Sn-based multilayer coated steel strip having improved corrosion resistance, weldability and lacquerability

The Sn based multilayer coated steel strip for container-use is improved by the provision of a novel underlying coating of ternary Fe-Ni-P alloy which, mainly because of the Ni component, ensures a uniform deposition of the Sn layer, and mainly because of the P and Fe contents ensure a satisfactory remaining amount of free Sn for improving the weldability. The steel strip has a thin Sn plated layer and a chromate coating layer on the underlying coating.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an Sn-based multilayer coated steel strip 
having improved electric-resistance weldability and improved 
corrosion-resistance for use in general-purpose food cans and beverage 
cans. The present invention also relates to a method for producing the 
Sn-based multilayer coated steel strip. 
2. Description of the Related Arts 
Recently, various manufacturing methods and design of the beverage cans and 
food cans have been increasingly developed. These container materials for 
such cans should be inexpensive and exhibit superior characteristics. 
The electric resistance welding process, e.g., Soudronic welding process, 
is widely employed in can manufacturing, because of advantages such as a 
high material yield, and a bonding strength high enough that leakages due 
to a bonding failure are kept to an extremely low level, and cans of 
various designs can be produced. Cans to be produced by the above welding 
process were heretofore made from Sn-plated steel sheet having an Sn 
plated amount of #10 (amount of plated Sn=1.12 g/m.sup.2) or more 
preferably #25 (amount of plated Sn=2.8 g/m.sup.2) or more. Nevertheless, 
a serious disadvantage of the Sn-plated steel sheet is that, due to 
increased tin prices, such a steel sheet has become expensive. Various 
attempts have been made to decrease the amount of plated Sn and thus 
attain a cost reduction, but the decrease in the amount of plated Sn cause 
a problem of degradation of the corrosion resistance and weldability. 
Recently, multilayer coated materials for container use have been 
developed, as disclosed in Japanese Unexamined Patent Publication Nos. 
57-23,091, 57-200,592 and 57-110,685, as alternative materials to the Sn 
plated steel sheet. The production methods of these multilayer coated 
materials utilize various combinations of surface treatments of the steel 
sheet, i.e., Ni-plating, Sn-plating with a thin deposition amount, 
alloying-diffusion treatment of Ni-Sn (heating-and melting-treatment), and 
chromate treatment. The steel sheets so produced have a dual coating. In 
these steel sheets, due to the superimposing effects of the dual coating, 
the number of pinholes is reduced, and due to a dense formation of an 
Ni-Sn alloy layer, the ATC (alloy tin couple) value is lessened, and hence 
the corrosion resistance is enhanced. In these steel sheets, particularly 
when the underlying Ni plating layer is formed, the formation of an 
alloying layer of Fe and Sn (FeSn.sub.2 alloying layer), which occurs 
during the high temperature-heating step of the can manufacture involving 
welding or at the high temperature-sterlization step subsequent to filling 
of the can, is suppressed and the weldability and the appearance of the 
welded parts are improved. 
When the known steel sheets for container use are considered in detail, it 
cannot be necessarily concluded that the requisite properties are ensured. 
Referring to FIG. 1, the dissolution speeds of Sn of the various Sn plated 
layers in the model corrosive liquid are shown. 
The model corrosive liquid was a solution containing 1.5% of citric acid 
and 1.5% of sodium chloride. The dissolution test was carried out under 
the measurement condition of a temperature of 27.degree. C. and an N.sub.2 
atmosphere. 
The test samples had the following coating structures. 
.cndot. . . . Undercoating: (Fe-18% Ni-1.7% P) alloy plating (160 
mg/m.sup.2).fwdarw.Sn plating (780 mg/m.sup.2).fwdarw.heating and melting 
treatment.fwdarw.chromate treatment (10 mg/m.sup.2). 
. . . Undercoating: (Fe-20% Ni) alloy plating (200 mg/m.sup.2).fwdarw.Sn 
plating (800 mg/m.sup.2).fwdarw.heating and melting 
treatment.fwdarw.chromate treatment (9 mg/m.sup.2) 
.DELTA. . . . Undercoating: Ni plating (25 mg/m.sup.2).fwdarw.Sn plating 
(800 mg/m.sup.2).fwdarw.chromate treatment (8 mg/m.sup.2) 
.quadrature. . . . Undercoating: (Fe-10% Ni) diffusion coating layer (Ni 
plating at 50 mg/m.sup.2 followed by diffusion treatment).fwdarw.heating 
and melting treatment.fwdarw.chromate treatment (8 mg/m.sup.2) 
x . . . Sn plating (850 mg/m.sup.2).fwdarw.heating and melting 
treatment.fwdarw.chromate treatment (9 mg/m.sup.2) 
. . . Undercoating: (Ni-16% P) alloy plating (60 mg/m.sup.2).fwdarw.Sn 
plating (850 mg/m.sup.2).fwdarw.chromate treatment 
As is understood from the o, .DELTA., and .quadrature. curves, when the 
dual layer plated steel sheets comprising the Ni undercoating plating and 
the Sn plating are exposed to corrosive environments, the dissolution 
speed of Sn lessens at the initial corrosion stage, and hence, the initial 
corrosion resistance of these steel sheets is excellent. Nevertheless, 
when they are exposed to a corrosive environment over a long period of 
time, the Sn is consumed and the alloy layer may be exposed. Under such 
circumstances, no matter how dense the alloy layer, it is not free of 
pinholes, so that a local cell is formed and the corrosion is promoted. In 
the local cell, the Ni-Sn alloy layer is electric potentially extremely 
noble or cathodic relative to the steel base, with the result that the 
parts of the steel base exposed by the pinholes are preferentially 
dissolved. As a result, the corrosion resistance is degraded and, 
occasionally, piercing corrosion occurs. The piercing corrosion may occur 
because the exposed alloy layer or base steel is exposed due to flaws 
formed during the can manufacture. In this case, the base steel dissolves 
and the corrosion resistance is degraded. 
