Technical Field:
[0001] This invention relates to a surface-treated steel sheet, an organic resin-coated
surface-treated steel sheet, a metal container, and a process for producing the surface-treated
steel sheet. More specifically, the invention relates to a surface-treated steel sheet
featuring excellent adhesion to the organic resin coating and excellent resistance
against the elution, and to a process for producing the same.
Background Art:
[0002] The treatment with the chromate has heretofore been known as a treatment for improving
close adhesion between a steel sheet and an organic coating in the field of domestic
appliances, building materials, vehicles, aircraft, containers and the like, and has,
therefore, been widely employed owing to its excellent corrosion resistance and close
adhesion.
[0003] The treatment with the chromate pertains to either the one of the type of containing
hexavalent chromium in the coating and the one of the type of containing no hexavalent
chromium in the coating. In recent years, however, from the standpoint of environment
and health in the working environment, it is a growing trend to ban the use of any
starting material that contains hexavalent chromium irrespective of the state of the
final products.
[0004] Materials for producing metal containers such as cans and lids are treated with the
chromate of the type that does not leave hexavalent chromium in the final products,
as a matter of course. Besides, the materials are, usually, coated with an organic
resin. For instance, the tin-plated steel sheet is cathodically electrolyzed in an
aqueous solution of sodium bichromate, a steel sheet is cathodically electrolyzed
in an aqueous solution of a fluoride-containing chromium anhydrite, and an aluminum
alloy is treated with a chromic phosphate, followed, further, by the coating with
an organic resin.
[0005] Metal containers such as cans and can lids are, in many cases, subjected to the retort
treatment with hot water in order to sterilize the contents. Therefore, the materials
are subjected to a severe environment arousing a problem of decrease in the adhesion
between the resin coating and the surface of the metal. To solve the problem, therefore,
various studies were so far made. At present, in order to improve close adhesion during
the treatment with hot water, the tin-plated sheet and the steel sheet electrolytically
treated with chromate that are used as materials for producing cans, are washed with
warm water or hot water in the step of finally treating the surfaces. Namely, anions
such as sulfuric acid ions and fluorine ions in the treated coating are controlled
to obtain a metal surface that features excellent adhesion to the organic coating
(non-patent document 1, patent document 2).
[0006] As the chromium-free surface treatment studied in recent years in connection with
the steel sheets, there has been proposed a dip treatment using a treating liquid
that contains Zr (zirconium) or Ti (titanium) (patent document 1). However, the steel
sheet treated for its surface by being dipped in the Zr- or T-containing solution
has poor corrosion resistance in the coating thereof. Besides, the rate of depositing
the coating is small as compared to the electrolytic chromate-treated steel sheet
(TFS) that has heretofore been used as a material for producing cans. Therefore, the
productivity is very poor. As a high-speed treatment to substitute for the dip treatment,
therefore, there have been proposed a Zr and/or Ti treatment and/or an Al treatment
by applying the cathodic electrolysis. It has been known that both of these treatments
are capable of forming an oxygen compound of a metal on the surface of the base material
at high speeds (patent documents 3, 4 and 5).
[0007] As a method of improving close adhesion of a coating of an oxygen compound of a metal
to an organic resin layer, further, there has been proposed a technology concerned
to a method of producing a steel sheet for containers having a conversion-treated
film that contains metal Zr in an amount of 1 to 100 mg/m
2 and F in an amount of not more than 0.1 mg/m
2 by forming, on the base material, a coating of an oxygen compound of a metal that
contains an oxygen compound of Zr, and washing the surface of the coating of the metal
oxygen compound with hot water of not lower than 80°C (patent document 6).
Prior Art Documents:
Patent Documents:
Non-Patent Document:
Outline of the Invention:
[0010] Problems that the Invention is to Solve:
[0011] If it is attempted to improve the corrosion resistance of a metallic base material
without forming a metal-plated layer thereon but, instead, directly forming a coating
of an oxygen compound of a metal comprising chiefly an oxygen compound such as of
Zr, Al or Ti on the surface of the metallic base material, it becomes necessary to
increase the thickness of the coating (amount of coating) as compared to that of the
metal-plated layer. Specifically, in the use for seamless cans that are worked to
a large degree, the underlying iron may be exposed due to the working or the adhesion
to the organic resin may decrease. Therefore, it has been urged to maintain corrosion
resistance by increasing the amount of the coating and, at the same time, to improve
close adhesion to the organic resin.
[0012] In addition to the above items related to close adhesion, there still remains another
problem which the present invention is to solve, i.e., to prevent the components constituting
the metal container from eluting out into the content. For the metal containers, it
is very important to maintain the quality of the content and, therefore, special attention
must be paid to the components that may elute out into the content from the metal
container. In general, elution of metallic components of the container can be represented
by elution of iron due to corrosion and elution of anions such as sulfuric acid ions
and fluorine ions in the coating. Therefore, attention must be paid to the amount
of the coating in the metal surface treatment, surface state, and adhesive force to
the organic resin coating such as film or coating, in addition to paying attention
to the pH of the content and the sterilizing conditions.
[0013] A patent document 2 is disclosing an example of improving the close adhesion by washing
the surface of the coating of an oxygen compound of a metal on the metal-plated layer
with hot water. However, in case it is required to form the coating in a large amount
as described above, it was discovered that the washing with hot water, that has heretofore
been used for the electrolytic chromate-treated steel sheets, is not enough for attaining
the surface properties and for suppressing the elution as desired. Therefore, if the
electrolytic chromate-treatment line is applied to the formation of the coating of
the oxygen compound of a metal, the washing must be conducted for further longer periods
of time than the conventional methods. It was, therefore, learned that there still
exist many problems such as an increase in the load in connection with the production
and an increase in the amount of energy consumption, imposing limitation on the speed
for operating the surface-treatment line, requiring an increased number of the tanks
for washing, and increased amount of hot water that must be used.
[0014] The present invention, therefore, was contrived in view of such circumstances, and
its object is to provide a surface-treated steel sheet which, when an organic resin
layer is formed on the surfaces thereof, features excellent adhesion to the organic
resin layer and excellent corrosion resistance and an organic resin-coated surface-treated
steel sheet, to provide an organic resin-coated metal container featuring excellent
adhesion to the resin on the inner and outer surfaces of the can and excellent resistance
against the elution of fluorine, and to provide a process for producing the surface-treated
steel sheet.
Means for Solving the Problems:
[0015] According to the present invention, there is provided a surface-treated steel sheet
having a steel sheet and a surface-treating layer on at least one surface of the steel
sheet, the surface-treating layer including zirconium, oxygen and fluorine, wherein
said surface-treating layer contains an element of the Group II on the surface side
thereof.
[0016] In the surface-treated steel sheet of the present invention, it is desired that:
- (1) The element of the Group II is present as a fluorine compound;
- (2) The element of the Group II is at least either calcium or magnesium;
- (3) A molar ratio AE/Zr of the element (AE) of the Group II and zirconium (Zr) in
the surface-treating layer is not less than 0.2; and
- (4) The thickness of zirconium in terms of weight in the surface-treating layer is
100 to 200 mg/m2.
[0017] According to the invention, further, there is provided an organic resin-coated surface-treated
steel sheet obtained by forming an organic resin coating on the surface-treated steel
sheet.
[0018] According to the invention, further, there is provided a metal container or a can
lid made from the organic resin-coated surface-treated steel sheet.
[0019] According to the invention, further, there is provided a process for producing a
surface-treated steel sheet having a steel sheet and a surface-treating layer on at
least one surface of the steel sheet, the surface-treating layer including zirconium,
oxygen and fluorine, said process comprising the steps of:
forming a coating by cathodically electrolyzing the steel sheet in an aqueous solution
that contains Zr ions and F ions; and
thereafter, adjusting the surfaces by conducting any one or more of a dip treatment,
a spray treatment or a cathodic electrolytic treatment by using an aqueous solution
that contains an element of the Group II for adjusting the surface.
[0020] In the process for producing the surface-treated steel sheet of the present invention,
it is desired that:
- (1) The element of the Group II is at least one of calcium or magnesium; and
- (2) In the step of adjusting the surfaces, a reduction ratio of fluorine from that
of the step of forming the coating is not more than 30%.
