Note: Descriptions are shown in the official language in which they were submitted.
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METHOD FOR IMPROVING A METAL COATING ON A STEEL STRIP
The invention concerns a method to improve a metal coating on a steel strip or
steel sheet
according to the preamble of Claim 1 and an apparatus to apply a metal coating
on a steel strip,
in particular a strip tin-plating unit, according to the preamble of Claim 10.
In the production of galvanically coated steel strips, for example, in the
production of
tinplate, a method is known to increase the corrosion resistance of the
coating by a melting of the
coating according to the galvanic coating process. To this end, the coating
galvanically deposited
on the steel strip is heated to a temperature above the melting point of the
coating material and
subsequently quenched in a water bath. By the melting of the coating, the
surface of the coating
receives a shiny appearance and the porosity of the coating is reduced,
wherein its corrosion
resistance is increased and its permeability for aggressive substances, in
particular organic acid,
is decreased.
The melting of the coating can, for example, take place by inductive heating
of the coated
steel strip. From DE 1 186 158-A, an arrangement for the inductive heating of
metal strips for
the melting, in particular, of electrolytically applied coatings, such as tin
layer on steel strips, is
known. This arrangement has several rollers, over which the coated strip is
conducted, and
several induction coils that are arranged one behind the other in groups and
comprising a moving
strip, with which the coated strip is inductively heated to temperatures above
the melting
temperature of the coating material so as to melt the coating. In order to
make it possible for the
melting temperature to be reached uniformly over the entire width of the
strip, additional
inductors with heating conductors acting in lines are placed on the strip
edges of the coated strip.
This measure is to prevent the temperature of the coated strip being raised by
the induction coils
to temperatures far above the melting temperature of the coating material, so
that the coating will
be heated uniformly over the entire width of the strip. In this way, in turn,
the formation of an
alloy intermediate layer is to be avoided, which is composed of iron atoms and
atoms of the
coating material, for example, tin.
With the known methods for the melting of metal layers on steel strips or
sheets, the
entire steel strip or sheet, including the applied coating, is, as a rule,
heated to temperatures
above the melting temperature of the coating material and subsequently again
cooled, for
example, in a water bath, to normal temperature. For this purpose, there is a
considerable energy
requirement.
Proceeding from this, the goal of the invention is to indicate a method and an
apparatus to
improve a metal coating on a steel strip or sheet, which, in comparison to the
known methods
and apparatuses, make possible a substantially more energy-efficient treatment
of the coated
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steel strip. The method and the apparatus should also attain an increased
corrosion stability of the
coating treated in accordance with the invention, even with thin coating
layers.
These goals are attained with a method with the features of Claim 1 and with
an
apparatus with the features of Claim 10. Preferred embodiments of the method
and the apparatus
in accordance with the invention are indicated in the subclaims.
With the method in accordance with the invention, the metal coating is
appropriately
melted over its entire thickness by heating to a temperature above the melting
temperature of the
coating material, wherein the heating is carried out by electromagnetic
induction by means of an
induction furnace with at least one induction coil or one inductor. The
maximum temperature of
the coating thereby attained is designated below as the maximum temperature.
After the
inductive heating, the temperature of the coating is held for a holding time
at a temperature
above the melting temperature of the coating material before the coated steel
strip is quenched in
a cooling device at a quenching temperature below the melting temperature. The
time period in
which the temperature of the coating is above the melting temperature of the
coating material is
regarded as the holding time. By moving at least one of the induction coils
relative to the cooling
device, the holding time is thereby adapted to the other process parameters,
in particular, the
maximum temperature, the strip speed, and the thickness of the coating, so as
to completely melt
the coating over its entire thickness down to the boundary layer with the
steel strip. In this way,
the process parameters can be coordinated with one another, so that the
coating (in an essentially
precise manner) is melted over its entire thickness down to the boundary layer
with the steel
strip, without the underlying steel strip being substantially heated. The
movement of at least one
of the induction coils relative to the cooling device provided in accordance
with the invention
makes possible thereby the adaptation of the holding time to the strip speed
(specified by the
production process in the galvanic coating method) and the thickness of the
coating applied in
the coating method. The latter is appropriately recorded by suitable thickness
sensors at the end
of the coating device. The holding times that are preferably maintained are in
the range of
150 ms to 800 ms with the typical strip speeds of strip tin-plating units
(which move between
300 m/min and 700 m/min). In order not to worsen the deformability of the
strip, it is preferable
that the holding time be set as low as possible (however, without thereby
setting the maximum
temperature to values above 360 C).