Regarding the welding procedure, because the speed of this procedure has 
been greatly increased, recently a higher weldability has become 
necessary. The amount of non-alloyed Sn (free Sn) is decisive when 
considering the weldability, and therefore, it is essential to suppress 
the reactions for the alloy formation occurring during the lacquer paint 
coating, thereby increasing the residual amount of free Sn. The underlying 
Ni plating of the current steel sheets used for containers is effective to 
a certain degree in suppressing the alloy formation, but because of the 
high diffusion speed of Ni and Sn, it is difficult to ensure a sufficient 
amount of free Sn is available for improving the weldability. 
Particularly, when the deposition amount of Sn is small, the excellent 
weldability needed to attain a high welding speed is not necessarily 
obtained by the underlying Ni plating. 
Note, cans having easy-to-open ends (EOE) do not require cutting and can be 
easily opened anywhere. The EOE cans are used for all beverage cans and 
will probably be used for all food cans in the future. Al sheets provide a 
good end openable property and are widely used for the EOE materials. 
Surface treated steel sheets (tin plate) are used for food cans to hold 
foods containing sodium chloride, for example, tomato juice, for which Al 
cannot be used because it is insufficiently corrosion resistant. Recently, 
however, the materials of steel sheets and the designs for can ends have 
been improved, and tin plates having as good an openable property as the 
Al sheet have been produced for the ends of EOE cans. New materials, which 
will make it possible to reduce costs, are now required. 
Not only a good weldability but also good lacquerability and corrosion 
resistance after baking are needed for the materials used for welded cans. 
The materials used for the ends of EOE cans are subjected to score 
forming, i.e., the formation on the surface of an end, of a V-notch which 
facilitates can opening and allows the formation of a satisfactory opening 
for removing the content of the can therethrough. These materials are 
further subjected to bulging and the drawing of a tab, which acts as the 
starting point of tearing and staking, i.e., rivetting, for fixing the 
tab. Since the bulging, drawing, and rivetting are severe working, the 
steel sheet must have a good formability. In addition, the following 
properties are required for the surface coating layers. 
A. No cracks formed on the surface coating layer due to rivetting or 
scoring, and even if cracks are formed, they do not reach the base steel. 
B. The lacquerability of the worked parts is not degraded. 
Regarding ends other than the ends of EOE cans, and the can drums, the 
materials are subjected to severe working, such as winding-fastening by 
winding or bending, and thus the materials used must satisfy the same 
properties as described above.

SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a material for use as a 
welded can, which is inexpensive and can replace the conventional tin 
plates having a high Sn deposition amount and which exhibits an improved 
weldability, corrosion resistance, and lacquer adherence property. 
Therefore, in accordance with the present invention, there is provided an 
Sn-based multilayer coated steel strip, characterized by having an Fe-Ni-P 
based, underlying coating layer, an Sn plated layer on the underlying 
coating layer, and a chromate coating layer on the Sn plated layer. 
DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The steel strip having the coating layers according to the present 
invention is used for welded cans which are subjected to lacquering and 
then an electric welding resistance process during manufacture, or for 
ends of EOE cans which are subjected to severe working during manufacture. 
The Fe-Ni-P based, underlying coating layer is described in comparison with 
the Ni or Ni-P underlying coating layer. 
(a) Ni underlying coating layer 
When an Ni-based steel sheet is subjected to the Ni-based undercoating 
treatment, such as formation of an Ni or Ni-Fe alloy, the electrolytic 
deposition of the Sn layer on the underlying layer is improved, and in 
addition, a uniform and dense layer alloy of Ni and Sn is formed. As a 
result, the pinholes are lessened even when only a small plating amount of 
Sn is used. Accordingly, a reduction in the dissolution speed of Sn, which 
will improve the corrosion resistance, can be expected. Also, when a steel 
sheet is subjected to lacquer baking, and thus heated at a temperature of 
from 160.degree. to 220.degree. C. for approximately 20 to 60 minutes, a 
considerable amount of the free Sn is expected to remain. 
But, since the speed of diffusion between metallic Ni and Sn is high, the 
amount of free Sn remaining is not necessarily satisfactory. 
(b) Fe-P or Ni-P underlying coating layer 
The undercoating treatment by Fe-P or Ni-P is extremely effective for 
causing the free Sn to remain after the lacquer baking treatment, but is 
not satisfactory for providing a uniform coating of the Sn plated layer. 
Also, the Sn plated layer has numerous pinholes. 
(c) Fe-Ni-P underlying coating layer 
The Sn based multilayer coated steel strip for welded-container-use is 
improved by the provision of a novel underlying coating of ternary Fe-Ni-P 
alloy which, mainly because of the Ni component, ensures a uniform 
deposition of the Sn layer, and mainly because of the P and Fe contents 
ensure a satisfactory remaining amount of free Sn for improving the 
weldability. This layer attains the same uniform electrolytic deposition 
of Sn as the Ni underlying coating layer (a), and in addition, suppresses 
the reaction between the P-based underlying coating layer (b) and the Sn 
plated layer, as does the P-based underlying coating layer (b). The 
Fe-Ni-P based underlying coating layer is effective for providing a 
uniform electrolytic deposition of the Sn plated layer and for reducing 
the number of pinholes by the formation of a dense alloy layer, even when 
the amount of Sn deposited is extremely small. 
The effects of the Fe-Ni-P based underlying coating layer for suppressing 
the diffusion reaction between the same and the Sn-plated layer during the 
lacquer baking, and hence for causing the free Sn to remain, are apparent 
from FIG. 2. 
The amount of free Sn remaining was measured under the following 
conditions. 