Effects of the Invention:
[0021] The present invention is capable of providing a surface-treated steel sheet which,
when an organic resin layer is formed on the surfaces thereof, features excellent
adhesion to the organic resin layer and excellent corrosion resistance and an organic
resin-coated surface-treated steel sheet, is capable of providing, as a container,
an organic resin-coated metal container featuring excellent resistance against the
elution of fluorine, excellent adhesion to the organic resin and excellent corrosion
resistance and is, further, capable of providing a process for producing the surface-treated
steel sheet.
[0022] In the present invention, specifically, the element of the Group II and, specifically,
a fluorine compound of the element of the Group II is made present in the surface-treating
layer on the surface side thereof. Therefore, fluorine is insolubilized and is suppressed
from eluting out. Besides, the structure of the surface-treating layer is stabilized
without permitting zirconium to be dissolved, and defective portions can be reduced
over the whole surface-treating layer.
[0023] Further, the organic resin layer that is formed on the surface can be effectively
prevented from peeling when it is subjected to the working or the heat treatment.
Therefore, the invention provides the surface-treated steel sheet which does not easily
corrode even in case the organic resin layer is cracked and the metal surface is exposed
under wet environment and which suppresses the elution of the metal components constituting
the container, provides the organic resin-coated metal container using the surface-treated
steel sheet, and provides the process for producing the surface-treated steel sheet.
[0024] In the process for producing the surface-treated steel sheet of the invention, the
step of adjusting the surface executes at least any one or more of the dip treatment,
spray treatment or cathodic electrolytic treatment by using the aqueous solution that
contains the element of the Group II for adjusting the surface. Therefore, the electrolytic
chromate-treated steel sheet that was so far washed with hot water can now be washed
with warm water or water of normal temperature, making it possible to shorten the
time for treatment as compared to when the washing was conducted with hot water only
and reduces the energy requirement.
[0025] Moreover, by employing the step of adjusting the surface by using the aqueous solution
containing the element of the Group II for adjusting the surface, fluorine in the
coating is not removed and discharged into environment unlike that of when the steel
sheet was washed with hot water but, instead, fluorine in the coating is reacted with
the element of the Group II so as to be insolubilized in the coating. After the step
of forming the coating, therefore, the reduction ratio of fluorine in the surface-treated
steel sheet is suppressed to be not more than 30%, the fluorine concentration in the
drain water is decreased, and a decreased load is exerted on the drain water. Thus
there is provided the process for producing the surface-treated steel sheet while
excellently maintaining the environment.
Brief Description of the Drawings:
[0026]
[Fig. 1] is a diagram showing changes in the atomic concentration in a surface-treating
layer on a surface-treated steel sheet of the present invention in the direction of
depth as found by using an X-ray photoelectron spectrometer.
[Fig. 2] is a diagram showing changes in the atomic concentration in a surface-treating
layer on a surface-treated steel sheet of Comparative Example 4 in the direction of
depth as found by using the X-ray photoelectron spectrometer.
[Fig. 3] is a view schematically illustrating the sectional structure of the surface-treated
steel sheet of the present invention.
Modes for Carrying Out the Invention:
[0027] The surface-treated steel sheet of the present invention has, on at least one surface
of the steel sheet, a surface-treating layer that chiefly comprises zirconium and
oxygen, and contains fluorine, wherein an important feature resides in that an element
of the Group II is contained in the surface-treating layer on the surface side thereof.
[0028] In the surface-treated steel plate of the invention, the element of the Group II
and the fluorine compound contained in the surface-treating layer on the surface side
thereof can be confirmed relying on various kinds of surface analyses or sectional
analysis such as X-ray photoelectron spectrometry (XPS), Auger electrophotometry (AES),
analytical electron microscope (SEM, TEM) or the like.
[0029] As described above, the surface-treated steel sheet 1 obtained by the present invention,
as shown in Fig. 3, has a surface-treating layer 3 on at least one surface (both surfaces
in the drawing) of a steel sheet 2, the surface-treating layer 3 containing an element
of the Group II and, specifically, a fluorine compound of the element of the Group
II on the surface side thereof. Usually, an organic resin layer is formed on the surface-treating
layer 3 to obtain an organic resin-coated surface-treated steel sheet which is then
used as a material of metal containers such as cans and the like.
[0030] Described below are the surface-treated steel sheet of the invention, the organic
resin-coated container using the surface-treated steel sheet, and the process for
producing the surface-treated steel sheet.
(Surface-treated steel sheet)
[0031] The surface-treating layer is formed on the surface-treated steel sheet of the invention,
comprises chiefly zirconium and oxygen, contains fluorine, and is considered to assume
a non-crystalline structure like ZrO
x(OH)
y-
zF
z. The coating dehydrogenates upon drying and firing, dispels F, turns into an oxidized
coating having much crystalline components and, if further heated, finally becomes
a coating of almost ZrO
2. However, the heating in excess of thermal hysteresis which the ordinary can materials
receive induces cracks in the coating due to a change in the structure and renders
the coating to become more like ceramics, inviting not only a decrease in the workability
but also a decrease in the adhesiveness to the resin coating, which is not desirable.
Further, if the amount of F extremely decreases in the surface-treating layer due
to washing with hot water or the like, then the structure of the coating tends to
be easily changed even by the heating of a slight degree, inviting a decrease in the
cohesive force of the coating, inducing a decrease in the corrosion resistance of
the resin-coated metal sheet in the cross-cut testing, and causing corrosion or decrease
in the adhesion in case the can body has received shocks.
[0032] It is, therefore, desired that the surface-treating layer maintains the structure
like ZrO
x(OH)
y-zF
z that contains F and OH.
[0033] The present inventors have studied over extended periods of time about the relationships
among the coating components such as amount of Zr, amount of F, cross-cut corrosion
resistance after the retort treatment, and adhesiveness to the coating resin. As a
result, the inventors have discovered that a surface-treating coating containing much
zirconium and fluorine is effective in attaining the above properties.
[0034] Concerning fluorine, however, if the surface-treated steel sheet containing F in
very large amounts in the coating is used for producing metal cans, then fluorine
that is abundantly present in the surface-treating layer elutes out into the content
when the can is retort-sterilized or is stored at high temperatures, and may spoil
the taste of the content. On the other hand, if fluorine in the coating is forcibly
removed by, for example, washing with hot water, then the surface-treated steel plate
is placed in a state that may easily induce a change in the structure of the coating,
causing a decrease in properties such as corrosion resistance and close adhesion.
In dealing with the surface-treated steel sheet of the present invention, therefore,
suppressing the elution of fluorine from the surface-treating coating by the treatment
for adjusting the surface, is effective in both maintaining the taste of the content
and maintaining properties of the metal cans.
[0035] On the other hand, if the amount of zirconium is small, defective parts are much
present in the surface-treating coating; i.e., the coating easily permits iron constituting
the base material to elute out. Iron elutes out in the anodic reaction. Due to the
cathodic reaction which is the counter-reaction thereof, however, an alkali forms
on the interface between the coating resin and the metal coating. The alkali that
is formed accelerates the elution of fluorine from the surface-treating coating and
becomes a cause of interfacial peeling between the coating resin and the surface-treating
layer. Therefore, it is desired to use a surface-treating coating that contains zirconium
much from the standpoint of cross-cut corrosion resistance after the retort sterilization
and close adhesion to the coating resin.
[0036] In the surface-treated steel sheet of the present invention, fluorine in the surface-treating
coating reacts with the element of the Group II in the aqueous solution for adjusting
the surface so that a fluorine compound in which fluorine is insolubilized is formed
on the surface side. Therefore, there is produced an effect of suppressing the elution
of fluorine as well as an effect of reducing the defective portions in the surface-treating
layer as will be described later. Accordingly, the surface-treated steel sheet of
the present invention is capable of maintaining corrosion resistance and close adhesion
despite the amount of Zr is smaller than those of the prior art.
