Note: Descriptions are shown in the official language in which they were submitted.
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FIELD OF THE INVENTION
The present invention relates to a method for producing a
thin tin and nickel plated steel sheet having excellent corrosion
resistance after lac~uering and excellent weldability. In
detail, the inv~ntion relates to a method for producing a thin
tin and nickel plated steel sheet which is characterized by an
anodic treatment of a pickled steel sheet in an àlkaline
electrol~te with a pH of above 10 before plating with a small
amount of nickel, plating with a small amount of tin on the
ni~kel plated steel sheet, reflowing, quenching, and chromate
treating the tin and nickel plated steel sheet.
By using this tin and nickel plated steel sheet, a welded
can body can be easily produced at high speed in spite of the
small amount of the plated tin and nickel without removing the
plated layer in the welded part.
, .
BACKGROU~D AND OBJECTIVE
Recently, electric welding has been widely used for the
seaming of tinplate can bodies in the field of food cans, aerosol
cans, and miscellaneous cans, instead of soldering. In the
seaming of the tinplate can body, it is desirable to decrease the
tin coating weight in the tinplate because tin used for
electrotinplate is very,expensive and there is concern over the
exhaustion of tin resources. However, the weldability of the
tinplate becomes gradually poor with a decrease of the tin
, coating weight.
From the background described above, the development of a
welded can material which is cheaper than conventional
electrotinplate
clcc~ eC-, is easily welded at high speed without the removal
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of the plated layer and i5.. excellent in corrosion
resistance after lacquering, has been required in the
field of food cans. Over the past few years, various
surface treated steel sheets have been prop~sed for
welded can materials which can be easily welded at
high speed without the removal of the plated layer
and are cheaper than tinplate. For instance, the
followins surface treated steel sheets have been
proposed: (a) Lightly tin plated steel sheet ~LTS)
with below about 1.0 g/m2 of tin which is reflowed or
unreflowed after tinplating (Japanese Patent
Publication Nos. Sho.56-3440 (Jan. 24, 1981;
Applicant: Toyo Kohan Co., Ltd.), Sho.56-54070 tDec.
23, 1981; Applicant: Toyo Kohan Co., Ltd.), Sho.
57-55800 (Nov. 26, 1982; Applicant: Nihon Kokan
,:
- Corporation) and Laid Open Japanese Patent Applica-
. .
~ tion Nos. Sho.56-75589 (June 22, 1981; Applicant;
, ~
Nihon Kokan Corporation), Sho.56-130487 (Oct. 13,
1981i Applicant: Toyo Kohan Co., Ltd.), Sho.56-156788
(Dec. 3, 1981; Applicant: Nlhon Kokan Corporation),
Sho.57-101694 (June 24, 1982; Applicant: Toyo Kohan
Co., Ltd.), Sho.57-185997 (Nov. 16, 1982; Applicant:
Toyo Kohan Co., Ltd.), Sho.57-192294 tNov. 26, 1982;
Applicant: Nihon Kokan Corporation), Sho.57-192295
. ': :~ :
Nov. 26, 1982; Applicant: Nihon Kokan Corporation)
and Sho.55-69297 (May 24, 1980; Applicant: Nippon
Steel Corporation)); (b) Nickel preplated LTS with
, ~." ~
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.: ~ . ' - . -' ~ ' '' :` ` '
below about 1 g/m of tin ~Laid Open Japanese Patent
Application Nos. Sho.57-23091 (Feb. 6, 1982;
Applicant: Nippon Steel Corporation), Sho.57-67196
(Apr 23, 1982; Applicant: Nippon Steel Corporation),
Sho.57-110685 (Jul. 9, 1982; Applicant: Nippon Steel
Corporation), Sho. 57-177991 (Nov. 1, 1982;
Applicant: Nippon Steel Corporation), Sho.57-200592
(Dec. 8, 1982; Applicant: Kawasaki Steel Corpora-
tion), Sho.57-203797 (Dec. 14, 1982i Applicant:
Kawasaki Steel Corporation), Sho. 60-33362 (Fe~. 20,
1985; Applicant: Nippon Steel Corporation) and
Sho.60-56074 (Apr. 1. 1985; Applicant: Nippon Steel
Corporation)); (c) Nickel plated steel sheet with
chromate film or phosphate film (Laid Open Japanese
Patent Application Nos. Sho.56-116885 (Sept. 12,
1981; Applicant: Nippon Steel Corporation),