Test pieces (Sn coating amount-690 mg/m.sup.2 Sn) were baked three times at 
205.degree. C. for 10 minutes and then the Sn coating was electrically 
dissolved in a 5% NaOH solution. The Sn amount was measured, before and 
after the dissolution, by fluorescent X ray analyzer. The difference in 
the Sn amount was taken as the amount of free Sn remaining. When the 
remaining amount of free Sn is high, and providing that there is an 
identical deposition amount of Sn, a steel sheet having such a large 
amount of free Sn remaining is advantageous, when compared with a steel 
sheet having only a small amount of Sn remaining, with regard to flaw 
generation in a coating layer and corrosion protection of defects in a 
coating layer. In addition, when such cans are exposed to a corrosive 
environment, the period in which the Sn plated layer is lost is prolonged, 
that is, the non-corrosion period is advantageously prolonged, when there 
is a large amount of free Sn remaining. 
The Fe-Ni-P underlying coating layer is also effective for preventing 
cracks occurring when the steel sheet is subjected to severe working, such 
as rivetting and scoring working. 
The preferred embodiments of the present invention are described 
hereinafter. 
The steel sheet to which the Sn-based multilayer coating according to the 
present invention is applied, may be a steel sheet which is generally and 
widely produced at present by the steel industry for use as a tin plate, a 
tin free steel (T.F.S), and the like. Such a steel sheet is produced by 
the steps of cold-rolling, annealing, and temper rolling, or occasionally 
second cold-rolling. The steel sheet so produced is referred to as a black 
plate. Various kinds of steel sheets processed as black plate can be used 
in the present invention. The steel sheet so processed is subjected to a 
pretreatment for surface activation, by alkali washing and then pickling. 
The Fe-Ni-P alloy is then plated on the surface-activated steel sheet. The 
plating bath for the Fe-Ni-P alloy may be one of a number of baths, such 
as sulfate bath, chloride bath, chloride-sulfate bath, cyanate bath, 
cirtric acid bath, and pyrophosphoric acid bath, but is preferably a 
sulfate bath, sulfate-chloride bath, or chloride bath in the light of bath 
operation and cost. An example of the sulfate bath contains ferrous 
sulfate, nickel sulfate, phosphorous acid, phosphoric acid, sodium 
acetate, and sodium sulfate. 
The Fe-Ni-P underlying coating layer preferably has a coating amount in the 
range of from 10 to 300 mg/m.sup.2 per one side of the sheet. When the 
coating amount is less than 10 mg/m.sup.2, the steel sheet (black plate) 
for plating is not satisfactorily uniformly covered by the Fe-Ni-P alloy 
layer, with the result that it becomes difficult to attain a uniform 
covering by the Sn plated layer and the desired suppression of alloy 
formation between the Ni and Sn so as to provide a large amount of 
remaining free Sn. If the coating amount is less than 10 mg/m.sup.2, the 
improvements in the corrosion resistance and weldability are attained only 
with difficulty. On the other hand, when the coating amount exceeds 300 
mg/m.sup.2, not only is there a saturation of the effects of the Fe-Ni-P 
based underlying coating layer but also the resultant hard characteristic 
becomes a source of cracks generation when the steel sheet is deformed, 
thereby degrading the corrosion resistance. The coating amount of Fe-Ni-P 
underlying coating layer is preferably in the range of from 30 to 250 
mg/m.sup.2. 
The Fe-Ni-P underlying coating layer has the following composition. The Ni 
content in the Fe-Ni-P underlying coating is preferably from 5 to 30%. 
When the Ni content is less then 5%, the effect of Ni for realizing a 
uniform Sn plated layer is practically nonexistent. On the other hand, 
when the Ni content exceeds 30%, the effect of Ni for realizing a uniform 
Sn plated layer tends to become saturated and the diffusion reaction 
between Ni and Sn is exceedingly enhanced during the lacquer baking to 
reduce the amount of remaining free Sn. In this case, the weldability and 
corrosion resistance are degraded. A preferable Ni content is from 10 to 
25%. 
When the P content is less than 0.1%, the effect of P for suppressing the 
diffusion reaction between the Sn and the Fe-Ni-P based underlying coating 
layer is small, and hence the amount of remaining free Sn also is small. 
In this case, the weldability and corrosion resistance are degraded. On 
the other hand, when the P content exceeds 10%, a uniform electrolytic 
deposition of the Sn plated layer is impeded and the formation of pinholes 
is increased, with the result that the corrosion resistance is greatly 
degraded. A preferred P content is from 1 to 5%. 
The Fe-Ni-P based underlying coating layer may contain, as unavoidable 
impurities, Co, Sn, and the like, which do not impede the effects of Fe, 
Ni, and P. 
When the Fe-Ni-P based underlying coating layer is formed, then the layer 
is rinsed with water and is subjected, directly or after activation by 
pickling, to the overlying coating by Sn. The Sn plating method is not 
limited with regard to the procedure thereof and the electrolytic treating 
conditions. Any of the ferrostan baths or halogen baths which are used at 
present for the production of tin plates, or any other electroplating 
bath, may be used. Since the Fe-Ni-P based underlying coating layer will 
realize, even at a small plating amount of Sn, the formation of a uniform 
and dense Ni-Fe-Sn-P alloy layer, an ensured free Sn coating layer, and a 
uniform electrolytic deposition of Sn coating, the deposition amount of Sn 
can be small, preferably not more than 2500 mg/m.sup.2, more preferably 
not more than 1500 mg/m.sup.2. If the deposition amount of Sn is 
exceedingly small, only a small amount of the free Sn remains when a steel 
sheet is subjected to heating during the process of manufacturing cans. In 
this case, the corrosion protection of defective plating parts by the Sn 
plated layer is not satisfactory. In addition, the Sn plated layer is 
converted to a virtually alloyed layer containing Ni, Fe, Sn, and P, 
thereby lessening the amount of remaining free Sn. Further, in this case, 
the contact resistance of the Sn plated layer is high, thereby degrading 
the weldability. A preferred deposition amount of Sn is not less than 50 
mg/m.sup.2, more preferably not less than 100 mg/m.sup.2. 