[0037] As for the amount of coating in the surface-treating layer on the surface-treated
steel sheet of the present invention, it is desired that the amount of Zr is in a
range of 10 to 350 mg/m
2. If the amount of Zr is less than 10 mg/m
2, it becomes difficult to attain the cross-cut corrosion resistance after having been
coated with the organic resin or to attain the close adhesion of the resin to the
inner and outer surfaces of the cans to a sufficient degree. On the other hand, use
of Zr in amounts in excess of 350 mg/m
2 is not economical and, besides, causes a decrease in the close adhesion correspondingly
during the working, and is not desirable.
[0038] The amount of F is desirably from 0.3 to 30 mg/m
2. If the amount of F exceeds 30 mg/m
2, it becomes difficult to suppress the elution of fluorine despite of forming a layer
of a compound of the element of the Group II on the surface side. If the amount of
F is less than 0.3 mg/m
2, on the other hand, the cohesive force of the coating decreases due to a change in
the structure being affected by hydration causing, therefore, a decrease in the close
adhesion and corrosion resistance, which is not desirable.
[0039] As the element of the Group II contained in the surface of the surface-treating layer
by the treatment for adjusting the surface, there can be exemplified beryllium, magnesium,
calcium, strontium, barium and radium. Among them, however, it is desired to use Ca
and Mg from the standpoint of safety, sanitation, availability and cost in addition
to forming a sparingly soluble compound upon the reaction with fluorine, and it is
most desired to use Ca.
[0040] Specifically, if the amount of Zr is large, then the coating contains fluorine much
and the resistance against the elution of fluorine becomes more important. It is,
therefore, desired that the molar ratio AE/Zr of the element AE of the Group II such
as calcium and zirconium in the surface-treating layer is not less than 0.2 and that
the thickness of the coating in terms of the weight of the element AE of the Group
II such as calcium is in a range of 7 to 150 mg/m
2. If the thickness is smaller than 7 mg/m
2, the effect is small for suppressing the elution of fluorine and for decreasing the
surface defects. If the thickness exceeds 150 mg/m
2, the cohesive force of the coating decreases, workability decreases and close adhesion
decreases, which is not desirable. If there are contained a plurality of kinds of
the elements of the Group II, AE represents the total amount thereof.
[0041] The surface-treated steel sheet of the present invention has the surface-treating
layer that chiefly comprises zirconium and oxygen, and contains fluorine, wherein
an important feature resides in that the surface-treating layer has a layer of a compound
of an element of the Group II formed on the surface side thereof.
[0042] Fig. 1 is a diagram showing changes in the atomic concentration in a surface-treated
steel sheet of the present invention in the direction of depth, the surface-treated
steel sheet having a layer of a compound chiefly comprising calcium and fluoride formed
on the surface side of the surface-treating layer that is obtained by treating a surface-treated
steel sheet having a surface-treating layer that chiefly comprises zirconium and oxygen
and contains fluorine with a calcium-containing aqueous solution through the step
of adjusting the surface.
[0043] In Fig. 1, peaks of C1s, O1s, F1s, Fe2p3, Zr3d, and Ca2p3 were measured by using
the X-ray photoelectron spectrometer (hereinafter referred to as XPS), the sum of
these elements were regarded to be 100%, and the depth of Ar sputtering (calculated
as SiO
2) from the surface was represented on the abscissa to express changes in the atomic
concentrations. For reference, further, Fig. 2 shows the results of the profiles of
atomic concentrations in the direction of depth examined by using the surface-treated
steel sheet of before being treated with the calcium-containing aqueous solution through
the step of adjusting the surface. Fig. 1 represents the analytical results of the
surface-treated steel sheet prepared in Example 11 appearing later and Fig. 2 represents
the analytical results of the surface-treated steel sheet prepared in Comparative
Example 4.
[0044] It will be learned from Fig. 1 that in the surface-treated steel sheet of the present
invention, Ca is present in the surface-treating layer on the surface side thereof.
As compared to Fig. 2 that is not conducting the step of adjusting the surface, the
surface-treating layer in Fig. 1 has an increased F concentration on the surface side
thereof but, contrary, has a decreased Zr concentration and a decreased O concentration
on the surface side. This is presumably due to that through the treatment with the
calcium-containing aqueous solution, fluorine and calcium reacts with each other,
and an insoluble compound is formed in the surface. In Fig. 1, further, the atomic
concentration of Fe rises little in comparison to Fig. 2 despite the surface-treating
layer is sputtered deep from the surface thereof and it is, therefore, learned that
the surface-treating layer has a structure that permits defects to be little exposed.
[0045] The similar trends are exhibited in Examples 1 to 15 and Comparative Examples 1 to
5 appearing later. Namely, in Examples, the elements of the Group II are present on
the surface side like in Fig. 1 indicating an increase in the F concentration on the
surface side as compared to Comparative Examples represented by Fig. 2.
(Method of producing surface-treated steel sheets)
<Step of forming the coating>
[0046] In the process for producing the surface-treated steel sheet of the invention, first,
the steel sheet, in the step of forming the coating, is cathodically electrolyzed
in an electrolytic treating liquid which is an aqueous solution containing Zr ions
and F ions so that the coating of a Zr compound chiefly comprising zirconium and oxygen
and containing fluorine is formed on at least one surface of the steel sheet in such
a fashion that the amount of Zr is in a range of 10 to 350 mg/m
2 and, more preferably, 10 to 200 mg/m
2 and the amount of F is in a range of 0.3 to 30 mg/m
2. Specifically, in the step of adjusting the surface that will be described later,
the reduction of fluorine can be greatly suppressed if the amount of the coating is
large. Therefore, it is desired that the amount of Zr is not less than 100 mg/m
2 and, specifically, is in a range of 100 to 200 mg/m
2.
[0047] The steel sheet after the surface-treating layer has been formed thereon is squeezed
through the rolls to remove the electrolytic treating solution, washed with water,
further, squeezed through the rolls to remove the washing water, and is sent to the
next step of adjusting the surface.
[0048] In the electrolytic treating solution used in the step of forming the coating, it
is desired that the concentration of Zr is 1,000 to 10,000 ppm and the concentration
of F is 600 to 13,000 ppm. It is desired that the electrolytic treating solution has
a pH of 2 to 5 and, more preferably, 2.5 to 4. The temperature of the electrolytic
treating solution is desirably 30 to 60°C.
[0049] Various kinds of compounds that will be described later can be added to the electrolytic
treating solution used in the step of forming the coating. Here, the electrolytic
treating solution basically contains nitric acid ions and ammonium ions for adjusting
the pH as well as Fe ions eluted out from the base material in addition to containing
Zr ions and F ions.
[0050] There is no particular limitation on the chemicals used for forming Zr ions that
constitute the electrolytic treating solution, and there can be used, for example,
K
2ZrF
6, (NH
4)
2ZrF
6, (NH
4)
2ZrO(CO
3)
2, H
2ZrF
6, ZrO(NO
3)
2 and ZrO(CH
3COO)
2. In the invention, the above chemicals may be used alone or in a combination of two
or more kinds.
[0051] If the coating of a Zr compound is to be formed by the cathodic electrolytic treatment,
it is, usually, desired to use a treating solution that contains F ions in addition
to the above-mentioned Zr ions as the electrolytic treating solution. F ions contained
in the electrolytic treating solution work as a complexing agent that enhances solubility
of Zr ions in the electrolytic treating solution. Therefore, the Zr compound can be
precipitated maintaining a uniform thickness on the base plate, and the adhesion can
be further improved between the coating and the organic resin layer.
[0052] If the electrolytic treating liquid contains F ions in small amounts, then Zr locally
precipitates; i.e., the coating includes a mixture of portions where Zr is thickly
present and portions where Zr is thinly present. Namely, the coating lacks uniformity
in the thickness and, as a result, has poor adhesiveness and corrosion resistance
after the working. In the step of forming the coating, therefore, it is important
that the molar ratio F/Zr of F atoms to Zr atoms in the coating is so controlled as
to be not less than 0.6.
[0053] There is no particular limitation on the chemicals used for forming F ions in the
electrolytic treating solution, and there can be used ammonium zirconium fluoride,
aluminum fluoride, titanium fluoride, sodium fluoride, ammonium fluoride, hydrofluoric
acid, calcium fluoride, hexafluorosilicic acid, and sodium hexafluorosilicate. Among
them, it is desired to use those chemicals that are highly soluble in water.
