Note: Descriptions are shown in the official language in which they were submitted.
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PROCESS FOR PRODUCING A CHROMIU~q--PLATED STEEL
STRlP HAVING ENHAMC~D WELDABILITY
FIELD OF THE INVENTION
The present invention relates to a process for pro- .
ducing a chromium-plated steel strip having an enhanced
- weldability, espec~ially regardlng s~arn weldability.
BACKGROUND OF THE INVENTION
The term "chromium-plated steel strip" used herein is
synonymous with the term ~electrolytically chromic acid-
-treated steel strip".
The chromium-plated steel strip has an excellent
lacquer-bonding property, a superior resistance to corrosion
under the lacquer-coating and a beautiful plated appearance,
and therefore, is useful for producing various steel pro-
ducts, for example: sanitary cans, such as beverage cans
and marine product cansf 18 liter cans, pails, motor oil
cans and other cans, in general; horne electric appliances;
and, steel made furn.iture. One type of chromium-plated
steel strip i5 commercially produced and sold under the
tradcmark, Can Super, by Nippon Steel Corporation.
However, despite its excellent properties, the con-
ventional chrornium-plated steel strip is limited in its
use, because i~ exhibits a relatively poor weldability.
For example, in the usual production of cans to be used for
beverayes, such as fruit juices, by using a soldering
method or an adhesive-bonding method, it is possible to
.~
produce 400 to 600 cans per minute. However, i~ the bodies
of the cans are produced from the conventional chromium-
-plated steel strip by using a welding method, it is
possible to weld only 150 to 200 cans per minute, and the
reliability in airtightness of the welded cans is poorer
; than that of the soldered or adhesive-bonded cans~
- The poor weldability of the conventional chromium-
-plated steel strip and the inferior reliablity in
airtightness of the welded can made from the conventional
chromium~plated steel strip are due to the following facts.
That is, in the conventional chromium-plated steel strip,
the plated coating layers comprise a metallic chromium and
a non-metallic chromium substance consisting essentially of
chromium oxides. Such plated coating layers exhibit poor
electric and thermal conductivities. Also, the plated
coating layers tend to generate heat locally and to
frequently produce splashes during the welding procedure.
Accordingly, in order to eliminate the above-mentioned
disadvantayes of the conventional chromium-platcd steel
strip, it is necessary -to enhance the electric and thermal
condllctivities o~ the plated surface layer~ th~reof. For
the purpose of eliminating the disadvantages, it was
attempted to form scratches on the plated coating layers,
so as to make the plated coating layers porous, or to
partially make the plated coating layers defective by
subjecting the chromium-plated steel strip to a temper
rolling procedure~ The above-mentioned attempts succeeded
in the elimination of the above-mentioned disadvantages.
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The scratches, pores or defects formed in the plated coa ing
layers are effective for enhancing the weldability of the
conventional chromium-plated steel strip. However, another
disadvantage resulted from these attempts. That is, the
above attempts all need an additional process for forming
the scratches or pores in the plated coating layer~ of the
conventional chromium-plated steel strip. This additional
process causes the-cost o~ the products made from the
conventional chromium-plated steel strip to be high.
SUMMARY OF THE INVENTION
An object ofthe present invention isto provide aprocess
for producing achromium-plated steel strip having anenhanced
weldability which can beused forproduciny various welded cans.
Another object ofthe present invention isto provide a
process for producing achromium-plated steel strip having an
enhanced weldability with a high producti~ity and low cost.
The above-mentioned objects can be attained by the
process of the present invention which comprises the steps
of (1) electroplating both the surfaces of a descaled steel
strip with 2.8 g or less of tin per m2 of each surface of
the steel strip, and; (2~ subjecting the tln-plated steel
strip to an electrolytic chromic acid-treatment to form a
plated chromium layer comprising 5 to 150 mg of metallic
chromium and 3 to 3~ mg, in terms of metallic chromium, of
a non-metallic, chromium-containing substance, per m2 of
each surface of the tin-plated steel strip.