The steel sheet having the Sn plated layer may be subjected to a heating 
and melting step (sometimes referred to as the melt treatment) as carried 
out in a conventional production step for producing a tin plate. This 
heating and melting treatment is particularly advantageous in the present 
invention, because the Sn under the melting state reacts, in a short 
period of time, with the Fe-Ni-P based alloy coating layer, and an 
extremely uniform and fine Ni-Fe-Sn-P alloy layer is formed, thereby 
extremely reducing the ATC value and greatly suppressing, by this 
Ni-Fe-Sn-P layer, the decrease in free Sn. This is particularly 
advantageous for the weldability and corrosion resistance. The decrease in 
the ATC value leads to a decrease in the dissolution speed of Sn in an 
environment corrosive to Sn as shown in the dot/dash curve of FIG. 1. This 
is advantageous in the light of under-lacquer corrosion when coated with 
paint. 
The uniform and dense Ni-Fe-Sn-P based alloy layer formed by the melt 
treatment contains, by weight percentage, from 2 to 20% of Ni, from 0.05 
to 5% of P, and from 20 to 50% of Fe, the balance being Sn. Within this 
composition range, the Ni-Fe-Sn-P based alloy layer exhibits the excellent 
properties described above. The present invention is not limited to 
formation of the uniform and dense Ni-Fe-Sn-P based alloy layer by means 
of Sn plating and then melt-treating, but also may be embodied in such a 
manner that this Ni-Fe-Sn-P based alloy layer is formed by electroplating 
and an Sn plated layer is formed on this alloy layer. 
The heating and melting treatment is carried out as follows. An Sn-plated 
steel sheet is rinsed with water and is subjected, directly or after 
application of an aqueous solution-flux, to heating at a temperature of 
from 240.degree. to 350.degree. C., preferably from 250.degree. to 
300.degree. C., to melt the Sn plated layer. The heating is carried out in 
air or a non-oxidizing atmosphere, e.g., N.sub.2 atmosphere. The aqueous 
solution-flux is usually an Sn plating bath in which the Sn concentration 
is reduced compared with that for electroplating. A steel sheet is 
immersed in this plating bath so that the aqueous solution in the bath is 
applied on the Sn-plated surface of the steel sheet, which is then heated 
to melt the plated Sn. This method of applying the flux, however, gives 
the steel sheet a blackish luster appearance, because the appearance of 
the steel sheet is influenced by the underlying Fe-Ni-P alloy coating 
layer. A white luster appearance is advantageously obtained when an 
Sn-plated steel sheet is immersed in city water or a dilute solution 
having a concentration one tenth or less that of the plating bath, and is 
then subjected to the melting treatment. Ni and P partly intrude into the 
steel when the heating and melting treatment or paint baking is carried 
out at a high temperature. The diffusion layer so formed does not impede 
the effects to be attained by the present invention. 
The Sn plated layer formed in the present invention may be a uniform layer 
or nonuniform Sn layer in the discontinuous distribution, which are 
inevitably formed due to the melt treatment. 
According to the present invention, the surface of the Sn-plated layer is 
subjected to a chromate treatment, so as to improve the paintability and 
the coating properties. The chromate treatment is outstandingly effective 
for improving the adhesion of paint for cans and for preventing the so 
called undercutting corrosion, according to which the content in the form 
of an aqueous solution permeates through the coating and promotes the 
corrosion at the interface between the plating surface and the lacquer. 
When the undercutting corrosion is prevented, the adhesion of the lacquer 
does not deteriorate and a good corrosion resistance is maintained for a 
long period of time. The chromate coating is extremely effective for 
preventing sulfide-staining, that is, the appearance of the steel sheet 
becomes blackish when the content is a sulfur-containing food, such as 
fish or live-stock products. The chromate coating is advantageous for 
lacquered cans but is disadvantageous for welding. The chromate coating 
herein indicates the single coating of hydrated chromium oxide, i.e., the 
chromate coating in the original meaning, and the dual coating consisting 
of underlying metallic Cr and overlying hydrated chromium oxide. The 
hydrated chromium oxide is electrically insulative, and hence exhibits a 
high electric resistance. The metallic chromium has a high electric 
resistance and a high melting point. Both hydrated chromium oxide and 
metallic chromium therefore degrade the weldability. A preferred amount of 
the deposition amount of the Cr-bearing material in terms of metallic Cr 
is in the range of from 5 to 50 mg/m.sup.2, in the light of corrosion 
resistance and weldability. A more preferred deposition amount of the 
Cr-bearing material is from 7.5 to 35 mg/m.sup.2. When the deposition 
amount of the chromium-bearing material is less than 5 mg/m.sup.2, the 
chromate coating is not very effective for improving the lacquer adhesion 
or for preventing the undercutting corrosion. On the other hand, when the 
deposition amount of the chromium-bearing material exceeds 50 mg/m.sup.2, 
the appearance degrades and the contact resistance becomes so high that it 
becomes necessary to enhance the welding current. In this case, expulsion 
and surface flash are liable to be generated and the welding conditions 
become restricted, which indicates that the weldability is degraded. 
The chromate treatment is carried out as follows. 