[0054] In order to improve electric conductivity in the treating solution and to adjust
the pH of the treating solution, further, the electrolytic treating solution may be
added with nitric acid ions and ammonium ions in ranges in which they do not impair
the formation of the Zr compound coating.
[0055] To the electrolytic treating solution, furthermore, there can be added one or more
kinds of additives selected from such organic acids as citric acid, lactic acid, tartaric
acid and glycolic acid or such high molecular compounds as polyacrylic acid, polyitaconic
aid and phenol resin. Upon adding additives such as organic acid and phenol resin
to the electrolytic treating solution, additives such as organic acid and phenol resin
can be contained in the Zr compound coating that is formed to thereby impart flexibility
of the coating of the oxygen compound of a metal and to further improve adhesiveness
to the organic resin layer.
[0056] In subjecting the base material to the cathodic electrolytic treatment, the current
density is not specifically limited but is, preferably, 1 to 30 A/dm
2.
[0057] If the base material is to be subjected to the cathodic electrolytic treatment, it
is desired to employ a discrete electrolytic system which repeats the cycle of flowing
the electric current and interrupting the electric current. In this case, the total
time for flowing the electric current to the base material (total time for flowing
the electric current in conducting the cycle of flowing and interrupting the electric
current a plurality of number of times) is, preferably, 0.3 to 30 seconds.
[0058] In subjecting the base material to the cathodic electrolytic treatment, further,
an opposing electrode of any kind may be installed on the base material if it does
not dissolve in the electrolytic treating solution while the cathodic electrolytic
treatment is being conducted. It is, however, desired to use a titanium plate coated
with iridium oxide from the standpoint of a small oxygen overvoltage and that electrode
plate sparingly dissolves in the electrolytic treating solution.
<Step of adjusting the surfaces>
[0059] An important feature of the present invention is to conduct the step of adjusting
the surfaces by using the element of the Group II after the above-mentioned step of
forming the coating.
[0060] That is, the surface-treated steel sheet forming the surface-treating layer that
that chiefly comprises zirconium and oxygen and contains fluorine obtained through
the step of forming the coating, is subjected to any one or more of the dip treatment,
the spray treatment or the cathodic electrolytic treatment by using an aqueous solution
that contains the element of the Group II for adjusting the surfaces. Through this
treatment, the element of the Group II is made present in the surface-treating layer
on the surface side thereof. Here, as described above, it is specifically desired
that fluorine reacts with the element of the Group II and is insolubilized so as to
be made present as a fluorine compound.
[0061] After the treatment such as the dipping treatment, the steel sheet is squeezed with
the rolls to remove the aqueous solution used for adjusting the surfaces, is washed
with water, is further squeezed with the rolls to remove the washing water and is,
thereafter, dried with the hot air or the like.
[0062] If the step of adjusting the surface is not conducted, the container made from the
organic resin-coated surface-treated steel sheet applied with the organic resin coating
permits fluorine in the coating to elute out into the content in the step of sterilization
treatment with hot water, such as retort treatment. As a result, the coating induces
a change in the structure thereof causing a decrease in the properties such as corrosion
resistance, close adhesion, etc.
[0063] Therefore, prior to applying the resin coating for obtaining the material for cans,
it becomes important to insolubilize the fluorine in advance by forming the layer
of the compound thereof that has reacted with the element of the Group II in the step
of adjusting the surfaces, the fluorine having been dispersed on the surface side
in the surface-treating layer that chiefly comprises zirconium and oxygen and contains
fluorine.
[0064] As the element of the Group II used for the aqueous solution for adjusting the surfaces,
there can be exemplified beryllium, magnesium, calcium, strontium, barium and radium.
In using these elements for the aqueous solution for adjusting the surfaces, however,
attention must be given to that if the chemicals are soluble in water, if the chemicals
easily bond to the fluorine, if the chemicals are capable of forming a sparingly soluble
fluorine compound, if the chemicals are capable of maintaining safety and sanitation
excellently, and if the chemicals are not expensive. From the above points of view,
therefore, it is desired to use calcium or magnesium capable of forming sparingly
soluble CaF
2 or MgF
2 after having reacted with fluorine.
[0065] Here, the aqueous solution for adjusting the surfaces may contain either calcium
ions or magnesium ions, or both of them. If the ions of either one type are to be
contained, the aqueous solution containing calcium can be most desirably used for
adjusting the surfaces.
[0066] If the aqueous solution containing calcium is used for adjusting the surfaces, any
chemicals can be used without limitation provided they can dissolve in water, and
there can be used calcium lactate, calcium hydroxide, calcium gluconate, calcium chloride,
calcium nitrate, calcium sulfate, calcium citrate, calcium carbonate and calcium phosphate
monobasic. Among them, it is desired to use the one having large water-solubility.
If magnesium is used, further, there is no particular limitation on the chemicals
that can be used provided they dissolve in water. Preferably, there can be used magnesium
chloride, magnesium nitrate, magnesium sulfate, magnesium citrate, magnesium acetate
and magnesium gluconate. If the aqueous solution is alkaline, magnesium gluconate
can be particularly preferably used.
[0067] The aqueous solution for adjusting the surfaces may have been containing fluorine
at the time of preparing the chemicals or may have been containing fluorine as it
is dissolved in the step of adjusting the surfaces, as will be obvious from the fact
that the aqueous solution for adjusting the surfaces has the role of insolubilizing
the fluorine contained in the surface-treating coating. The invention, however, does
not dare to add fluorine thereto or contain fluorine therein.
[0068] In the step of adjusting the surfaces, further, it is desired that the aqueous solution
for adjusting the surfaces has a pH in a range of 2 to 13, preferably, 5 to 11 and,
more preferably, 5.5 to 7. Within this range, the fluorine in the surface-treating
layer assumes the state of free F ions but not complex ions, and are capable of being
more efficiently bonded to the element of the Group II that is forming the fluorine
compound with the element of the Group II in the surface-treating layer on the surface
side thereof maintaining stability improving, therefore, resistance against the elution
of fluorine and lowering the fluorine concentration in the drain water.
[0069] If the pH is lower than 2, the peripheral equipment and the steel sheet which is
the base material itself are adversely affected in terms of corrosion resistance.
If the pH exceeds 11 to become alkaline, the ability decreases for stably forming
the fluorine compound in the surface-treating layer on the surface side thereof. If
the pH exceeds 13, in particular, fluorine dissolves at an increased rate in the aqueous
solution for adjusting the surfaces eventually causing an increase in the fluorine
concentration in the drain water, which is not desirable.
[0070] As the alkaline chemicals for adjusting the pH of the aqueous solution for adjusting
the surfaces, there can be most simply used a hydroxide compound of the element of
the Group II that exhibits alkalinity upon being dissolved in water, such as Ca(OH)
2 or Mg(OH)
2. These chemicals, however, dissolve in water in relatively small amounts. If the
step of adjusting the surfaces is continuously conducted by dipping or cathodic electrolysis
or if the cathodic electrolysis is conducted, therefore, the chemical must be fed
frequently often requiring laborious work for maintaining and controlling the aqueous
solution. In such a case, it is desired to employ a spray system which sprays at all
times a new aqueous solution onto the steel sheet for adjusting the surfaces from
the standpoint of easy processing. Even if the chemical of the element of the Group
II does not dissolve in water and does not produce alkalinity, the pH can be adjusted
by adding a chemical that contains one or two or more kinds of sodium, ammonium and
potassium, and the solution can be used as an alkaline aqueous solution for adjusting
the surfaces.
[0071] As the chemical other than the element of the Group II used for adjusting the pH
of the aqueous solution for adjusting the surfaces, there can be exemplified ammonia,
ammonium zirconium carbonate, sodium hydroxide, sodium carbonate, sodium hydrogencarbonate,
sodium phosphate, sodium hydrogenphosphate, potassium hydroxide, potassium carbonate,
sodium borate and sodium silicate, which may be used in two or more kind.
[0072] As required, further, various surfactants and chelating agents may be added to the
aqueous solution for adjusting the surfaces.