Sho.56-169788 (Dec. 26, 1981; Applicant: Nippon Steel
Corporation), Sho.57-2892 (Jan. 8, lg82; Applicant:
Nippon Steel Corporation), shO.57-2895 (Jan. 8, 1982;
Applicant: Nippon Steel Corporation), Sho.57-2896
(Jan. 8, 1982; Applicant: Nippon Steel Corporation),
Sho.57-2897 (Jan. 8, 1982; Applicant: Nippon Steel
Corporation), Sho.57-35697 (~eb. 26, 1982; Applicant:
Nippon Steel Corporation) and Sho.57-35698 (Feb. 26,
1982; Applicant: Mippon Steel Corporation)).
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However, LTS and nickel preplated LTS above
identified as (a) and ~b) have a narrower current
range for sound welding than that in tinplate,
although these can be welded without the removal of
the plated layer. The reason why the current range
for sound welding in LTS and nickel preplated LTS is
narrower than in tinplate is considered to be that
the amount of free metallic tin in these is smaller
than that in tinplate and also further decreases
because of the change of plated free metallic tin to
iron-tin alloy or iron-tin-nickel alloy by heating
for lacquer curing or reflowing after tinplating.
Although the weldability -and the corrosion
resistance after lacquering are improved with an
increase of coating tin, it is difficult to increase
the expensive tin coating weight in LTS and nickel
preplated LTS in view of increase in production cost.
.~,
An increase in the amount of nickel in nickel
preplated LTS lmproves the corrosion resistance after
lacquering, but does not improve the weldability,
beoause the amount of free metallic tin decreases by
the formation of tin-nickel alloy during aging at
room temperature or by the formation of iron
tin-nickel alloy during reflowing nickel preplated
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~ LTS.
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Nickel plated steel sheet with chromate film
or phosphate film identi~ied ~bove as (c) also has a
narrower current range for sound welding than that in
tinplate, LTS, or nickel preplated LTS.
Furthermore, the corrosion resistance of
nickel plated steel sheet is poor, although the
lacquer adhesion is good. Particularly, pitting
corrosion in the defective part of the lacquered
nickel plate steel sheet may occur easily in acidic
foods such as tomato juice because the electric
potential of nickel is more noble than that of steel
sheet.
As described above, the various surface
treated steel sheets proposed in ~a), ~b) and (c)
have various problems in production cost and
characteristics as a welded can material which can be
easily welded without the removal of the plated layer
at high speed.
It is therefore an object of the present
~ invention to provide a thin tin and nickel plated
- steel sheet having excellent corrosion resistance
after lacquering and excellent weldability.
It is another object of the present
invention to provide a method for the continuous
production of a thin tin and nickel plated steel
`,~ sheet having excellent characteristics as described
~ ~ above.
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! In accordance with the invention, there is
provided a method of producing a tin and nickel
plated steel sheet having a surface structure in
which the distribution of numerous nodules of
metallic tin can be observed by using an electron
microscope (1000 magnifications) on an iron-tin-
. nickel alloy layer formed on a steel sheet, which
comprises the steps of:
a) anodically treating a substantially clean
steel sheet in an alkaline electrolyte having a pH
above 10 under a quantity of electricity ranging from
3 to 50 coulombs/dm2;
b) electroplating the steel sheet treated in
step (a) with nickel in an amount of S to 20 mg/m i
c) electroplating the nickel plated steel
:~ sheet obtained in step (b) with tin in an amount of
::
400 to 900 mg/m2;
: : d) reflowing and quenching the tin and
nickel plated steel sheet obtained in step (c); and
e) chromate treating the tin and nickel
plated steel sheet to form thereon a film containing
::,
hydrated chromium oxide in an amount of 3 to 20 mg/m2
as chromium.