The aqueous solution containing a chromic acid, and Na, K or ammonium salt 
of various chromic acids is used for either the immersion treatment, 
spraying treatment, or electric cathodic treatment. The electric cathodic 
treatment is preferred, particularly when the aqueous solution used for 
this treatment contains CrO.sub.3, SO.sub.4 ions, and F ions including 
complex ions, or a mixture thereof. The concentration of CrO.sub.3 is in 
the range of from 20 to 100 g/l but is not specifically limited. When the 
total of anions added is from 1/300 to 1/25 times, preferably from 1/200 
to 1/50 times, the ion concentration of hexa valent-Cr, the optimum 
chromate coating is obtained. When the ion concentration of anions is less 
than 1/300 times the Cr ions, it is difficult to obtain a chromate film 
which is dense and uniform and exerts a favourable influence upon the 
painting characteristics. On the other hand, when the ion concentration of 
anions exceeds 1/25 times the Cr ions, the amount of anions trapped in the 
chromate layer being formed is so increased that the chromate coating 
properties are degraded. The temperature of the chromate treatment-bath is 
not specifically limited but is advantageously from 30.degree. to 
70.degree. C. in the light of suitable operation. The current density of 
the electric cathodic treatment is sufficient when in the range of from 5 
to 100 A/dm.sup.2. The treatment time is adjusted in combination with the 
above described treating conditions so as to attain a predetermined 
deposition amount of chromium bearing material. 
A preferred treating condition of the chromate treatment includes the 
following provisos: the solution contains CrO.sub.3 ; the concentrations 
of SO.sub.4.sup.-2 and F.sup.- are within the ranges as described above; 
the current density is in the range of from 50 to 100 A/dm.sup.2 ; and, 
the treatment is for a short period of time such as 0.2 second or less. As 
shown in FIG. 3, the metallic Cr layer is deposited on the Sn-plated layer 
at an amount of from 5 to 15 mg/m.sup.2, and the hydrated chromium oxide 
layer is formed on this Cr layer. Thus, a dual chromium layer is formed. 
The amount of hydrated chromium oxide layer is regulated by adjusting the 
immersion time of in the solution, in which a workpiece is subjected to 
chromate treatment and is then immersed for adjusting the amount of 
hydrated chromium oxide. Alternatively, the workpiece is immersed in a 
separate tank containing a CrO.sub.3.sup.- -anionic bath having a 
CrO.sub.3.sup.- concentration different from that of the bath for chromate 
treatment. In FIG. 3, the relationship between the electrolytic conditions 
for the chromate treatment and the deposition amount of chromium is shown. 
When the metallic chromium, as shown in FIG. 3, is deposited uniformly over 
the Sn-plated layer, the lacquerability is enhanced, particularly in the 
case of an Sn-plating followed by the melt treatment. In this regard, when 
a steel sheet is used for the container for an aqueous solution of organic 
acid, such as a citric acid, and hence is exposed to a corrosive 
environment, the corrosive aqueous solution intrudes through the lacquer 
layer, so that the corrosion of the metallic Sn becomes relatively 
serious. The Cr layer, which precipitates in the metallic form, 
advantageously suppresses the corrosive aqueous solution from reaching the 
metallic Sn surface. Provided that the deposition amount of the chromium 
bearing layer is within the range as described above, the ratio of 
metallic chromium layer to the chromium oxide layer is preferably within 
the range of from 0.2.ltoreq.chromium oxides/metallic chromium .ltoreq.3. 
When the amount of chromium oxides mainly compose of Cr.sup.+3 is smaller 
than the amount of metallic Cr, the lacquer adhesion becomes poor since 
the oxide chromium exhibits a poorer property for a uniform coating than 
does the metallic chromium. On the other hand, when the amount of chromium 
oxide is greater than the amount of metallic chromium, the anions and 
Cr.sup.+6 ions contained in the oxide chromium increase and are dissolved 
when exposed, after lacquering, to a high temperature, corrosive 
atmosphere, thereby making it easy to form minute swellings in the 
lacquer, i.e., so called blisters. A more preferred ratio of a chromium 
oxide to metallic chromium is from 0.5 to 2.5. 
At the melt treatment, a trace amount of metallic Ni diffuses to the 
surface of the Sn plated layer and precipitates there. The precipitated Ni 
advantageously suppresses the undercutting corrosion and greatly improves 
the lacquer adhesion on the chromate coating. The anions are preferably 
added to the chromate treating bath in the form of sulfuric acid, chromium 
sulfate, ammonium fluoride, and sodium fluoride. 
The surface treated steel strip according to the present invention as 
described above can be produced in the various continuous plating lines 
used at present for the production of tin plates, but these lines must be 
equipped with the Fe-Ni-P plating apparatus. Accordingly, the production 
of the surface treated steel strip according to the present invention is 
efficient. 
The Sn-based multilayer coating structure according to the present 
invention may be present on the steel sheet in any form, such as an entire 
coating of the steel sheet, which is the most general form, or a partial 
coating provided by a partial coating process. 
The present invention is hereinafter described by way of examples. 
The surfaces of the cold-rolled steel strips were cleaned and then 
subjected to the formation of underlying coating of ternary alloy of 
Fe-Ni-P. The electroplating conditions for forming the underlying coating 
of ternary Fe-Ni-P alloy were as given in (A). 
Subsequently, a predetermined amount of Sn-coating layer is formed on the 
underlying coating. Subsequently, rinsing with water or a melt treatment 
at 260.degree. C. for 5 seconds was carried out, and then the chromate 
treatment under the conditions (C) was carried out. After an application 
of oil, various tests were carried out for evaluating the properties of 
the multilayer coating, as described in .circle.A - .circle.F , below. 
______________________________________ 
(A) Treatment for underlying coating of Fe--Ni--P 
based alloy 
Composition of NiSO.sub.4.6H.sub.2 O 
75 g/l 
plating bath NiCl.sub.2.6H.sub.2 O 
140 g/l 
FeSO.sub.4.7H.sub.2 O 
64 g/l 
H.sub.3 PO.sub.3 
44 g/l 
H.sub.3 BO.sub.3 
45 g/l 
Bath temperature 50.degree. C. 