The energy input produced by the electromagnetic induction preferably takes
place into
the melting coating and into the uppermost layers of the underlying steel
strip in the method in
accordance with the invention. The penetration depth of the induction current
can be controlled
thereby via the operating frequency of the induction coil or the inductor. The
range of the
frequencies that can be used with the required induction performances is
thereby in the high
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frequency range (50 kHz to 1 MHz), wherein frequencies are preferably around
150 kHz to attain
penetration depths in the range 10 to 100 gm.
It has been shown that the coated steel strips have particularly good values
for their
corrosion resistance if the metal coating is inductively heated to a maximum
temperature of more
than 310 C so as to melt the coating over the holding time. The range from 310
C to 360 C has
proved to be particularly advantageous, and the range from 320 to 350 is
particularly preferred
for the maximum temperature. With a heating to temperatures above 360 C, the
deformability of
the strips or sheets treated in accordance with the invention worsens as a
result of a reduction of
the yield strength.
By comparative experiments, it was surprisingly possible to show that in
maintaining a
maximum temperature of more than 310 C, essentially independent of the
selected holding time,
an alloy layer that is thin (in comparison to the thickness of the coating)
and that consists of iron
atoms and atoms of the coating material is formed on the boundary layer
between the coating and
the steel strip or the steel sheet, if the coating is completely melted over
its entire thickness down
to the boundary layer with the steel strip. With tin-plated steel strips
(tinplate), therefore, a very
thin iron-tin alloy layer (FeSn2) is formed, for example, on the boundary
layer of the tin coating
with the steel.
By measuring the ATC value ("Alloy Tin Couple" value), which, as an
electrochemical
test, is a measure for the porosity of the alloy layer, it was determined that
the alloy layer formed
by the inductive melting has a lower porosity and a substantially higher
density in comparison to
the alloy layers that result during the traditional process operation (that
is, the melting of the
coating in an annealing furnace, for example, by electrical resistance heating
at temperatures just
above the tin melting temperatures of 232 C). Therefore, it is suspected that
this thin and low-
pore alloy layer influences the corrosion stability in a particularly positive
manner. The method
in accordance with Claim 2 is therefore regarded as a stand-alone invention,
independent of the
features of the characterizing part of Claim 1.
The method parameters for the inductive melting of the coating, in particular,
the
maximum temperature and the holding time, are appropriately selected and
adapted to the strip
speed and the thickness of the coating in such a manner that only one part of
the coating is
alloyed with the iron atoms of the steel strip or the steel sheet and,
therefore, after the melting,
still unalloyed coating and, underneath, a thin alloy layer are present.
Depending on the selected
process parameters, the thickness of the alloy layer thereby corresponds to
approximately a
weight per unit area or a coating of only 1.3 gim2 or less. With regard to the
corrosion stability
and the formability, alloy layers that are thinner than 1.0 g/m2 have proved
to be particularly
suitable, and alloy layers with a thickness in the range of 0.05 to 0.6 g/m2
have proved to be
particularly preferred. With thicker alloy layers, corresponding to a coating
of more than
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1.3 g/m2, the formability of the coated steel sheet worsens, for example, for
the production of
cans for beverages or food.