The tin and nickel plated steel sheet
produced according to the method of the present
invention is clearly different from the nickel
preplated LTS, which has already been disclosed in
5b -
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various Laid-Open Japaneses Patent Applications, in
the surface structure, particularly in the form of
metallic tin on the iron-tin-nickel alloy layer
formed on the steel sheet, although it nevertheless
is a nickel preplated LTS. Namely, in the tin and
nickel plated steel sheet according to the present
invention, numerous nodules of metallic tin are
present on the iron-tin-nickel alloy layer formed on
: the steel sheet. On the other hand, it is considered
that a uniform and thin metallic tin layer is formed
on the
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ixon tin-nickel alloy layer or the tin-nickel alloy layer ~ormed
on the steel sheet in the known nickel preplated LTS.
DETAILED DESCRIPTION OF THE INVENTION
The steel sheet used for the production of the tin and
nickel plated steel sheet according to the present invention can
be any cold rolled steel sheet customarily used in manufacturing
electro-tin plated and tin-free steel. Preferably the thickness
of the steel sheet is from about Q.l to about 0.35mm.
The tin and nlckel plated steel sheet according to the
present invention is produced by the following process:
degreasing with an alkali and pickling with an acid ~ water
rinslng ~ an anodic treatment in an alkaline electrolyte ~ ~ater
rinsing ~ nickel plating ~ water rinsing ~ tinplating ~ water
rinslng ~ drying ~ reflowing ~ quenching . chromate
treatment ~ water rinsing ~ drying.
:,
In this process, it is possible that the anodically treated
s~tee~l sheet is plated with tin-nicke~l alloy, nickel-iron alloy,
nickél-~zinc alloy, or nickel containing boron and phosphorus
instead of plating with nickel. In the present invention, an
anodic treatment of a pickled steel sheet in an alkaline
electrolyte wlth a pH of above 10 is indispensable in order to
obtain the thin tin and nickel plated steel sheet having a
surface~seructure~ln whlch the distribution of numerous nodules
Qf`mëta`lllC~`till lS observed by uslng an electron microscope on
the~iron-tin-nic-kel alloy layer formed on the steel sheet after !
reflowlnq~of~the tin and nickel plated steel sheet. The alkaline
e1ectrolyte having a pH of above 10 used for the anodic treatment
of the pickled steel sheet is made up by the dissolution of at
least one alkallne compound selected from the group cansisting of
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- ~ydroxide, a carbonate, a bicarbonate, a silicate, a phosphate,
anl a borate of an alkali metal and ammonium compounds in water.
Furthermore, the effect of the anodic treatment o~ the pickled
steel sheet in the alkaline electrolyte is not reduced as long as
the pH of the alkaline elèctrolyte is maintained above lO, even
if at least one compound such as an acidic phosphate, an oxalate,
a citrate, and an acetate of an alkali metal, and an ammonium
compound is added , if the surface active agent which is usually
added t~ tAe alkaline solution for degreasing of the steel sheet
is added or a small amount of sulfuric acid or hydrochloric acid
is brought into the alkaline electrolyte of the present invention
because of insufficient rinsing after pickling.