In theprocess ofthe present invention, theelectrolytic
chromic acid-treatment iscarried outto anextent that each
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surface of the tin-plated steel strip iscoated with aplated
chromium layer comprislny 5to 150mg ofmetallic chromium and
3 to 30 mg, in terms ofmetallic chromium, o a non-metallic
chromium-containing substance, per m2 of each surface,
BRIEF DESCRIPTION OF THE INVENTION
Fig. lA shows an explanatory surface view of a welded
portion of a conventional chromium-plated steel strip;
Fig. lB shows an ex~lanatory cross-sectional view of
the welded portion shown in Fig. lA;
Fig. 2A shows an explana~ory surface view of a welded
portion of a chromium-plated steel strip produced in ac-
cordance with the process of the present invention
Fig. 2B shows an explanatory cross-sectional view of
the welded portion shown in Fig. 2A;
Fig. 3 is a microscopic photograph of a welded portion
of a chromiu~-plated steel strip of the present invention
as described in the sole Example;
Fig. 4 is a microscopic photograph of a welded portion
of a conventional chromium-plated steel strip as described
in Comparative Example l;
Fig. 5 is a m.icroscopic photograph of another con-
ventional chromium-plated steel strip, as described in
Comparison ~xample 2;
Fig~ 6 is a graph showing a relationship between the
lacquer-bonding intensity of a chromium-plated steel strip
and the amount of the tin layer plated on the steel strip
surface; and
Fig. 7 is a microscopic photograph of still another
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conventional chromium-plated steel strip as described in
Comparison Example 3.
DETAILED DESCRIPTION OF THE INVENTION
The chromium-plated steel strip of the present in-
vention is useful for producing various cans, for example,sanitary cans such as beverage cans, 18 liter cans, pails
and other cans, in general, by means of weldiny. It is
necessary for the resultant cans to exhibit a satislactory
airtightness in the welded portion thereof. Therefore, the
welding operation is usually carried out by an electric
resistance welding method, such as seam welding. However,
the welding operation may be carxied out by another method,
for example, spot welding. Therefore, the weldability of
the chromium-plated steel strip of the present invention
should cover not only the seam weldability, but also, other
weldabilities, for example, spot weldability.
In order to apply the process of the present invention,
a steel strip suitable for the plating process is descaled
by degreasing it with an alkali solution and, then, by
pickling it in an ordinary manner. The thus descaled steel
strip is termed "loam plate" hereinafter.
The loam plate is subjected to an ordinary electro-
plating procedure with tin. In this case, it is necessary
that both the surfaces of the loam plate are plated with
2.8 g or less of tin per m of each surface of the loam
plate. It is preferable that no reflow (xe-melting)
treatment is applied to the tin-plated steel strip, because
the reflow tr~atment, sometimes, causes the weldability of
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the finally resultant chromium-plated steel ~trip to
decrease. However, i~ the reflow treatment is carried out
to an extent that an alloy layer formed on the steel strip
is not exposed to the outside of the plated tin layer on
the steel strip, the reflow treatment does not affect the
weldability of the resultant chomium-plated steel strip.
- As sta-~ed above, the amount of the plated tin layer
- should be 2.8 9 or less, preferably, not less than 0.1 g,
per m of each surface of the loam plate. More preferably,
the amount of the plated tin layer is in the range of from
0.5 to 1.0 g per m2 of each surface of the loam plate.
Even if the amount of the plated tin layer is increased
to more than 2.8 g per m of each surface of the steel
strip, -the weldability of the resultant chromium-plated
steel strip does not increase with the increase in the
amount of the plated tin layer, and the welding current
necessary for welding the steel strip undesirably increases
with the increase in the amount of the plated tin layer.
Also, in -the welding procedure, the amount o;~ tin kha-t
flows out from the welded portion of the steeL strip in-
creases with the increa~e in the amount of the plated tin
layer, and causes the appearance of the resultant welded
product -to be unsatisfactory.
Further~ore, an excessively large amount of the plated
tin layer causes the following disadvantage. Usually, the
welded portion of the chromium-plated steel strip is spray-
-coated with a lacqu~r, so as to enhance the resistance or
the welded portion to corrosion. When the amount of the
~J ~l'79~
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plated tin layer is excessively large, the surface of the
welded portion is covered wi~h a relati~ely thick layer of
a tin-iron alloy which is fragile. r~en the alloy layer is
coated with a lacquer, the alloy layer is frequently broken
while the welded portion of the steel strip is worked, and
this breakaye of the alloy layer results in a separation
of the lacquer coating from the surface of the base steel.