With the method in accordance with the invention, it is possible to ensure
that, for
example, in the tin-plating of steel sheets, even those with thin total tin
coatings of 1.0 g/m2 or
less, a thin and, at the same time, essentially pore-free and thus very dense
alloy layer with an
optically attractive (that is, shiny) coating surface is attained. The alloy
layer, which, in
comparison to the thickness of the coating, is very thin and at the same time
dense, leads to an
increased corrosion resistance of the coated steel and to an improved adhesion
of the coating on
the steel strip or sheet. In accordance with the invention, this is made
possible in that the process
parameters can be adapted to one another during the melting of the coating, so
as to undertake a
purposeful adjustment of the thickness of the alloy layer forming during the
melting of the
coating. In particular, with the method in accordance with the invention,
according to Claim 1,
the thickness of the forming alloy layer is decoupled from the distance
between the melting
device and the cooling device, which has been firmly established in the method
up to now. In the
method in accordance with the invention, on the other hand, the distance from
the induction coil
to the cooling device can be appropriately adjusted continuously so as to
adjust the holding time
to the desired value. Via an adaptation of the holding time to the other
process parameters, such
as the maximum temperature and the thickness of the coating deposited on the
steel strip, it is
finally possible to purposefully control the thickness of the alloy layer and
thus, ultimately, the
material characteristics of the coated steel strip, such as its corrosion
resistance and formability.
The best results can thereby be attained if the maximum temperature was
established at values
between 310 C and 360 C and the holding time, between 0.1 sand 1.0 s, and
preferably between
0.2 s and 0.3 s.
The goal of the invention is, furthermore, attained with an apparatus to apply
a metal
coating on a steel strip. In the apparatus, an endless steel strip is moved at
a strip speed in the
movement direction of the strip and is electrolytically provided with a metal
coating in a coating
device. The apparatus can be, in particular, a strip tin-plating unit with an
electrolytic coating
device in which the steel strip is moved through a tin-containing electrolyte
at the strip speed, so
as to deposit a tin layer on the steel strip. In the movement direction of the
strip, a melting device
in which the coating is melted by inductive heating at a maximum temperature
above the melting
temperature of the material of the coating comes subsequent to the coating
device. A cooling
device in which the coating steel strip is cooled to a quenching temperature
below the melting
temperature follows the melting device in the movement direction of the strip.
In accordance
with the invention, the melting device can move, relative to the cooling
device, so as to be able
to adjust the distance between the melting device and the cooling time to a
desired value in the
movement direction of the strip.
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For this purpose, the melting device comprises at least one induction coil
arranged so it
can move in the movement direction of the strip. In addition to this movable
induction coil, the
melting device can also contain additional induction coils, which are arranged
one behind the
other in the movement direction of the strip. These additional induction coils
can be thereby
fixed in situ relative to the cooling device or can also be movable.
Appropriately, however, in an
arrangement of several induction coils connected one behind the other, at
least the last induction
coil, which is next to the cooling device, or the entire coil device are
designed so they can move.
With the induction coil(s), the coated steel strip can be heated inductively
to the
maximum temperature at adjustable heating rates. For the purpose, heating
rates between
600 K/s and 1300 K/s, and preferably between 900 K/s and 1100 K/s, have proved
to be
appropriate.
The cooling device can be a quenching tank, filled with a cooling liquid, for
example,
water. However, another cooling device, for example, blower cooling or gas
cooling, in
particular, an air cooling, can also be used.
The invention is explained in more detail below with the aid of an embodiment
example,
with reference to the accompanying figures. The figures show the following:
Figure 1: schematic representation of an apparatus for the application of a
metal coating
on a steel strip;
Figure 2: schematic representation of the melting device and the cooling
device of the
apparatus of Figure 1;
Figure 3: perspective representation of the movable melting device of the
apparatus of
Figure 1.
The apparatus shown schematically in Figure 1 is, for example, a strip tin-
plating unit
with a coating device, in which a tin coating is deposited on a fine or very
fine sheet, in which
the steel strip is conducted through a tin-containing electrolyte at a strip
speed vB. The
application area of the invention, however, is not limited to this embodiment
example. The
invention can also be used appropriately, for example, in methods for the
electrolytic coating of
steel strips with other metals, such as zinc, so as to produce a so-called
special, very fine, zinc-
plated sheet. The use of the method in accordance with the invention is also
not limited to the
coating of steel strips in strip zinc-plating units, but rather can also be
appropriately used, for
example, in the immersion coating of strip sheets in the form of tablets, in
which the metal
coating is not applied electrolytically on the steel strip.