[0073] It is desired that the ionic concentration of the element of the Group II contained
in the aqueous solution for adjusting the surfaces is in a range of 0.002 to 0.5 mols/l.
If the ionic concentration is less than 0. 002 mol/l, the reaction efficiency becomes
poor in forming the fluorine compound with the element of the Group II in the surface-treating
layer on the surface side thereof. If the ionic concentration exceeds 0.5 mols/l,
the element of the Group II precipitates too much, and the cohesive force of the coating
decreases, which is not desirable.
[0074] In the step of adjusting the surfaces as described above, the dip treatment, spray
treatment or cathodic electrolytic treatment can be conducted by using the aqueous
solution containing the element of the Group II for adjusting the surfaces. From the
standpoint of quick treatment, it is desired to also add the cathodic electrolytic
treatment in the aqueous solution for adjusting the surfaces. The spray treatment
and the dip treatment, on the other hand, are means desirable from the standpoint
of simplicity. The aqueous solution for adjusting the surfaces should have a pH in
the above-mentioned range and, specifically, in the range of 5.5 to 7 from the standpoint
of lowering the dissolution of fluorine in the aqueous solution for adjusting the
surfaces and lowering the load exerted on the drain water.
[0075] If the cathodic electrolytic treatment is to be conducted, it is desired that the
aqueous solution containing the element of the Group II for adjusting the surfaces
has an electric conductivity of not less than 2 mS/cm from the standpoint treatment
efficiency.
[0076] As described above, it is made possible to make present the element of the Group
II and, specifically, the compound of the element of the Group II in the surface-treating
layer on the surface side through the step of adjusting the surfaces. Here, the compound
of the element of the Group II formed in the surface-treating layer on the surface
side thereof is, preferably, a fluorine compound and, more preferably, is sparingly
soluble. The sparingly soluble compound formed by using the element of the Group II
is, preferably, a compound of calcium and/or magnesium, or may be a single compound
of either calcium or magnesium, or a compound of both calcium and magnesium. In the
case of the single compound, the most desired is the fluorine compound of calcium
forming the sparingly soluble compound.
[0077] The molar ratio AE/Zr of the element AE of the Group II and zirconium Zr in the surface-treating
layer is, preferably, not less than 0.2 and, specifically, in a range of 0.4 to 1.8.
It is more effective if the thickness of the element of the Group II in terms of the
weight is not less than 7 mg/m
2 and, specifically, in a range of 7 to 150 mg/m
2.
[0078] In the step of adjusting the surfaces, there is no particular limitation on the temperature
of the aqueous solution for adjusting the surfaces. From the standpoint of the reactivity
and controlling the temperature, however, it is desired that the temperature lies
in a range of 30 to 80°C and, specifically, 30 to 60°C. Further, the total treating
time such as of the dip treatment, spray treatment and cathodic electrolytic treatment
using the aqueous solution for adjusting the surfaces is in a range of 0.1 to 5 seconds
and, more preferably, 0.5 to 3 seconds.
[0079] After the treatment with the aqueous solution for adjusting the surfaces in the step
of adjusting the surfaces, it is also allowable to add the washing treatment by dipping
in, or spraying with, the warm water or hot water heated at about 40°C to about 95°C.
(Steel sheet as the base material)
[0080] As the steel sheet for use as the surface-treated steel sheet of the present invention,
there can be used, for example, a hot-rolled steel sheet based on a continuously casted
aluminum killed steel, a cold-rolled steel sheet obtained by cold-rolling the hot-rolled
steel sheet, and a steel sheet obtained by plating metals inclusive of Zn, Sn, Ni,
Cu, Al, etc. on the hot-rolled steel sheet or the cold-rolled sheet.
[0081] It is, further, allowable to use a steel sheet having, on part or on the whole surface
thereof, an alloy layer such as of an Sn-Ni-Fe alloy, an Sn-Fe alloy or an Ni-Fe alloy,
as well as a steel sheet having a layer of a metal such as Sn or Ni, further, plated
on the above alloy layer. Among them, a steel sheet is most desirably used as the
base material without having a metal-plated layer or having a metal-plated layer but
permitting iron to be locally exposed in a dispersed manner from the standpoint of
cost.
[0082] The thickness of the base material is not specifically limited and may be suitably
selected depending on the use, but is, preferably, 0.07 to 0.4 mm.
(Organic resin coating)
[0083] As described above, the surface-treated steel sheet obtained by the present invention
has an organic resin coating formed on the surface-treating layer. The organic resin
coating excellently adheres to the organic resin layer. Even in case the surface-treated
steel sheet is retort-treated, the organic resin coating prevents the organic resin
layer from peeling. The organic resin coating, further, effectively prevents the corrosion
from proceeding even in case the organic resin layer is cracked and the metal surface
is exposed in wet environment and, therefore, suppresses the metal components constituting
the container from eluting out.
[0084] The resin that constitutes the organic resin coating is not specifically limited
and may be suitably selected depending on the use of the surface-treated steel sheet
of the invention (depending on the use such as cans and containers for containing
specific contents). Namely, there can be exemplified resin coatings made from various
thermoplastic resins and films made from thermosetting coating materials or thermoplastic
coating materials. As the resin coating made from the thermoplastic resin, there can
be exemplified olefin resin films such as of polyethylene, polypropylene, ethylene-propylene
copolymer, ethylene-vinyl acetate copolymer, ethylene-acrylic ester copolymer and
ionomer; polyester films such as of polyethylene terephthalate and polybutylene terephthalate;
polyamide films such as nylon 6, nylon 6,6, nylon 11, and nylon 12; and thermoplastic
resin films such as polyvinyl chloride film and polyvinylidene chloride film, which
may not have been stretched or may have been biaxially stretched. Among them, particularly
preferred is an unoriented polyethylene terephthalate obtained by copolymerizing an
isophthalic acid. The resins for constituting the organic resin coating may be used
in a single kind or in a blend of different resins.
[0085] If the thermoplastic resin coating is formed as the organic resin coating, the coating
may be of a single resin layer or a multiplicity of layers formed by the simultaneous
extrusion. Multiplicity of polyester resin layers offer such an advantage that a polyester
resin having excellent adhesiveness can be used as the underlying layer, i.e., on
the side of the surface-treated steel sheet and that a polyester resin having resistance
against the content, i.e., having resistance against being extracted or having property
of not adsorbing flavor component can be used as the surface layer.
[0086] Examples of the multiplicity of polyester resin layers include, being expressed as
surface layer/lower layer, polyethylene terephthalate/polyethylene terephthalate·
isophthalate; polyethylene terephthalate/polyethylene· cyclohexylenedimethylene·terephthalate;
polyethylene terephthalate having a small isophthalate content· isophthalate/polyethylene
terephthalate having a large isophthalate contentisophthalate; polyethylene terephthalate·isophthalate/[blend
of polyethylene terephthalate·isophthalate and polybutylene terephthalate· adipate]
and the like, to which only, however, the invention is in no way limited. It is desired
that the thickness ratio of the surface layer: lower layer is in a range of 5:95 to
95:5.
[0087] The above organic coatings can be blended with known blending agents for resins,
such as anti-blocking agent or amorphous silica, inorganic filler, various antistatic
agents, lubricant, antioxidant and ultraviolet-ray absorber according to a known recipe.
[0088] Among them, it is desired to use a tocopherol (vitamin E). It has heretofore been
known that the tocopherol is used as an antioxidant and works to prevent a decrease
in the molecular weight caused by the oxidation and decomposition when the polyester
resin is being heat-treated and works to improve resistance against being dented.
Specifically, if added to a polyester composition obtained by blending the polyester
resin with the above ethylene type polymer as a resin reforming component, the tocopherol
not only provides resistance against being dented but also works to prevent corrosion
from occurring due to cracks formed in the coating as a result of being subjected
to severe conditions during the retort sterilization and stored in a hot vending machine,
offering an effect of greatly improving the corrosion resistance.
[0089] The tocopherol is added in an amount of, desirably, 0.05 to 3% by weight and, specifically,
0.1 to 2% by weight.