Next, the tin plated steel strip is subjected to an
electrolytic chromic acid~treatment to form a plated
lQ chromium layer on each surface of the tin-plated steel
strip. The resultant plated chromium layer preferably
comprises 5 to 150 mg of metallic chromium and 3 to 30 mg,
in terms of metallic chromium, of a non-metallic, chromium-
-containing substance, per m2 of each surface of the tin-
-plated steel strip. The non-metallic, chromium-contalning
substance consists essen-tially of chromium oxides.
In the electrolytically plated chromium layer, ik is
preferable that the amount of -the metallic chromium is in
the range o~ from 5 to 150 mg, more preferably, ~rom 10 to
~0 mg, per m2 o each surEace oE the tin-platecl steel
strip. When the amount o~ the metallic chromium is less
than 5 mg per m oE each steel strip surface, sometimes,
the resultant chromium-plated steel strip may exhibit
unsatisfactory qualities, for example, an inferior
resistance to corrosion under a coating of lacquer and a
poor close~onding property to the coa-ting of lacquer.
Also, when the amount o the metallic chromium is more than
150 mg per m of each surface of the steel strip, the
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resultant pla-ted chromium layer, sometimes, may exhibit a
poor electric conductivity in the seam welding procedure.
Also, the excessively large amount of the metallic
chromium sometimes may cause the resultant plated chrmium
layer to exhbit an enhanced tendency of generating heat
locally and forming splashes in the welding operation, in
spite o~ the fact that the plated tin layer is located
- under the plated chromi~ layer.
In the plated chromium layer, it is preferable that
the amount of the non-metallic, chromium-containing sub-
stance is in the range of from 3 to 30 mg, more preferably,
from 7 to 12 mg, per m2 of each surface of the tin-plated
steel strip. If the amount of the non-metallic, chromium-
-containing substance is less than 3 mg per m2 of each
lS surface, sometimes the resultant chromium-plated steel
strip may exhibit an unsatisfactory close-bonding property
to the lacquer coating, and a poor resistance to corrosion
unless the chromium plated steel strip is coaked with
lacquer. Also, usually, it i~ very dlf~iclllt to produce
the plated chromlum layer containing less than 3 mg of the
non-metallic chromium-containincJ substance unless an
excessive amount of the substance is remo~ed ~rom the
plated chromium layer. The removal of the substance can be
effected only by using a special removing device or the
substanceO
Also, when the amount of the non-metallic chromium-
-containing substance i5 more than 30 mg per m of each
surface, the resultant plated chromium layer, sometimes,
3~
may exhibit poor electric and thermal conductivities and an
enhanced tendency of generating heat locally~ The local
heat generation results in the formation of splashes. This
phenomenon causes the workability and weldability of the
chromium-plated steel strip to be decreased. Furthermore,
the excessively large amount of the non-metallic, chromium-
-containiny substance sometimes may result in the forma-tion
~f undesirable pores or hollows in nugget portions and in
the vinicity of the nugget portions in the welded product.
Therefore, the stability and reliability of the welded
portion of the chromium-plated steel strip may become
unsatisfactory.
The tin-electroplating procedure and the electrolytic
chromic acid-treatment can be carried out in the usual
manner. That is, the tin-electroplatiny procedure may be
carried out using a usual ferro-stannous platiny bath
containing stannous sulfate and phenolsulfonic acid, at a
temperature of from 35 to 55C at a curren-t density of from
10 to 50 A/dm2 for 0.03 to 10 seconcls. The ~erro-stannous
platin~ bath preferably cvntains 10 to 30 g/Q of stannous
ions and 4 to 16 y/~, in terms of sulfaric acid, Oe a free
acid. Also, the electrolytlc chromic acid-treatment may be
carried out by using a usual chromic acid electrolytic bath
containing 30 to 200 g/Q of CrO3 , 0 to 6 g/Q of a fluoride
type additive, for example, Na2SiF6 and 0 to 2 g/Q of
sulfuric acid, at a temperature of 35 to 55C at a current
density of from 10 to 100 A/dm2 for 0.01 to 100 seconds~
In -the resultant chromium-plated steel strip of the
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present invention, the plated chromium layer which comprises
a metallic chromium having a melting point of 1905C and a
non-metallic chromium-containing substance that consists
essentially of chromium oxides, and.has a low electric
conductivity, is formed as an upper plated layer on an
under plated layer which consists of tin having a melting
point of 231.9C and a high electric conductivity.