Current density 10 A/dm.sup.2 
(B) Treatment for Sn plating 
Composition of 
plating bath 
tin sulfate 20.about.30 g/l 
phenol sulfonic acid 
25.about.35 g/l 
(65% solution) 
Bath temperature 50.degree. C. 
Current density 15 A/dm.sup.2 
(C) Chromate treatment (Electric cathodic method) 
Treatment (a): Bath composition 
Na.sub.2 Cr.sub.2 O.sub.7 - 25 g/l 
Bath temperature 60.degree. C. 
Current density 
5.about.8 A/dm.sup.2 
Time 2 seconds 
Treatment (b): Bath composition 
100 g/l CrO.sub.3 - 
0.6 g/l SO.sub.4.sup.-2 
Bath temperature 45.degree. C. 
Current density 
60.about.80 A/dm.sup.2 
Time 0.1 second 
______________________________________ 
.circle.A Uniform coating of plated Sn layer 
The plated test pieces 10 mm.times.10 mm in size were immersed in the test 
liquid which contained 0.2 mol of sodium carbonate and 0.005 mol of sodium 
chloride, and the pH of which was adjusted to pH 10 by an addition of 
sodium hydrogen carbonate. The test pieces were connected as an anode 
having a constant potential of 1.2 V relative to a standard calomel 
electrode. The constant anode potential was adjusted by a potentiostat. 
The test pieces were thus electrolyzed at the potential of 1.2 V. After 3 
minutes of electrolysis, the current was measured to evaluate the 
uniformity of the coating of the plated Sn layer. 
.circle.B Seam Weldability 
The cans were welded under the conditions of a lapping amount of 0.5 mm, 
welding applied force of 45 kg, and a welding speed of 420 cans/minute. 
During the welding, the welding current was varied to ascertain the 
minimum welding current for obtaining a satisfactory welding strength, and 
the range of welding current, in which such welding defects as splashing 
noticeably occur, as well as the circumstances of the generation of 
welding defects. The seam weldability was evaluated by considering the 
above collectively. 
.circle.C UCC Test (Undercut film corrosion test) 
An epoxyphenol resin lacquer (phenol enriched lacquer) used for can 
manufacture was applied on the tested surface of test pieces at a dry 
weight of 50 mg/dm.sup.2 per one side, and was then baked at 205.degree. 
C. for 10 minutes, and further, at 180.degree. C. for 20 minutes for 
postbaking. The lacquered surface was scratched with a knife. The test 
pieces were immersed in the corrosive liquid (1.5% citric acid and 1.5% 
sodium chloride) and held at 55.degree. C. for 4 days in an open ambient 
atmosphere. Tapes adhered to the scratched part and flat part were peeled 
from these parts. The peeling states of the lacquer on the flat and 
scratched parts as well as the pitting corrosion of the scratched parts 
were investigated. 
.circle.D Sulfide stain test 
The test pieces were lacquered as in .circle.C and were subjected to the 
1 t bending (bent around 1 mm thick sheet). Water-boiled mackerel 
available in the market was uniformalized by a mixer. The test pieces were 
immersed in the uniformalized mackerel and treated in a retort at 
115.degree. C. for 90 minutes. After the treatment in the retort, the 
sulfide staining of the test pieces were evaluated at the bent and flat 
parts. 
.circle.E Filiform corrosion test 
The test pieces were lacquered and scratched as in .circle.C and then 
subjected to bulging to 4 mm at the central portion thereof. The samples 
were then sprayed with 5% NaCl for 3 hours by a sodium chloride-water 
spraying machine. 
The test pieces were rinsed with water and then put in a thermostat testor, 
in which the constant temperature in terms of a dry bulb temperature was 
38.degree. C. and a wet bulb temperature was 35.5.degree. C. and the 
constant humidity in terms of relative humidity was 85%. The test pieces 
were allowed to stand in the thermostat testor for 60 days. The scratched 
parts of the lacquer were observed with the naked eye to detect and 
evaluate the generation of filiform corrosion. 
.circle. F Evaluation of properties of material to be worked as EOE 
An epoxy phenol resin-based lacquer was applied on the samples in an amount 
of 45 mg/dm.sup.2 so as to provide corrosion protection of the inner 
surface of the samples which were later worked to form EOEs. The 
occurrence of cracks on the rivetted, and scored (75 .mu.m of the score 
remaining thickness) parts, as well as the counter sunk parts, was 
observed. 
The test under the same conditions as in .circle.C -U.C.C. test- was 
carried out. 
The occurrence of corrosion under the lacquer was observed and evaluated. 
The sulfide staining was observed and evaluated under the same conditions 
as in .circle.D -sulfide stain test-. 
The test pieces, which were worked as EOEs, were subjected to a retort 
treatment while immersed in a 5% NaCl solution at 125.degree. C. for 1 
hour. An adhesive tape was attached to and then peeled from the lacquer of 
test pieces to evaluate the peeling of lacquer. 
The results of observation and evaluation as above were collectively 
evaluated to determine the material properties after the EOE working. 