[0090] The organic resin coating applied to the surface-treated steel sheet obtained by
the present invention, in the case of the thermoplastic resin coating, has a thickness
in a range of, usually, 3 to 50
µm and, specifically, 5 to 40
µm and, in the case of a film, has a thickness in a range of 1 to 50
µm and, specifically, 3 to 30
µm after fired. If the thickness is smaller than the above range, corrosion resistance
becomes insufficient. If the thickness exceeds the above range, on the other hand,
problems tend to occur in regard to workability.
[0091] The surface-treated steel sheet obtained by the present invention can be coated with
the organic resin by any means such as, in the case of the thermoplastic resin coating,
an extrusion-coating method, a cast film heat-adhesion method or a biaxially stretched
film heat-adhesion method. In the case of the extrusion-coating method, the polyester
resin in a molten state is extruded onto the surface-treated steel sheet and is thermally
adhered thereto. Namely, the polyester resin is melt-kneaded by an extruder, extruded
into the form of a thin film through a T-die, and the molten resin film that is extruded
is passed together with the surface-treated steel sheet through a pair of laminating
rolls so as to be pressed together into a unitary structure under cold condition followed
by quenching. If a multiplicity of polyester resin layers are to be extruded, use
is made of an extruder for extruding the surface resin layer and an extruder for extruding
the lower resin layer. The flows of resins from these extruders are met together in
a multi-layer die. Thereafter, the resultant flow of resins may be extruded like in
the case of extruding the single resin layer. Further, by passing the surface-treated
steel sheet between the pair of laminating rolls in a vertical direction and by feeding
the webs of molten resins to both sides thereof, it is made possible to coat both
surfaces of the base material with the polyester resins.
[0092] Concretely described below is the production of the organic resin-coated surface-treated
steel sheet having an organic coating of polyester resin based on the extrusion-coating
method. The surface-treated steel sheet, as required, is preheated by a heating device,
and is fed to a nipping position between the pair of laminating rolls. The polyester
resin, on the other hand, is pushed into the form of a thin film through the die head
of the extruder, fed into between the laminating rolls and the surface-treated steel
sheet, and is press-adhered onto the surface-treated
steel sheet by the laminating rolls. The laminating rolls are maintained at a predetermined
temperature. Thin films of the thermoplastic resin such as polyester are pressed onto
the surface-treated steel sheet and are thermally adhered thereto followed by cooling
from both sides thereof so as to obtain an organic resin-coated surface-treated steel
sheet. Usually, the organic resin-coated surface-treated steel sheet that is formed
is, further, introduced into a water tank for cooling, and is quenched therein to
prevent thermal crystallization.
[0093] In the extrusion-coating method, the polyester resin layer assumes the crystallinity
of a low level, i.e., has a density which is different from the amorphous density
thereof by not more than 0.05 g/cm
3 due to the resin composition that is selected and due to quenching by the rolls and
in the cooling tank. Therefore, the polyester resin layer is sufficiently guaranteed
for its workability in the subsequent steps of forming cans and lids. The quenching
operation is not limited to the above example only but may be to spray the cooling
water onto the organic resin-coated surface-treated steel sheet that is formed so
as to quench the laminated sheet.
[0094] The polyester resin is thermally adhered to the surface-treated steel sheet by utilizing
the quantity of heat possessed by the molten resin layer and the quantity of heat
possessed by the surface-treated steel sheet. A proper range of the temperature (T1)
for heating the surface-treated steel sheet is, usually, 90°C to 290°C and, specifically,
100°C to 280°C while a proper range of the temperature of the laminating rolls is
10°C to 150°C.
[0095] The organic resin coating can also be formed on the surface-treated steel sheet obtained
by the production method of the present invention by thermally adhering, onto the
surface-treated steel sheet, a polyester resin film that is formed in advance by a
T-die method or an inflation method. As the film, there can be used an unstretched
film formed by a cast-forming method by quenching the film that is extruded. It is,
further, allowable to use a biaxially stretched film obtained by biaxially stretching
the film sequentially or simultaneously at a stretching temperature, and thermally
setting the film after having been stretched.
(Metal containers)
[0096] As for the metal container (can body) formed by using the surface-treated steel sheet
of the invention, it is desired that the container is formed by using the organic
resin-coated surface-treated steel sheet obtained by coating the surfaces of the surface-treated
steel sheet with the organic resin as described earlier relying on any can-producing
method. Concretely speaking, the organic resin-coated surface-treated steel sheet
can be used for forming a three-piece can (welded can) having a seam on the side surface
thereof and a seamless can (two-piece can). From the standpoint of close adhesion
to the organic resin as described above, however, the surface-treated steel sheet
containing Zr in large amounts is most desirably used for forming seamless cans.
[0097] The seamless can is produced relying on a conventional means such as draw working,
draw·redraw working, bend-elongation working (stretching) based on the draw· redrawing,
bend-elongation·ironing working based on the draw ·redrawing, or draw· ironing working
in a manner that the organic resin coating is on the inner surface side of the cans.
[0098] When it comes to a seamless that is subjected to a high degree of working such as
bend-elongation working (stretching) based on the draw·redrawing, bend-elongation·ironing
working based on the drawn· redrawing or the like, it is desired that the organic
resin coating is a thermoplastic resin coating formed by the extrusion-coating method.
The organic resin-coated surface-treated steel sheet features excellent close adhesion
during the working. Namely, the coating remains excellently adhered even if it is
subjected to severe working, and makes it possible to provide a seamless can having
excellent corrosion resistance.
(Lids)
[0099] The can lid formed by using the surface-treated steel sheet of the invention is,
desirably, formed by using the organic resin-coated surface-treated steel sheet like
the metal container described above, and is formed by a known lid-forming method.
Concretely, the lids may be a flat lid, an easy-open can lid of the stay-on-tub type,
and an easy-open can lid of the full-open type.
[0100] According to the invention, the can lids of a variety of types can be formed without
limitation by using the organic resin-coated surface-treated steel sheet of the present
invention.
EXAMPLES
[0101] The invention will now be concretely described by way of Examples to which only,
however, the invention is in no way limited. The materials to be coated, dewaxing
agents and organic coatings are those arbitrarily selected from those placed in the
market, and are not to impose limitation on the process for producing the surface-treated
steel sheet of the present invention.
[0102] The process for producing the surface-treated steel sheet and the methods of evaluating
the properties thereof are as described below.
(Step of forming the coating)
[0103] As a starting steel sheet, use was made of a low-carbon steel sheet 0.225 mm in thickness
and 200 mm in width. Next, as a pre-treatment, the steel sheet was dewaxed by the
electrolysis with an alkali and was washed with an acid by being dipped in sulfuric
acid. Thereafter, the steel sheet was dipped in an electrolytic treating solution
and was cathodically and electrolytically treated so that the steel sheet was coated
on its both surfaces with a compound that chiefly comprised of Zr and contained F.
Next, the steel sheet was squeezed with the rolls, washed with water and, further,
squeezed with the rolls to remove the washing water to thereby form the coatings.
Electrolytic treating solution:
An aqueous solution in which ammonium zirconium fluoride was dissolved as a Zr compound,
the concentration of Zr being 6,000 ppm and the concentration of F being 7,500 ppm.
pH of the electrolytic treating solution:
3.0 (pH was adjusted with nitric acid and/or ammonia).
Temperature of the electrolytic treating solution:
40°C
Opposing electrode:
Titanium plate coated with iridium oxide.
Method of flowing electric current during the cathodic electrolysis:
The electric current was flown one time or a plurality of times (hereinafter called
number of cycles) at a current density of 3 A/dm2 for 0.15 seconds each time.
(Step of adjusting the surfaces)
[0104] The steel sheet after the step of forming the coating was treated with the aqueous
solution for adjusting the surfaces for a predetermined period of time, squeezed with
the rolls, washed with water, further, squeezed with rolls and was, thereafter, dried
with the hot air to obtain a surface-treated steel sheet.
[0105] In the step of adjusting the surfaces according to the present invention, there can
be conducted any one or more of the dip treatment, spray treatment and cathodic electrolytic
treatment by using the aqueous solution containing the element of the Group II for
adjusting the surfaces. In Examples of the invention, however, there were conducted
the dip treatment, the spray treatment and the cathodic electrolytic treatment by
using the aqueous solution containing calcium or magnesium for adjusting the surfaces.