Therefore, when the chromium-plated steel strip of the
present invention is welded, a portion of the upper plated
chromium layer is pressed by a welder ~lectrode and a
portion of the under plated tin layer located under the
pressed portion of the upper layer is molten in the initial
stage of the welding procedure. Therefore, in the initial
stage of the~ welding procedure, the portion of the upper
plated chromium layer, which is located on ~he molten
portion of the under plated tin layer, is broken and removed
~rom the plated steel strip, and in the subsequent staye,
the steel strip is firmly welded.
That is, the under plated tin layer can eliminate all
the disadvantac3es o~ the chromium layer, such as low
electri.c conductivity, local generation of heat and poor
thermal conductivity thereof, and allo~ the steel strip to
be firmly welded.
In this connection, it should be noted that the ~ntire
25 amount of tin in the undex layer should not be converted
into a tin-iron alloy when heating the under layer -to a
temperature above the melting point of th~ tinl because the
tin-iron alloy has a high melting point of about 1130C and
6~9
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therefore, cannot be molten in the initial stage of the
welding procedure. The tin-iron alloy layer cannot
cause the upper plated chromium layer to be partially
eliminated, Since the upper plated chromium layer is
not partially eliminated, the welding procedure for the
steel strip is carried out in the presence of the upper
plated chromium layer which has a high melting point and
low electric and thermal conductivities. Therefore,
heat is generated locally in the welded portion and the
local generation of heat results in the formation of
splashes. The resultant nuggets are small and discon-
tinuous. Accordingly, the conversion of the entire
amount of tin in the under layer would result in a poor
seam weldability of the steel ~trip and, therefore,
should be avoided.
The features and advantages of the process of the
present invention will be further described with refer-
ence to the accompanying drawings.
In Figs, lA and lB, two pieces 1 and 2 oE a con-
ventional chromium-plated steel strip are welded to each
other by an ordinary seam welding method. The plated
chromium layers on the steel strip surfaces contain
97.8 mg/m2 of metallic chromium and 12.5 mg/m of a non-
metallic chromium-containing substance. As a result of
the welding procedure, a p]urality of spots 3, each con-
sisting of a residue of molten metal discharged from the
welded portions of the pieces 1 and 2, are formed in the
- lla -
portion la of the surface of the piece 1~ The slze and
configuration of the spots 3 are di~erent ~rom each
other. The configuration of the spot is termed "flow
line". That
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,t3~
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is, the flow lines of the spots are unstable.
Also, a number of marks 4 were foxmed on the surface 2a
of the piece 2 when a welding current was applied to the
surface 2a using a welder electrode consisting of a copper
wire. This welding procedure caused the surface 2a to be
very slightly fused locally to make the marks 4.
In view of Figs. lA and lB, it is clear that the
- surface 2a, which contacted the welder electrodes in the
welding procedure, was substantially not fused, and ~he
pieces l and 2 were fuse-bonded by the heat generated in the
interface between the pieces l and 2. Also, the heat-
-affected zone formed by the welding procedure was very
small and there was no fuse-bonding between nugget portions.
Therefore, a portion of the molten metal, consisting es-
lS sentially of iron in the nuggets, flowed out from thenon-fuse bonded nugget portions. The ilowed out metal
formed the spots 3 on the surface la. The flow lines of
the spots are different from each other.
In E'igs. 2~ ancl 2B, two pieces 21 and 22 o~ a chromium-
-plated steel strip of the present invention are welded to
each other by means of a seam welding method by using a
copper wire electrode. The chromium-plated steel strip has
under layers each consisting of l.l g/m of tin and upper
layers ~ach consisting of 92.5 mg/m2 of metallic chromium
and 8.9 mg/m of a non-metallic, chromium-containing
substance~
Referring to Figs. 2A and 2B, a portion of molten
metal in the nugget portions flows out from the nugget
- 13 -
portions and forms a continuous molten metal layer 23
having a stable flow line. Also, the contac-t of the wire
electrodes with the surface 22a causes the contacting
portion of the surface 22a to be well fused to make clear
marks 24~ The heat-affected zone formed on the welding
portions of ~he pieces 21 and 22 is relatively large, and
the interface portions of the pieces 21 and 22 are well
- fused. The stable flow line o the molten metal layer 23
shows that the interface between the nuggets is satis-
factorily heated to the melting temperature so as to fuse
bond firmly the pieces 21 and 22 to each other.
