downward convex portions
to the average thickness TC of the upward concave
portions of the surface coating layers is 1 or more and
less than 1.2. Also the term "uneven distribution"
refers to a distribution of thickness of the inter-
mediate tin coating layers in such a mannex that the
ratio o~ the average thickness TV of the downwaxd
convex portions to the average thickness TC of the
upward concave portions of the surface coating layers is ~
1.2 or more. ;
Preferably, the surface coating layers have an
uneven thickness distribution.
The results of the seam welding test and the rust
resistance test in the examples and comparative examples
are shown in Table 1. ~ ~
Examples 6 and 7 ~`
In each of Examples 6 and 7, the same procedures as
those described in Example 1 were carried out with the
following exception.
The nickel-plating step was carried out so that the
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- 19 -
.
resultant nickel coating layers were as indicated in
Table 1.
The tin-coating step was carried out in an aqueous
plating solution containing 75 g/l of stannous chloride,
25 g/l of sodium fluoride, 50 g/l of potassium hydrogen
fluoride, and 45 g/l of sodium chloride at a temperature
in the range of from 40 to 50C and at a cathodic :
current density in the range of from 20 to 40 A/dm2,
so that the resultant tin coating layers had the average
amount as indicated in Table 1~
No flux treatment was applied to the tin-coated
steel strip. The tin-coated steel strip was washed with
water and then subjected to the heat treatment.
The results of the tests are shown in Table 1.
Comparative Examples 4 and 5
In each of Comparative Examples 4 and 5, the same
procedures as those mentioned in Example 1 were carried
out except that. the nickel plating step was omitted, and
in Comparative Example 5, the average amount of the tin
coating layer was 1100 mg/m per surface of the
substrate.
The results of the tests are indicated in Table 1~ . . .
Referential Example :
.,
An ordinary tinplate #25 having tin coating layers
in an amount of 2800 mg/m2 per surface of the tinplate
was subjected to the same electrolytic chromate treatment
and tests as those mentioned above. ~
The results are shown in Table 1. ~:
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-- I331962
- 22 -
In Examples 1, 2, 3 and 6, the resultant multilayer
coated steel strips exhibited excellent seam weldability
and corrosion resistance compatible with those of the
ordinary tinplate, although the amounts of the tin
coating layers in Examples 1, 2, 3 and 6 are in a low
level of from 800 or 1000 mg/m2, whereas the ordinary
tinplate had a large amount of tin coating layers of
2800 mg/m2.
Also, from Examples 1 to 7 in comparison with
Comparative Examples 1 to 5, it is clear that the
presence of the uneven nickel coating layers on the
substrate surfaces is very effective for enhancing the
seam weldability and corrosion resistance of the
resultant coated steel strip.
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