In the cathodic electrolysis in the step of adjusting the surfaces, a titanium plate
coated with iridium oxide was used as the opposing electrode, and the electric current-flowing
cycle was repeated a plurality of times, each cycle comprising flowing the electric
current for 0.15 seconds followed by the interruption of 0.1 second.
(Producing the organic resin-coated surface-treated steel sheets)
[0106] An organic resin-coated surface-treated steel sheet was obtained by thermally adhering
a 19
µm-thick stretched film a polyethylene terephthalate/isophthalate copolymer composition
containing 11 mol% of isophthalic acid component onto one surface, that becomes the
inner surface of the can, of the surface-treated steel sheet obtained above and by
thermally adhering a 13
µm-thick stretched film of a polyethylene terephthalate/isophthalate copolymer composition
containing 12 mol% of isophthalic acid component and, further, containing titanium
oxide and colored white onto the other surface that becomes the outer surface of the
can, by using laminating rolls followed readily by cooling with water while paying
attention such that the film was oriented to a suitable degree. The obtained organic
resin-coated surface-treated steel sheet was partly used for evaluating cross-cut
corrosion resistance, but the rest of it was used for producing metal cans.
(Producing the metal cans)
[0107] Paraffin wax was electrostatically applied onto both surfaces of the organic resin-coated
surface-treated steel sheet obtained above. The steel sheet was punched into a circle
143 mm in diameter and was draw-formed into a cup 91 mm in diameter and 36 mm in height
according in a customary manner. The draw-formed cup was at the same time subjected
to the draw-ironing working repetitively two times to form a cup having a small diameter
and a large height. The thus obtained cup possessed properties as described below.
Diameter of cup: 52.0 mm
Height of cup: 111.7 mm
Reduction ratio of sheet thickness in the can wall relative to the initial sheet thickness:
30%
[0108] After the doming, the cup was heat-treated at 220°C for 60 seconds to remove strain
from the resin film, followed by trimming for the open end, printing on the curved
surface, necking into a diameter of 50.8 mm and flanging to thereby obtain a seamless
can having a capacity of 200 ml.
(Measuring the amount of Zr and the amount of AE (amount of the element of the Group
II))
[0109] By using an X-ray fluorometric analyzer (Model: ZSX100e manufactured by Rigaku Co.),
the surface-treated steel sheet obtained above was measured for its amount of Zr and
the amount of AE (amount of Ca or amount of Mg in Examples) contained in the metal
compound coating. The molar ratio AE/Zr was found according to the following formula,
(Measuring the amount of F)
[0110] Microanalysis of the amount of F in the obtained surface-treated steel sheet based
on the X-ray fluorometry poses limitation in regard to quantitative precision. Specifically,
it is difficult to determine the amount of F from the surface-treated steel sheet
containing F in amounts of less than 1.5 mg/m
2. After having studied variously, therefore, we have measured the amount of F in a
manner as described below. That is, by using a special cell capable of holding 160
cm
2 of one surface of the surface-treated steel sheet in a state of being contacted to
183 g of very pure water, the surface-treated steel sheet was retort-treated at 130°C
for 30 minutes. Thereafter, fluorine ions released into very pure water were measured
by the ion chromatography (DX-320 manufactured by DIONEX Co.). The amount of F present
in very pure water was found from the obtained concentration of F and was converted
into the amount of F present in the surface-treated steel sheet per a unit area, and
was regarded to be the amount of F in the coating.
[0111] Fluorine does not almost elute out despite the surface-treated steel sheets that
have passed through the step of adjusting the surfaces shown in Examples are subjected
to the retort-treatment and, therefore, the amounts of F in the surface-treated steel
sheets cannot be known. For the surface-treated steel sheets that have passed through
the step of adjusting the surfaces, therefore, the amounts of F were measured by the
X-ray fluorometry. In case the amount of F was less than 1.5 mg/m
2, however, the peak in the X-ray fluorometry was not clear. Therefore, the scraped
powder of the surface-treating coating was collected in amounts equivalent to more
than 10 times the areas that are usually measured by the X-ray fluorometry, and fluorine
was measured by the X-ray fluorometry and was converted into an amount thereof per
a unit area.
(Measuring the reduction ratio of F)
[0112] The percentage reduction in the amount of F in the surface-treated steel sheet that
has passed through the step of adjusting the surface was found from the amount of
F in the surface-treated steel sheet that was formed through only the step of forming
the coating. The evaluation thereof serves as an index of fluorine load exerted on
the drain water in the step of adjusting the surfaces. It is desired that the index
is not more than 30%.
(Evaluating the cross-cut corrosion resistance)
[0113] By using a cutter knife, a portion of the obtained surface-treated steel sheet that
would become the inner surface side of a can was engraved over a length of 4 cm in
a crossing manner deep enough to reach the steel sheet to thereby prepare a test piece.
The test piece was put in a bottle and was dipped in a commercially available coffee
(trade name, Blendy, bottled coffee, low sugar, produced by Ajinomoto General Foods
Co.). The bottle was deaerated, stored at 37°C for 4 weeks to evaluate the state of
corrosion. During this period, the coffee was regularly renewed to suppress the generation
of mold as much as possible. The corroded state was evaluated by taking the test piece
out of the coffee. Namely, the cross-cut portion and the surroundings thereof were
observed with the eye in regard to if the organic resin layer was peeled or if the
color has changed due to the formation of corroded product.
[0114] A test piece whose color has changed or whose film has peeled by a maximum width
of not less than 3 mm around the cross-cut portion was counted to be one point, a
test piece having a maximum width of peeling of not less than 2 mm but less than 3
mm was counted to be two points, a test piece of not less than 1 mm but less than
2 mm was counted to be three points, a test piece of not less than 0. 5 mm but less
than 1 mm was counted to be four points, and a test piece of less than 0.5 mm was
counted to be five points. Test pieces of counts of three or more points were regarded
to be acceptable.
(Evaluating the adhesion of the resin to the inner surface of the can)
[0115] A seamless can that was obtained was filled with distilled water, double-wrap-seamed
with a lid, and was retort-treated at 125°C for 30 minutes. Thereafter, the lid was
removed from the can body, the content was removed from the can, and the can was cut
into halves with the direction of rolling the surface-treated steel sheet at 45 degrees
as a boundary. Next, the can cut into halves was dipped in a solution obtained by
adding 0.02% by weight of a surfactant to an aqueous solution containing 1% by weight
of sodium chloride for one hour. By using a pair of scissors, the can was further
cut into halves from the side of the can bottom with the rolling direction of 135
degrees as a boundary. The cross section of a radial portion of the bottom of the
finally cut can on the inner surface side thereof was observed in regard to if the
resin was peeled to thereby evaluate the close adhesion of the resin. The can with
peeling of not less than 10 mm near the cut surface was counted to be one point, the
can with peeling of less than 10 mm but not less than 5 mm was counted to be two points,
the can with peeling of less than 5 mm but not less than 2 mm was counted to be three
points, the can with peeling but less than 2 mm was counted to be four points, and
the can with no peeling was counted to be five points. The cans of counts of three
or more points were regarded to be acceptable.
(Evaluating the resistance against the elution of F)
[0116] The obtained seamless can was filled with 183 g of very pre water, double-seamed,
and was retort-treated at 130°C for 30 minutes. Thereafter, fluorine ions released
into very pure water was measured by the ion chromatograph (DX-320 manufactured by
DIONEX Co.). The cans releasing F by not less than 0.1 ppm were evaluated to be X
and the cans releasing F by less than 0.1 ppm were evaluated to be ○.
(Evaluating the load exerted on the drain water)
[0117] The load exerted on the drain water was evaluated from the reduction ratio of F.
The cases of when the reduction ratios of the amounts of F were not more than 30%
were evaluated to be ○ and the cases of when the reduction ratios thereof were not
less than 30% were evaluated to be Δ. ○ is preferred to Δ.