TABLE 1 
__________________________________________________________________________ 
Electrolytic chromate 
(Ni--Fe--P) Alloy treatment 
plating layer Sn Amount of Amount 
Plating Coating metallic 
of total 
amount 
Ni P amount 
Melting 
Treating 
chromium 
chromium 
Example 
(mg/m.sup.2) 
wt % 
wt % 
(mg/m.sup.2) 
treatment 
method 
(mg/m.sup.2) 
(mg/m.sup.2) 
__________________________________________________________________________ 
Example 1 
12 18 1.6 620 yes (b) 3 7.9 
Example 2 
110 22 2.1 780 yes (a) 2 7.8 
Example 3 
290 13 2.8 890 yes (b) 7 14.5 
Example 4 
50 5.3 1.3 750 yes (b) 9 12.0 
Example 5 
150 17 2.6 670 yes (b) 10 13.1 
Example 6 
210 28 1.9 810 yes (b) 4 15.0 
Example 7 
120 14 0.12 
820 yes (b) 7 12.0 
Example 8 
180 17 2.2 690 yes (b) 11 18.0 
Example 9 
160 16 5.0 730 yes (b) 12 21.0 
Example 10 
190 22 9.8 750 yes (b) 9 34.0 
Example 11 
200 8 1.6 55 yes (b) 8 17.0 
Example 12 
180 11 2.2 220 yes (b) 7 16.0 
Example 13 
210 13 2.4 560 yes (b) 9 21.0 
Example 14 
220 9 3.6 1120 yes (a) 2 5.4 
Example 15 
170 18 4.8 1780 yes (b) 8 18.0 
Example 16 
180 22 2.6 2480 yes (a) 1 6.1 
Example 17 
90 26 1.9 60 no (a) 2 5.5 
Example 18 
120 14 2.7 670 no (b) 8 14.0 
Example 19 
150 18 2.3 910 no (b) 9 18.0 
Example 20 
110 18 1.7 1480 no (a) 2.5 7.7 
Example 21 
190 21 4.8 2320 no (b) 7 16.0 
Example 22 
150 22 1.9 720 yes (a) 1.5 6.0 
Example 23 
140 24 2.3 740 yes (a) 2 7.3 
Example 24 
190 19 1.4 820 yes (a) 2 12.1 
Example 25 
220 18 2.1 820 yes (b) 4 5.5 
Example 26 
160 13 1.8 770 yes (b) 10 12.1 
Example 27 
120 12 2.3 630 yes (b) 19 31 
Example 28 
180 11 1.3 690 yes (b) 30 48 
Example 29 
40 10 2.0 690 yes (b) 9 15.0 
160 20 3.0 
Example 30 
180 15 2.4 890 yes (b) 11 19.0 
70 10 1.8 
__________________________________________________________________________ 
.circle.A 
Coating .circle.C UCC 
.circle.D Sulfide 
.circle.F 
uniformity evaluation test 
stain test 
.circle.E 
Evaluation 
of Sn .circle.B 
Cross Bending Filiform 
test for 
plated 
Seam cut Flat 
formed 
Flat 
corrosion 
EOE forming 
Example 
layer weldability 
parts 
parts 
parts 
parts 
test materials 
__________________________________________________________________________ 
Example 1 
.circle. 
.circleincircle. 
.circleincircle. 
.circleincircle. 
.circleincircle. 
.circleincircle. 
.circleincircle. 
.circleincircle. 
Example 2 
.circleincircle. 
.circleincircle. 
.circleincircle. 
.circleincircle. 
.circleincircle. 
.circleincircle. 
.circle. .about..circleincircle. 
.circle. 
Example 3 
.circleincircle. 
.circleincircle. 
.circleincircle. 
.circleincircle. 
.circleincircle. 
.circleincircle. 
.circleincircle. 
.circleincircle. 
Example 4 
.circleincircle. 
.circleincircle. 
.circleincircle. 
.circleincircle. 
.circleincircle. 
.circleincircle. 
.circleincircle. 
.circleincircle. 
Example 5 
.circleincircle. 
.circleincircle. 
.circleincircle. 
.circleincircle. 
.circleincircle. 
.circleincircle. 
.circleincircle. 
.circleincircle. 
Example 6 
.circleincircle. 
.circleincircle. 
.circleincircle. 
.circleincircle. 
.circleincircle. 
.circleincircle. 
.circleincircle. 
.circleincircle. 
Example 7 
.circleincircle. 
.circle. 
.circleincircle. 
.circleincircle. 
.circleincircle. 
.circleincircle. 
.circleincircle. 
.circleincircle. 
Example 8 
.circleincircle. 
.circleincircle. 
.circleincircle. 
.circleincircle. 
.circleincircle. 
.circleincircle. 
.circleincircle. 
.circleincircle. 
Example 9 
.circleincircle. 
.circleincircle. 
.circleincircle. 
.circleincircle. 
.circleincircle. 
.circleincircle. 
.circle. 
.circleincircle. 
Example 10 
.circle. 
.circleincircle. 
.circle. 
.circle. 
.circleincircle. 
.circleincircle. 
.circle. 
.circleincircle. 
Example 11 
.circle. 
.circle. 
.circleincircle. 
.circleincircle. 
.circleincircle. 
.circleincircle. 
.circle. 
.circleincircle. 
Example 12 
.circle. .about..circleincircle. 
.circle. 
.circleincircle. 
.circleincircle. 
.circleincircle. 
.circleincircle. 
.circleincircle. 
.circleincircle. 
Example 13 
.circleincircle. 
.circle. .about..circleincircle. 
.circleincircle. 
.circleincircle. 
.circleincircle. 
.circleincircle. 
.circleincircle. 
.circleincircle. 
Example 14 
.circleincircle. 
.circleincircle. 
.circleincircle. 
.circleincircle. 
.circle. 
.circleincircle. 
.circle. .about..circleincircle. 
.circleincircle. 
Example 15 
.circleincircle. 
.circleincircle. 
.circleincircle. 
.circleincircle. 
.circleincircle. 
.circleincircle. 
.circleincircle. 
.circleincircle. 
Example 16 
.circleincircle. 
.circleincircle. 
.circle. 
.circleincircle. 
.circle. 
.circleincircle. 
.circle. .about..circleincircle. 
.circleincircle. 
Example 17 
.circleincircle. 
.circle. 
.circleincircle. 
.circleincircle. 
.circle. 
.circleincircle. 
.circle. 
.circleincircle. 