The strip tin-plating unit for the electrolytic tin-plating, shown
schematically in Figure 1,
comprises a decoiler group 10, in which a steel strip, cold-rolled to form a
fine or very fine sheet,
is drawn off from a roll (coil) and is welded together, in a welding device
11, to form an endless
steel strip. The endless strip is conducted in a loop tower 12 in order to
form a supply of strips.
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The supply of strips held by the loop tower 12 also makes possible a
continuous passage of the
strip through the strip tin-plating unit at a prespecified strip speed during
the necessary idle times
in the welding together or, later, during the separation of the coated steel
strip and the rolling
onto wound coils. The loop tower 12 is followed by a pretreatment device 13
and a coating
device 4. In the pretreatment device 13, there is a cleaning and degreasing of
the steel strip
surface, which is described in more detail below, and in the coating device,
4, the strip that is
moving through the strip tin-plating unit at the strip speed (vB) is conducted
through a tin-
containing electrolyte so as to deposit a tin layer on the steel strip. The
coating device 4 is
followed in the movement direction of the strip by a melting device 5, in
which the coating
deposited on the steel strip is heated to temperatures above the melting
temperature of the
coating material (with tin, this is 232 C), so as to melt the deposited
coating. The melting device
is followed by a cooling device 3 and a post-treatment device 14 and a second
loop tower 15.
Finally, the coated steel strip is wound, in a winding group 16, on rollers
(coils).
The still uncoated steel strip coming from the first loop tower 12 is first
subjected to a
pretreatment in the pretreatment device 13 before it is provided with a tin
layer in the coating
device 4. In the pretreatment device 13, the uncoated steel strip is first
degreased and then
pickled. In addition, the still uncoated steel strip is conducted through an
alkaline degreasing
bath, for example, a sodium carbonate or sodium hydroxide solution at the
strip speed (vB). The
degreasing bath was freed at regular intervals of soiling that was produced by
the introduction of
grease and iron wear. It was shown that for the subsequent carrying out of the
improving method
in accordance with the invention, a sufficient cleanliness of the degreasing
bath is present; if the
bath murkiness (bath extinction) of the degreasing bath has an extinction
value of < 1 (according
to the Lambert-Beer Law, corresponding to a light weakening of less than
factor 10), with an
optical measurement with light with a wavelength of 535 nm.
After the degreasing, a first rinsing takes place with a rinsing liquid and
subsequently, the
steel strip is pickled in an acidic solution, for example, in a sulfuric acid
solution, and rinsed
once again. For the subsequent carrying out of the improving method in
accordance with the
invention, it is appropriate to rinse the steel strip after the degreasing and
pickling with a rinsing
liquid, which preferably has a conductivity of < 20 p.S/cm.
In the coating device 4, which follows the pretreatment device 13, the
degreased and
pickled steel strip is conducted through a tin-containing electrolyte bath and
is connected there as
a cathode and conducted through between two rows of tin anodes. In this way,
the tin of the
anodes is dissolved and deposited on the steel strip as a tin coating. The tin
can be thereby
applied in any thickness and, if required, on both sides of the steel strip.
The thickness of the
applied tin layer is regularly between 1.0 g/m2 and 5.6 g/m2. However, a
coating of the steel strip
with thinner or with thicker tin layers is also possible.
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To increase the corrosion resistance of the coated steel strip, it is
subjected to an
improving method in accordance with the invention after the coating process in
the coating
device 4. The improving method is carried out in the melting device 5 and the
cooling device 3,
which follows it in the movement direction of the strip. The details of the
improving method in
accordance with the invention and the devices used for the purpose are
described in detail below
with reference to Figures 2 and 3.