Referring to Fig. 3, showing a microscopic view of a
welded portion o a chromium-plated steel strip of the
presen~ invention, the nuggets formed in the welded portion
are remarkably large and firmly fuse-bonded to each other,
and therefore, the welded portion is extremely airtight.
Referring to Fig. 4, showin~ a rnicroscopic view of a
welded portion of a conventional chromium-plated steel
strip having no under plated tin layer, the nu~ets ~ormed
in the welded portion ar~ small and the fuse-bondiny of the
steel strip sur~aces i5 effected only just under the wire
electrodes brought into contact with the steel strip
surface.
Refexring to Fig. 5 showing a microscopic view of a
welded portion of another chromium-plated steel strip
having excessively thick under plated tin layers, the
nuggets formed in the welded portions are unsatisfactorily
fuse-bonded~ Therefore, the airtightness of the welded
llt7g~z9
portion is, sometimes, unsatisfactory.
Fig. 6 sho~7s a relationship between the bonding
strength of the lacquer coating to a chromium-plated steel
strip of the present invention having under plated tin
layers on both surfaces thereof, and the total amount of
the under plated tin layers.
The total amount of the under plated tin layers was
varied from 0 to 1~.0 g/m2 as indicated in Fig-. 6, and the
upper plated chromium layers consisted of 20 mg/m2 of
metallic chromium and 10 mg/m of a non-metallic, chromium-
-containing substance. The chromium-plated steel strip was
shaped into a can having a diameter o~ 65 mm and an overlap
width of 0.8 mm, by means of an electric resistance seam
welding method using wire electrodes, at a welding current
of 21A (in the primary side) under a pressure of 50 kg.
The resultant can was subjected to a flanging
procedure. The 1anged can was coated by sprayincJ a paint
in an amount of 80 mg/dm onto the surfaces of the can.
The coated can was heated at a temperature o 170C for
5 minutes to cure the lacquer coating.
In order to -test the bonding strength of the lacquer
coating to the surface of the can, an adhesive tape was
adhered onto the inside surface of the welded, flanged
portion of the coated can, and, then, the adhesive tape was
peeled off from the inside surface. The area of a portion
of the lacquer coating separated from the inside surface of
the can, together with the adhesive tape, was measured.
The bonding strength was represented by -the ratio (O~ of
.
~ ~t~9 ~
- 15 -
the measured area o the separated portion of the lacquer
coating to the entire area of the inside sur~ace of the
welded, flanged portion. The larger the ratio (~), the
lower the bonding strength of the lacquer coating to the
welded chromium-plated steel strip surface.
From Fig. 6, it is evident that in order to obtain a
practical satisfactory bonding strength, the total amount
of the plated tin ~ayers on both surfaces should be
5.6 g/m or less. Also, the excessively large amount of
plated tin layers causes the resultant product to be
expensive. Therefore, on each of the surfaces of the
chromium-plated steel strip of the present invention, the
amount of the plated tin layer should be 2.8 mg or less per
m2 of each surface.
lS In Fig. 7, showing a microscopic view of the welded
portion of still another chromium-plated steel strip in
which the tin layers are entirely converted into tin-iron
alloy layers, the nuggets formed in -the w~lded portion ~re
small and splashes are formed in the welded portion. That
is, this type oE chromiur~-plated steel strip exhibits an
unsatisfactory weldability.
The following example illustrates, but does not limit,
the present invention.
A loam plate, having a thickness of 0.23 mm which had
been degreased and pickled in a usual ~anner, was subjected
to an electroplating procedure by using a ferro-stannous
plating bath containing 30 g/Q of stannous ions and 14 to
~t71~
- 16 -
15 g/~, in terms of sulfuric acid, of a free acidj at a
temperature of 43C at a current density o 25 A/dm2 for
O.9 seconds, whexeby 1.2 g/m2 of tin were plated on both
the surfaces of the loam plate.