It is an essential condition that the alkaline electrolyte
used for the anodic treatment of the pickled steel sheet
be maintained above a pH of lO, although it is not necessary to
strictly control the concentration of hydroxide, carbonate, etc.,
salts of an alkal~ m~tal and ammonium compounds. If the
concentration of the alkaline electrolyte is restricted, it is
. " , ,
preferably in the range of lO to lOO g/l. At below lO g/l, a
waste of electric power resul~s because of tne higher electric
resistance of the alkaline electrolyte. The concentration is
". ~
~ limited to lOO g/l from the viewpoint of economy, although the
. .
effect of the anodic treatment in the alkaline electrolyte is not
increased even if the concentration is above lOO g/l. Although it
:
is not necessary that the temperature of the alkaline electrolyte
,~
be strictly controlled, it is preferably below 90C from the
; viewpoint of energy savings.
rn order to obtain the effect of the anodic treatment in the
aIkaline electrolyte, it is necessary that the quantity of
. ~
~ electricity for the anodic trea~ment be in the range of 3 to 50
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coulom~s/dm2, more preferably 5 to 30 coulombsldm2. If the
q~_ntity o~ electricity is below 3 coulombs~dm2,,the effect o~ -
the anodic treatment is not obtained because thin and dense iron
oxide film is not uni~ormly formed on the surface of the pickled
steel sheet The quantity of electricity is limited to 50
coulombs/dm ~rom the viewpoint of the high speed production of
the tin and nickel plated steel sheet and from the viewpoint of
ener5y savings, although the effect of the anodic treatment in
the al~31ine electrolyte is not reduced.
In the case o~ the alkaline electrolyte having a pH of below
, a large quantity of electricity is necessary in order to
obtain the effect of the anodic treatment in the present
invention. Therefore, a 12rge qùantity of electricity for the
.
:, anodic treatment is not desirahie from the viewpoint of the high
speed production of the tin and nickel plated steel sheet
: accarding to the present invention~
~ ,;
For the high speed production of tin and nickel plated steel
sheet:~according to the presen~ invention, it is reasonable ~hat
the~electrolytic time be 0.1 to S seconds, and the range of '
~r ~ current density be 1 to 50 A/dm in the anodic treatment of the
pickled~steel sheet,
The amount o~ nickeI plated on.the'steel sheet which, is
anodically treated beforehand in the al~aline electrolyte with a
pH~of ;above 10 lS one of the lmportant factors in the present
inventlon. The:amount of.plated nic~el should be controlled in
.` ,'~the~range of~5 to 20 mg/'m2.
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5.~,7~
BRIEF DESCRIPTION OF THE DRAWINGS
. . . _ .
Figure 1 shows the magnified photograph
wherein numerous nodules of metallic tin 1 are
distributed on the uniform iron-tin-nickel alloy
layer 2 in the surface of the tin and nickel plated
steel sheet according to the present invention.
Figure 2 shows the effectof the amount of
plated nickel on the-disbribution of nodular metallic
tin on the iron-tin-nickel alloy layer formed on the
steel sheet after tinplating and then reflowing.
Figure 3 shows the effect of the amount of
plated nickel on the electric contact resistance of
the tin and nickel plated steel sheet after heating
at 2LOC for 20 minutes.
:.
Figure 4 shows the effect of the amount of
plated nickel on the filiform corrosion resistance
after lacquering of the tin and nickel plated steel
sheet.
In Figure 2 to Figure 4, Curve A shows
examples wherein nickel plating, tinplating, and then
reflowing were carried out after the anodic treatment
of the pickled steel sheet in an alkaline electro-
lyte, and Curve s shows examples without the anodic
treatment before nickel plating. Samples used for
Example A and s in Figure 2 to Figure 4 were produced
by the following same conditions in each step except
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~ for the anodic treatment of samples for Example A.
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Furthermore, samples shown--in Figure 4 were
cathodically treated in chromic acid electrolyte
after reflowing.
Degreasing...Cathodic electrolysis in 70 g/l
of NaOH under 5 A/dm for 2 seconds at 70 C.
Pickling...Immersion into 100 g/l of H2SO4
for 2 seconds at 25C.