<Example 1>
[0118] In the step of forming the coating, first, a titanium plate coated with iridium was
used as the opposing electrode in the electrolytic treating solution, a steel sheet
was used as the cathode, an electric current was flown one time at a current density
of 3 A/dm
2 for 0.15 seconds, the steel sheet was squeezed with the rolls to remove the electrolytic
treating solution, washed with water of normal temperature and was, further, squeezed
with the rolls to remove the washing water. Next, the surfaces were adjusted by the
cathodic electrolytic treatment. As the aqueous solution for adjusting the surfaces,
use was made of an aqueous solution containing calcium lactate in an amount of 0.1
mol/l, and having an electric conductivity of 6.57 mS/cm and a pH of 6.96. In the
aqueous solution maintained at a liquid temperature of 30°C for adjusting the surfaces,
the step of adjusting the surfaces was conducted by repeating twice the cycle of flowing
an electric current at a current density of 4 A/dm
2 for 0.15 seconds followed by an interruption of 0.1 second. The steel sheet after
the step of adjusting the surfaces was squeezed with the rolls to remove the aqueous
solution, washed with water, squeezed again with the rolls to remove the washing water,
and was dried to obtain a surface-treated steel sheet.
[0119] The obtained surface-treated steel sheet was measured for the amount of Zr, amount
of AE and amount of F by the methods described above. Measured amounts of the coating
were as shown in Table 1 which also shows a molar ratio AE/Zr of the element (AE)
of the Group II and Zr calculated from the amounts of the coating, and the reduction
ratio of F. In Table, "-" stands for that the measurement was not taken.
<Example 2>
[0120] A surface-treated steel sheet was obtained in the same manner as in Example 1 but
flowing the electric current at a current density of 10 A/dm
2 and repeating the cycle twice in the step of forming the coating, and flowing the
electric current at a density of 1 A/dm
2 in the step of adjusting the surfaces.
<Example 3>
[0121] A surface-treated steel sheet was obtained in the same manner as in Example 2 but
flowing the electric current at a density of 6.5 A/dm
2 in the step of adjusting the surfaces.
<Example 4>
[0122] A surface-treated steel sheet was obtained in the same manner as in Example 2 but
dip-treating the steel sheet at 60°C for 3 seconds in the step of adjusting the surfaces.
<Example 5>
[0123] A surface-treated steel sheet was obtained in the same manner as in Example 3 but
flowing the electric current at a density of 10 A/dm
2 and repeating the cycle 4 times in the step of forming the coating.
<Example 6>
[0124] A surface-treated steel sheet was obtained in the same manner as in Example 5 but
preparing an aqueous solution for adjusting the surfaces having a pH of 11.0 and an
electric conductivity of 7.13 mS/cm by adding ammonia to an aqueous solution containing
calcium lactate in an amount of 0.1 mol/l, and conducting the step of adjusting the
surfaces by repeating two times the cycle of cathodic electrolysis at a liquid temperature
of 40°C and flowing the electric current at a density of 2 A/dm
2.
<Example 7>
[0125] A surface-treated steel sheet was obtained in the same manner as in Example 6 but
conducting the cathodic electrolysis by flowing a current at a density of 7 A/dm
2 in the step of adjusting the surfaces.
<Example 8>
[0126] A surface-treated steel sheet was obtained in the same manner as in Example 6 but
spray-treating the steel sheet for 3 seconds in the step of adjusting the surfaces.
<Example 9>
[0127] A surface-treated steel sheet was obtained in the same manner as in Example 2 but
repeating the cycle 8 times in the step of forming the coating.
<Example 10>
[0128] A surface-treated steel sheet was obtained in the same manner as in Example 9 but
flowing the electric current at a density of 4 A/dm
2 and repeating the cycle 4 times in the step of adjusting the surfaces.
<Example 11>
[0129] A surface-treated steel sheet was obtained in the same manner as in Example 10 but
flowing the electric current at a density of 6.5 A/dm
2 in the step of adjusting the surfaces.
<Example 12>
[0130] A surface-treated steel sheet was obtained in the same manner as in Example 11 but
preparing an aqueous solution for adjusting the surfaces having an electric conductivity
of 15.1 mS/cm and a pH of 5.61 by using a magnesium nitrate hexahydrate in an amount
of 0.1 mol/l, and conducting the step of adjusting the surfaces by repeating two times
the cycle of cathodic electrolysis at a liquid temperature of 40°C and flowing the
electric current at a density of 4 A/dm
2.
<Example 13>
[0131] A surface treated steel sheet was obtained in the same manner as in Example 10 but
repeating the cycle 12 times in the step of forming the coating.
<Example 14>
[0132] A surface treated steel sheet was obtained in the same manner as in Example 11 but
repeating the cycle 12 times in the step of forming the coating.
<Example 15>
[0133] A surface-treated steel sheet was obtained in the same manner as in Example 13 but
using an aqueous solution containing calcium nitrate in an amount of 0.1 mol/l and
having a pH of 5.6 as the aqueous solution for adjusting the surfaces and repeating
two times the cycle of cathodic electrolysis at a liquid temperature of 40°C and flowing
the electric current at a density of 4 A/dm
2 in the step of adjusting the surfaces.
<Comparative Example 1>
[0134] A surface-treated steel sheet was obtained in the same manner as in Example 1 but
without conducting the step of adjusting the surfaces.
<Comparative Example 2>
[0135] A surface-treated steel sheet was obtained in the same manner as in Example 2 but
without conducting the step of adjusting the surfaces.
<Comparative Example 3>
[0136] A surface-treated steel sheet was obtained in the same manner as in Example 5 but
without conducting the step of adjusting the surfaces.
<Comparative Example 4>
[0137] A surface-treated steel sheet was obtained in the same manner as in Example 9 but
without conducting the step of adjusting the surfaces.
<Comparative Example 5>
(Consideration)
[0139] As will be obvious from Table 1, in Examples 1 to 15, the amount of Zr in the coating
was set to be 12 to 182 mg/m
2, and the steel sheets were treated with an aqueous solution containing an element
of the Group II in the step of adjusting the surfaces to obtain the steel sheets containing
F in amounts of 0.4 to 19.8 mg/m
2 in the coating. In Comparative Examples 1 to 3 in which no step was conducted to
adjust the surfaces, the steel sheets were satisfactory in regard to the resistance
against the elution of F but were poor in regard to the cross-cut corrosion resistance
and the close adhesion. As the amount of Zr increases, these properties are improved
but the resistance against the elution of F decreases. The organic resin-coated metal
sheets obtained from the materials that were subjected to the step of adjusting the
surfaces of Examples 1 to 15, exhibited excellent cross-cut corrosion resistance,
excellent adhesiveness on the inner surface of the metal cans, excellent resistance
against the elution of F, and high degree of adhesion of the organic resin layer.
Even in case the organic resin layer was cracked after the working for forming cans
and the retort-treatment, it was confirmed that the organic resin layer remained closely
adhered, and the containers excellently maintained the quality of the contents.
[0140] In Examples 1 to 15, further, the reduction ratio of F was small if the amount of
Zr in the coating was large. Specifically, the reduction ratio of F was not more than
30% if the amount of Zr was not less than 100 mg/m
2. That is, it was learned that even if the reduction ratio of F was small, the steel
sheets could be obtained having excellent corrosion resistance, close adhesion and
resistance against the elution of F.
[0141] As will be obvious from Table 1, further, in Examples 1 to 15, the amounts of AE
were 7.7 to 141 mg/m
2 whereas in Comparative Examples, the amounts of AE were 1.4 to 5.1 mg/m
2. In Comparative Examples, no step was conducted for adjusting the surfaces, and no
element of the Group II was intentionally added to the aqueous solution or to the
washing water in the step of forming the coating. It is considered that the elements
of the Group II are stemming from Ca and Mg that were unavoidably contained as impurities
in the aqueous solution or in the washing water in the step of forming the coating.
Giving attention to the AE/Zr ratios in Table 1, the materials of Examples 1 to 15
that exhibited favorable properties all possessed the AE/Zr ratios of not less than
0.2 whereas the materials of Comparative Examples 1 to 5 failing to satisfy properties
all possessed the AE/Zr ratios of less than 0.2. Therefore, use of the AE/Zr ratio
makes it possible to distinguish Ca ad Mg that are unavoidably contained as impurities.