Figure 2 schematically shows the melting device 5 and the cooling device 3,
which
follows in the movement direction of the strip. The moved steel strip is moved
at the strip speed
over deflection rollers 19 and conducted into the melting device 5 and, from
there, into the
cooling device 3. The moved steel strip essentially moves between the melting
device 5 and the
cooling device 3 in a vertical direction from top to bottom, as shown in
Figure 2. The melting
device 5 is an induction furnace with at least one induction coil 2. The
induction furnace can also
comprise several induction coils or inductors, arranged one behind the other
in the movement
direction of the strip. The assumption below is that the induction furnace
contains only one
induction coil 2. The induction coil 2 is impinged on by an electric
alternating current, preferably
in the high frequency range (50 kHz to 30 MHz), and the coated steel strip 1
is moved through
the induction coil 2 at the strip speed (vB). In this way, alternating
currents are induced in the
coated steel strip that heat the coated steel strip. In order to melt the
coating applied on the steel
strip, the coated steel strip is heated in the induction furnace to
temperatures above the melting
temperature of the coating material T9; this is 232 C with tin). The maximum
temperature
thereby attained is designated as the maximum temperature (peak metal
temperature, PMT). It
has been shown that for the execution of the improving method in accordance
with the invention,
maximum temperatures that are higher than 310 C are to be preferred, and are
preferably in the
range between 320 C and 350 C. The maximum temperature can be controlled by
the output of
the induction coil 2. The penetration depth of the induction current produced
by the
electromagnetic induction into the surface of the coated steel strip can be
controlled by the
frequency of the electromagnetic alternating current with which the induction
coil 2 is impinged.
The outputs of the induction coil 2 required for the carrying out of the
improving method in
accordance with the invention are in the range of 1500 to 2500 kW.
With the induction furnace, the coated steel strip can be heated to
temperatures above the
melting temperature T, of the coating material at heating rates between 600
K/s and 1300 K/s.
The heating rates of the induction furnace are appropriately set between 900
K/s and 1100 K/s.
The melting device 5 (induction furnace) or the induction coil 2 extends in
the movement
direction of the strip, between the coil inlet 2a and the coil outlet 2b, over
a length L, which is
appropriately in the range from 2 to 3 m. This length L represents the
effective heating zone in
which the coated steel strip is heated in the melting device 5.
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A cooling device 3 follows the melting device 5 in the movement direction of
the strip
and at a distance to the melting device 5. In the embodiment example shown
here graphically,
the cooling device 3 comprises a quenching tank 6 filled with a cooling
liquid. Another
deflection roller 19 is located in the quenching tank 6; the quenched steel
strip is conducted out
of the cooling device 3 by means of this deflection roller. The liquid level
of the cooling liquid is
designated, in Figure 2, with the reference symbol 7. On the stretch between
the rinsing outlet 2b
and the liquid level 7, the melted coating is slightly cooled by heat
conduction and convection
between the melting device 5 and the cooling device 3. Since the coating,
however, was heated
to temperatures far above the melting temperature T, in the melting device 5,
the melted coating
still remains in a melted state on its way between the melting device 5 and
the cooling device.
The time over which a prespecified point on the strip traverses between the
rinsing outlet 2b and
the liquid level 7 of the cooling liquid is determined by the distance D
between the rinsing outlet
2b and the liquid level 7 and the strip speed (vB), and is calculated as tH =
D/vB. This time period
tH is designated below as the holding time.
If the strip is immersed in the cooling liquid, there is a rapid quenching of
the strip heated
in the melting device 5 to the temperature of the cooling liquid, which, as a
rule, is in the area of
the room temperature. By the melting and rapid quenching of the coating, a
shiny surface of the
coated strip is produced. Furthermore, the adhesive capacity of the applied
coating on the steel
strip is increased by the melting and the rapid quenching.
In accordance with the invention, provision is then made so that the entire
melting device
or at least one induction coil 2, located therein, can be moved relative to
the cooling device 5
so as to be able to set the distance D between the rinsing outlet 2b and the
inlet of the cooling
device 3, in particular the liquid level 7, at a desired value suitable for
carrying out the method in
accordance with the invention. To this end, the entire melting device 5, or at
least its induction
coil 2, is arranged so it can move in a frame 8, as shown in Figure 3.