Anodic treatment...Anodic treatment in 70
g/l of NaOH (pH 14~ at 5 A/dm for 2 seconds at 70 C.
(Only samples for Example A)
Nickel plating...Plating with various
amounts of nickel under 3 A/dm at 40C by using
Watts bath.
Tinplating...Plating with 700 mg/m2 of tin
under 10 A/dm at 40 C by using a phenolsulfonic acid
bath.
Reflowing...Raising the temperature of the
tin and nickel plated steel sheet up to 28QC during
1.6 seconds by using resistance heating.
Quenching...Rapid immersion into water after
reflowing.
Chromate treatment...Cathodic treatment in
chromic acid electrolyte containing 30 g/l of CrO
and 0.3 g/l of H2SO4 under 10 A/dm for 0.5 seconds
at 50C.
Water rinsing was carried out between
succeeding steps.
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As shown ln Flgure-2, in the case of Ex~mple --
A with ~r~odic treatment in alkaline solution,
numerous nodules of metallic tin were observed by an
electron microscope on the iron-tin-nickel alloy layer
Eormed on -the steel sheet in the range of S to 20
mq/m2 of plated nickel, but nodular metallic tin is
almost not present at above 20 mg/m and at below 5
mg/m2 of plated nickel.
On the other hand, in the case of Example B
without anodic treatment in alkaline solution, almost
all of the deposited tin forms a uniform iron-tin-
nickel alloy layer irrespective of the amount of nickel
prior to tinplating.
Generally, tne weldability is evaluated by
an available range of secondary current in welding as
shown in the report by N.T. Williams (Metal
Construction, April 19?7, pages 157-160), that is to
say, the wider the secondary current range in
. welding, the better the weldability. The upper limit
. ~ . . .
~ in the available secondary current range corresponds
.:
~ to the welding conditions in which some defect such
~ ..
as splashing is found and the lower limit corresponds
- to the welding conditions in which the breakage
~ occurs in the welded part by the tearing test.
-~ ~ However, in order to obtain data wherein the
available range of secondary current in welding is
;-` decided for each sample, a large amount of samples
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are necessary. Therefore, the weldability is evaluated by
electric contact resistance, because electric contact resistance
has an apparent correlation with the available range of secondary
current in welding as shown in the report by T. Fu~imura (Journal
of The Iron and Steel Institute of Japan, Vol. 69, No. 13,
September 1983, page 181), that is, the lower the electric
contact resistance, the wider the secondary current range in
welding Accordingly, ~f the electric contact resistance is
lower, the weldability is better.
It is found from Figure 3 that the electric contact
than
resistance of samples for Example A is lower that in the sample
for Example B at below 20 mg/m2 of plated nickel wherein nodular
metallic tin is observed on the iron-tin-nickel alloy layer
formed on the steel sheet in the sample for Example A. At below
5 mglm2 plated nickel, the electric contact resistance becomes
slLghtly high because of the decrease in the amount of metallic
t~in by the formation of an alloy consisting mainly of iron-tin
alloy. The electric contact resistance increases with an
increase in the amount of plated nickel because of the decrease
Ln the amount of metallic tin caused by the formation of an alloy
consisting mainly of tin-nickel alloy during aging at ordlnary
tèmpérature-
As shown in Figure 4, the filiform corrosion resistance is
lmproved with an increa$e in the amount of plated nickel. In the
range of~5 to ~30 mg/m2 of plated nickel, the filiform corrosion
resi~stance after laoquering of samples for Example A is excellent
compared with that of Example B because the iron-tin-nickel alloy
having excellent lacquer adhesion is present with numerous
nodules of metallic tin on the surface of samples for Example A,
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~-t the surface of the samples for Example ~ is covered with
uniform metallic tin layer having poor lac~uer adhesion.
As described above, the anodic treatment of the pickled
steel sheet in an alkaline electrolyte and the plating with the
limited amount of nickel are indispensible factors in the present
invention.