Example 18 
.circleincircle. 
.circleincircle. 
.circleincircle. 
.circleincircle. 
.circleincircle. 
.circleincircle. 
.circle. 
.circleincircle. 
Example 19 
.circleincircle. 
.circleincircle. 
.circleincircle. 
.circleincircle. 
.circleincircle. 
.circleincircle. 
.circle. 
.circleincircle. 
Example 20 
.circleincircle. 
.circleincircle. 
.circle. 
.circleincircle. 
.circle. 
.circleincircle. 
.circleincircle. 
.circle. 
Example 21 
.circle. .about..circleincircle. 
.circleincircle. 
.circleincircle. 
.circleincircle. 
.circleincircle. 
.circleincircle. 
.circleincircle. 
.circleincircle. 
Example 22 
.circleincircle. 
.circleincircle. 
.circle. 
.circleincircle. 
.circle. 
.circleincircle. 
.circle. .about..circleincircle. 
.circleincircle. 
Example 23 
.circleincircle. 
.circleincircle. 
.circleincircle. 
.circleincircle. 
.circle. 
.circleincircle. 
.circle. .about..circleincircle. 
.circleincircle. 
Example 24 
.circleincircle. 
.circleincircle. 
.circleincircle. 
.circleincircle. 
.circle. 
.circleincircle. 
.circle. .about..circleincircle. 
.circleincircle. 
Example 25 
.circleincircle. 
.circleincircle. 
.circleincircle. 
.circleincircle. 
.circle. 
.circleincircle. 
.circleincircle. 
.circleincircle. 
Example 26 
.circleincircle. 
.circleincircle. 
.circleincircle. 
.circleincircle. 
.circleincircle. 
.circleincircle. 
.circleincircle. 
.circleincircle. 
Example 27 
.circleincircle. 
.circle. 
.circleincircle. 
.circleincircle. 
.circleincircle. 
.circleincircle. 
.circleincircle. 
.circleincircle. 
Example 28 
.circleincircle. 
.DELTA..about. .circle. 
.circleincircle. 
.circleincircle. 
.circleincircle. 
.circleincircle. 
.circleincircle. 
.circleincircle. 
Example 29 
.circleincircle. 
.circleincircle. 
.circleincircle. 
.circleincircle. 
.circleincircle. 
.circleincircle. 
.circleincircle. 
.circleincircle. 
Example 30 
.circleincircle. 
.circleincircle. 
.circleincircle. 
.circleincircle. 
.circleincircle. 
.circleincircle. 
.circleincircle. 
.circleincircle. 
__________________________________________________________________________ 
.circleincircle. - extremely good 
.circle. - relatively good 
.DELTA. - relatively poor 
x extremely poor 
TABLE 2 
__________________________________________________________________________ 
Electrolytic chromate 
(Ni--Fe--P) Alloy treatment 
plating layer 
Sn Amount of 
Amount 
(per one Coating metallic 
of total 
surface) amount chromium 
chromium 
Plating (per one (per one) 
(per one 
amount 
Ni P surface) 
Melting 
Treating 
surface) 
surface) 
(mg/m.sup.2) 
wt % 
wt % 
(mg/m.sup.2) 
treatment 
method 
(mg/m.sup.2) 
(mg/m.sup.2) 
__________________________________________________________________________ 
Comparative 1 
200 16 12 870 yes (a) 0 6.8 
Comparative 2 
180 14 0.05 
780 no (a) 0 8.8 
Comparative 3 
220 3.6 3.5 770 no (b) 3 12.0 
Comparative 4 
240 48 2.7 860 yes (b) 7 18.5 
Comparative 5 
800 15 6.5 900 yes (b) 6 14.0 
Comparative 6 
18 17 3.1 890 no (b) 4 11.0 
Comparative 7 
120 14 1.5 270 no (b) 5 12.0 
Comparative 8 
200 15 -- 650 yes (b) 6 18.0 
Comparative 9 
100 -- 2.0 680 no (b) 5 14.0 
Comparative 10 
-- -- -- 770 no (a) 0 7.4 
__________________________________________________________________________ 
.circle.A 
Coating .circle.C UCC 
.circle.D Sulfide 
.circle.F 
uniformity evaluation test 
stain test 
.circle.E 
Evaluation 
of Sn .circle.B 
Cross Bending 
Filiform 
test for 
plated 
Seam cut Flat 
Flat 
formed 
corrosion 
EOE forming 
layer weldability 
parts 
parts 
parts 
parts 
test materials 
__________________________________________________________________________ 
Comparative 1 
x .circleincircle. 
x x .circle. 
.DELTA. 
x x 
Comparative 2 
.circleincircle. 
.DELTA. 
.DELTA. 
.DELTA. 
.circle. 
.DELTA. 
.DELTA. 
x 
Comparative 3 
x .circle. 
.DELTA. 
.DELTA. 
.circle. 
.DELTA. 
.DELTA. 
x 
Comparative 4 
.circleincircle. 
x .DELTA. 
.DELTA. 
.circleincircle. 
.circle. 
.circle. 
x 
Comparative 5 
x .circleincircle. 
.DELTA. 
.DELTA. 
.circle. 
.circle. 
.DELTA. 
x 
Comparative 6 
.DELTA. 
.DELTA. 
.DELTA. 
.DELTA. 
.circleincircle. 
.circle. 
.DELTA. 
x 
Comparative 7 
x x x x .circle. 
.DELTA. 
x x 
Comparative 8 
.circleincircle. 
.DELTA. 
.circleincircle. 
.circleincircle. 
.circleincircle. 
.circle. 
.circle. 
x 
Comparative 9 
x .circleincircle. 
.DELTA. 
.DELTA. 
.circle. 
.circle. 
.DELTA. 
x 
Comparative 10 
x x x x x x x x 
__________________________________________________________________________