Appropriately, the entire
melting device 5 is arranged on the frame 8 so that it can be moved
continuously in the
movement direction of the strip. When using a melting device 5 with an
induction coil series
(consisting of a plurality of induction coils that are appropriately arranged,
one behind the other,
in the movement direction of the strip), at least the induction coil that is
last seen in the
movement direction of the strip (that is, the induction coil that is adjacent
to the cooling device
3) is to be designed so that it can be moved in the movement direction of the
strip, so as to be
able to set its distance to the adjacent cooling device 3 at a suitable value.
The suitable distance
between the melting device 5 or the (last) induction coil of an induction
rinsing series is thereby
determined so that the coating is melted just so over its entire thickness
down to the boundary
layer with the steel strip without thereby introducing (by the electromagnetic
induction) excess
energy into the coating.
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Figure 3 shows the frame 8 with the melting device 5 (induction furnace)
arranged
thereon. The melting device 5 thereby comprises a housing 9, in which the
induction coil 2 is
located. The housing 9 is located on the frame 8 over sliding tracks so that
it can move between
an upper end position 2c and a lower end position 2d. The movement of the
frame 9
appropriately takes place via a motor drive.
With this arrangement, it is now possible to adapt the holding time after the
melting of
the coating to the quenching of the melted coating in the cooling device 3 to
the other process
parameters, such as the maximum temperature, the strip speed, and the
thickness of the coating
applied in the coating device 4. In this way, it is possible to set the
aforementioned process
parameters and the holding time so that the coating is melted under defined
conditions. It is
possible, in particular, for the coating to be melted (right) over its entire
thickness down to the
boundary layer with the steel strip. It has been shown that a melting of the
coating down to the
boundary layer with the steel strip is very advantageous, because,
simultaneously, a very dense
and thin, in comparison to the thickness of the coating, alloy layer is formed
thereby on the
boundary layer between the coating and the steel strip. This alloy layer
consists of iron atoms of
the steel strip and atoms of the coating material (that is, for example, with
a tin coating
consisting of tin and iron atoms, in the FeSn2 stoichiometry). The formation
of this alloy
intermediate layer has a considerable effect on the characteristics of the
coated steel strip. In
particular, the formation of the alloy layer increases the corrosion
resistance of the coated steel
strip and improves the adhesion of the coating to the steel strip.
By comparative experiments, it was possible to determine that with the
improving
method in accordance with the invention, especially if the maximum temperature
is higher than
310 C, a particularly stable and dense alloy layer is formed. By measuring the
ATC value, it was
possible to determine that this alloy layer is particularly low-pore and thus
dense in comparison
to the intermediate layers formed with a traditional method operation. This
dense alloy layer with
a lower porosity leads to an improved corrosion stability of the coated steel
strip.
For comparison purposes, tinplates produced according to traditional methods
were
compared to tinplates which were improved with the method in accordance with
the invention.
To this end, tinplates coated with a tin coating of 2.0 to 8.6 g/m2 were
treated in accordance with
the invention, wherein in one embodiment example, a heating rate of 963 C/s
and a maximum
temperature (PMT) of 330 C were established in the inductive melting of the
coating. The
distance of the movable melting device to the cooling device was set at D =
3.9 m and the strip
was moved at a strip speed of 700 m/min through the strip tin-plating unit. An
alloy layer with a
layer thickness was thereby produced; it corresponds to a coating of 0.8 g/m2.
The tinplate thus
produced was tested with the standardized ATC method with regard to its
corrosion resistance
and compared to the traditionally produced tinplate. A traditionally produced
tinplate has typical
CA 02858292 2014-06-05
values of 0.12 ttA/cm2 or more for the ATC value ("Alloy Tin Couple" value).
The tinplates
treated in accordance with the invention, on the other hand, have
substantially lower ATC values
of less than 0.08 [tA/cm2. With the improving method in accordance with the
invention, it was
even possible to produce tinplates that now have ATC values of merely 0.04
A/em2. By
comparative experiments, it was possible to determine that such low ATC values
can be attained
especially if the maximum temperature (PMT) is above 310 C.