The reason why the anodic treatment of the pickled steel
sheet in an alkaline electrolyte and the plating with the limited
amount of~ni~ckel is necessary in order to produce the tin and
nickel plate steel sheet having numerous nodules of metallic tin
on ~he iron-tin-nickel alloy layer formed on the steel sheet is
not clear. However, it is assumed that the reason is the
formation of numerous nodules of metallic tin by dewetting of
~ .~
:~j metallic tin electrodeposited in the part wherein iron oxide is
: formed on the pickled steel sheet by the anodic treatment in an
. :~
~alkaline electrolyte and the concentration of.metallic tin to the
~part wherein nickel is electrodeposited during the formation of an
iron-tin-nickel alloy layer by reflowing after tinplating.
Furthermore, it is assumed that the reason why the tin and nickel
plat:ed steel sheet having numerous nodules of metallic tin on the
iron-tin-nickel alloy layer formed on the steel sheet is
excellent in welda~ility and corrosion resistance after
laequering, particularly filiform corrosion resistance, is that
excellent weldability is maintained in the area having a large
amount~of metallic tin in noduIar form compared with an average
amount~ of plated metallic tin and that excellent corrosion
rcsistance after lacquering is maintained in the area where
iron-tin-nickel alloy having excellent lacquer adhesion is
exposed~on the surface without being covered by a uniform
metallic tin làyer.
12-
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Therefore, an amount of nickel above 20 mg/m2 is
not desirable in the present lnvention becaus~ the tin
iron oxide coating formed on the steel sheet substrate
by the anodic treatment in the alkaline electrolyte
p~ior to nickel plating is almost completely removed
during nickel plating. In the present invention, a
known nickel plating electrolyte such as a Watts bath
containing 200 to 300 g/l of nickel sulfate, 20 to
50 g/l of nickel chloride, and 20 to 40 g/l of boric
acid or a nickel sulfamate bath containing 300 to
500 g/l of nickel sulfamate, and 20 to 40 g/l of boric
acid is used for nickel plating of the steel sheet
treated anodically in an alkaline electrolyte. The
plating 5 to 20 mg/m2 which is required in the present
invention is carried out by using these electrolytes
described above under 2 to 30 A/dm2, 30 to 70C of the
electrolyte temperature,-and 1 to 10 coulombs~dm2 of a
quantity of electricity.
In the present invention, the amount of tin
plated on the nickel plated steel sheet is also one
of the important factors. The amount of plated tin
should be controlled in the range of 400 to 900 mg/m2.
If the amount of plated tin is below
,.
400 mg/mG number of nodules of metallic tin is not
:.
enough to improve the weldability after reflowing
because a large part of electrodeposited metallic
,~,::: : :
tin readily changes to an iron-tin-nickel alloy on
reflowing. At above 900 mg/m2 of plated tin, few
- 13 -
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nodules o~ metall-Lc tin are obtained on re~lowing
bcc~use the iron tin-nic~el alloy layer formed on
reflowi.ng is uniformly covered with a large amount of
metallic tin.
Tinplating on the nickel plated steel sheet
: in the present invention is carried out by using a
known tinplating electrolyte
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uc~ rQr ~h~ production of electrotinplate. For instance, a
phenolsulfonic acicl ~ath colltailling 10 to 30 g/l
of phenolsulfonic acicl as ~ulfuric acid, 10 to 40 g/l of stannous
sulfate or stannous phenolsulfonate and 0.5 to 10 gtl of
et.hoxylated a-naphthol sulronic acid, halo5enide ba~h containing
stannous chloride, alkali haloserlide, and additives or an
alkaline bath containing al~ali stannate ~nd alkali hydro~ide is
used in the present invention.
The conditions for tinplating in the present invention are
also almost the same as that for the production of conventional
electrotinplate. For instance, tinplating by using a FERROSTAN
(trade ~ark; phenolsulfonic acid bath) bath is carried out under
5 to 50 A/dm2 of current density at 30 to 55C of electrol~e t~erature.