The tin-plated loaJn plate was subjected to an elec-
trolytic chromic acid treatment by using a chromic acid-
-treating bath containing 150 g/Q of CrO3 , 4.5 g/Q of
Na2SiF6 and 0.3 g/~ of sulfuric acid, at a tempçrature of
43~ at a current density of 70 A/dm for 0.4 seconds. The
resultant plated chromium layers on both the surfaces of
the loam plate consisted of 41.8 mg/m of metallic chromium
and 12.4 mg/m2 of a non-metallic, chromium-containing
substance.
The plated loam plate was coated with 3.0 to
4.0 mg/m of DOS, and, then, subjected to an electric
resistance welding procedure at a frequency of 180 Hz at a
welding speed of 23~5 m/min under a pressure of 5 Kg at a
weldiny current ~primary side) of 19 to 21h~ The overlap
width of the welded portion was O.B mm.
The microscopic photograph (magnification:25) of a
cross-section of the resultant welded portlon along the
seam portion thereof, is shown in Fig. 3.
Since the under tin layer was molten in the initi~1
stage of the welding procedure, the electric and thermal
conductivities of the welded portion were enhanced, the
nuggets formed in the welded portion grew satisfactorily
and the resultant welded portion exhibited a satisfactory
airtightness.
7~3~3
- 17 -
--o~5Y~ L~-Læ,~ e 1
The same procedures as those described in Example 1
were carried out with the following exception. No tin-
-electroplating procedure was applied to the loam plate.
The plated chromium layers on both surfaces of the loam
plate consisted of 43.3 mg/m2 of metallic chromium, and
11.7 mg/m , in terms of metallic chromium, of a non-
-metallic, chromium-containing substance. Also, the DOS
was uséd in an amount of 3.0 to 4.0 mg/m2.
During the welding procedure, the phenomena of ex-
plusion and surface flash were obser~ed in the weldedportion.
Fig. 4 shows a microscopic view (magnified 25 times)
of a cross-section of the welded portlon along the seam
portion thereof. From Fig. 4, it is clear that the nuggets
formed in the welded portion are small, and welding was
effected only in the portions in which khe welding current
become maximum.
~ arative Ex~ le 2
. .
The same procedures as those described in Example l
were carried out, except that the plated -tin layers on both
the surfaces of the loam plate were in a total amount of
9.2 g/m ; t~e electrolytic chromic acid-treating both
contained 100 g/Q of CrO3 and 3 g/Q of Na2SiF~ ; the
chromium plating procedure was carried out at a current
density of 35 ~/dm2 at a temperature of 43C for
0.7 seco~ds; the resultant chromium layers on both the
surfaces consisted of 14.h mg/m of metallic chromium and
3~$J
11.5 mg/m2, in terms o~ metallic chromium, of a non-
-metallic, chromium-containing su~s~ance, and; DOS was used
in an amount of about 3~0 mg/m2.
The excessively large amount of the plated tin layer
s caused the loss of heat during the welding procedure to be
large due to the melting of the tin and, there~ore, the
fuse bonding of the nuggets to each other/ in the welded
portion, was unsatisfactory. Accordingly, the reliability
in airtightness of the welded portion was poor.
1~ Comparative Example 3
The same procedures as those described in Example 1
were carried out, except that the tin-plated loam plate was
heated up to a temperature of 240C at a heating rate of
90C/sec, for 2.4 secondss and, then, the heated loam plate
was immediately cooled with cold water, to convert the
entire amount of the tin into a tin-iron alloy; and the
plated chromium layers on both the surfaces o~ the loam
plate consisted of 38.5 mg/m2 of metallic chromium and
12.5 mg/m2, in terms of metallic chromium~ of a non-metallic
chroTniuTn-containing subs-tance.
The cross-section o~ the welded portion along the
seam portion thereof is indicated in Fig. 7. The -tin-iron
layers having a high mel-ting point and a poor thermal
conductivity caused the nugge-ts formed in the welded portion
to be small and resulted in the formation of undesirable
splashes. That is, the resultant chromium-plated steel
strip exhibited a poor weldability.