Reflowing, that is flow melting of elect~odeposited metallic
tin after nickel and tin plating, is also indispensible in order
;~ to form numerous nodules of metallic tin on the iron-tin-nickel
alloy layer which is an inventive feature in the present
invention.
The known method in which a temperature above the melting
point of tin is maintained for a short time by resistance neating
and induction heating can be used for reflowing of the tin and
nickel plated steel sheet in the presen~ invention.
It is suitable in the present invention that the tin and
~; nlc~el plated steel sheet is heated from 235 to 350C for 0.5 ~o
3 seconds and then immediately quenched into water.
Reflowing at a higher temperature for a lon~er time is not
desirable because of the poor welda~ility caused by the change
:
of a large part of plated metallic tin to iron-tin-nickel alloy,
particularly in the case of a lower amount of plated metallic
tin. Furthermore, reflowing at lower temperature for a short
r 1 4
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~ .~79(~1
time is not also desirable because of the poor corrosion
resistance after lacquering caused by insufficient formation of
the iron-tin-nickel alloy layer, particularly in the case of a
higher amount of plated metallic tin.
After reflowing, the tin and nickel plated steel sheet
according to the present invention is cathodically treated in a
known electrolyte such as a sodium dichromate solution which is
used for conventional post-treatment of an electrotinplated, or a
chromic~`acid solution containing a small amount of sulfuric acid,
fluoric acid, fluoboric acid, fluosilicic acid, an alkali salt
thereof, and a combination thereof which is used for the
production of conventional tin free steel having a upper layer of
hydrated chromium oxide and a lower layer of metallic chromium,
in order to ensure excellent characteristics in lacquer adhesion,
corrosion resistance before Qr after lacquering. For instance,
the tin and nickel plated steel sheet according to the present
invention is cathodically treated in 20 to 100 g/l of a
dichromate of an alkali metal or ammonium or chromic acid
;,
solution containing 0.01 to 5% of sulfuric acid, fluoric acid,
fluoboric acid, fluosilicic acid, an alkali metal salt thereof,
or a combination thereof based on the amount of chromic acid
under 5 to 40 A/dm2 of a eurrent density for 0.1 to 5 seconds of
a treating time at 30 to 70C of electrolyte temperature.
The amount of total chromium in the film formed on the tin
and~nickel plated steel sheet by cathodic treatment in dichromate
or chromic acid solution described above should be limited to 3
to~20~mg/m2, preferably 5 to 15 mg~ . If the amount of total
chromium is below 3 mg~m2, the excellent corrosion resistance
bsfore or after lacguering is not obtained, althou~h the
weldability does not change.
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At above 20 mg/m2 of total chromium in the film formed by
cathodic treatment in dichromate or chromic acid solution, the
current-range for sound welding becomes narrow because of the
formation of thic~er chromium oxide having high electric
resistance by the dehydration of the formed hydrated chromium
oxide or the oxidation of metallic chromium during heating for
lacquer curing.
In the present invention, the presence of hvdrated chromium
oxide is indispensable in order to obtain the excellent corrosion
resistance before or after lacquerin5. Moreover, the presence of
metallic chromium is desirable for the improvement of filiform
corrosion resistance after lacquering. Thèrefore, in the case
: ~
- w~ere the film formed by chromate treatment consists of an u??er
iayer of hydrated chromium oxide and a lower layer of metallic
chromium, the amount of metallic chromium should be limited to 2
to 17 mg/m and the amount of hydrated chromium oxide should be
limited to 3 to 18 mg/m as chromium~
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The present invention is illustrated by the rollowing
examples.
In Example 1 to Example 4 and comparative Example 1 to
` Comparative Example 4, a cold rolled stèeI sheet having a
- thickness of 0.2 mm was basically treated by the following
process after electroiytically degreasing in a solution of 70 g/l
of sodium hydroxide, water rinsing, pickling by an immersion into
:, .
100 g/l of sulfuric acid, and then water rinsing.
Anodic treatment in an alkaline electrolyte ~ water rinsing
-~ nickel plating , water rinsing ~ tinplating . water rinsing
drying ~ reflowing . ~uenching I chromate treatment
:
water rinsing ~ drying.
In Comparative Example 1, anodic treatment in an alXaline
~ . .
electrolyte was omitted in the present scheme as described above.
In Comparative Example 2, reflowing after tinplating was omitted.
In~Comparative Example 3, the anodically treated steel sheet was
plated with nickel above the upper limited amount in the present
:
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invention. In Comparative Example 4, the nickel plated steel
sheet was plated with tin below the lower limited a~ount in the
present invention.
In Example l to Example 4 and Comparative Example 1 to
comparative Example 4, a watts bath containing 250 g/l of
NiSO4-6H2O, 30 g/l of NiCl2 6H2O, and 40 g/l of H3BO3 or sulfamic
acid bath containing 350 q/l of nickel sulfamate and 40 g~l of
H3BO3 was ~sed for nickel plating. Furthermore, a Ferrostan bath
containing 60 g/l of SnSO4, 3~ g/l of phenolsulfonic acid and 5
g/1 of ethoxylated a-naphthol sulfonic acid or alkaline bath
containing 80 g/l of Na2SnO3 and 15 g/l of NaOH was used for
tinplating after nickel plating.
The tin and nickel plated steel sheet was immediately
reflowing
quenched in water after ~ ~or~, and the temperature of the tin
and nickel plated steel sheet dropped to 280C over 1.6 seconds
except in Comparative Example 2. The other conditions in every
Example are shown in the attached table.
:
The weldability, and corrosion resistance after lacquering
of the tin and nickel plated steel sheet in~the above described
Examples and Comparative Examples were evaluated by the following
te;stinq methods after the measurement of the amounts nickel, tin,
metalIic chromium, and chromium in a hydrated chromium oxide by
~:,
~ the~fluorescent X-ray methodj and the results were shown in the
:~ attached TabIe.
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(1) Weldability 1~ 7 9~
The weldability was evaluated by electric contact resistance
for the reason already described.
At first, the sample plated on both sides was cut to a size
of 20mm x lOOmm after heating at 210C for 20 minutes. The
electric contact resistance was calculated from the change of
voltage in a pair of copper disk electrodes tdiameter: 65mm,
thickness: 2mm) to which 5 amperes of direct current were
supplied'~and SOkg of load was added, when two sample pieces were
inserted between a pair of the copper disk electrodes rotating
at.5m/min.
~ (2) Filiform corrosion resistance ~after lacquering (Test l)
-~ The sample was baked at 200C for 10 minutes after coating with
;- ~ 75 mg/dm2 of a vinyl organosol. The coated-sample was immersed
into 3%~sodium chloride solution for 1 hour and then was left in
a chamber having 85% of relative humidity at 45C for lO days
àter the~surface of the coated samplé was cross-hatched by a
razor~and ~then expanded for Smm by an ~richsen~tester.
resùIt~of filiform~rusting spread from the scratched
part~of~the coated sample was divided i~nto 5 ranks, namely,
excellent,~ goad, f~air, poor,~and bad.
3~ Underoutting corrosion resistance after lacquering (Test
The sample was baked at 210C for 10 minutes after coating
~-w~l-;th `6~5~mg~m2~of~an epoxy-phenolic type of lacquer. The coated
immersed into the deaerated solution containing 1.5
-id -nd l.S~ of sodium chlorlde for lS days at 37C
fber~thé~sùrfaae of the coated sample was cross-hatched by a
zo~r.~
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The result of corrosion in the scratched part of the coated
sample was divided into S ranks, namely, excellent, good, fair,
poor, and bad.
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