U.S. patent application number 14/373448 was filed with the patent office on 2015-01-01 for method for improving a metal coating on a steel strip.
The applicant listed for this patent is THYSSENKRUPP RASSELSTEIN GMBH. Invention is credited to Dirk Matusch, Helmut Oberhoffer, Markus Opper, Thomas Rainer, Reiner Sauer.
Application Number | 20150001089 14/373448 |
Document ID | / |
Family ID | 47630278 |
Filed Date | 2015-01-01 |
United States Patent
Application |
20150001089 |
Kind Code |
A1 |
Matusch; Dirk ; et
al. |
January 1, 2015 |
METHOD FOR IMPROVING A METAL COATING ON A STEEL STRIP
Abstract
A method for improving a metal coating on a steel strip or a
steel sheet or plate. The coating is melted to a maximum
temperature above the melting temperature of the material of the
coating by inductive heating performed by at least one induction
coil and subsequently cooled to a quenching temperature, below the
melting temperature, in a cooling device. In order to improve the
corrosion stability of the coating, even in the case of thin
coating layers, the coating is kept at a temperature above the
melting temperature during a holding time and the holding time is
adapted to the maximum temperature and the thickness of the coating
by moving at least one of the induction coils with respect to the
cooling device, in order to melt the coating completely over its
entire thickness to the boundary layer with the steel strip.
Inventors: |
Matusch; Dirk; (Neuwied,
DE) ; Sauer; Reiner; (Neuwied, DE) ;
Oberhoffer; Helmut; (St. Johann, DE) ; Rainer;
Thomas; (Kletten, DE) ; Opper; Markus;
(Neuwied, DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
THYSSENKRUPP RASSELSTEIN GMBH |
Andernach |
|
DE |
|
|
Family ID: |
47630278 |
Appl. No.: |
14/373448 |
Filed: |
January 22, 2013 |
PCT Filed: |
January 22, 2013 |
PCT NO: |
PCT/EP2013/051077 |
371 Date: |
July 21, 2014 |
Current U.S.
Class: |
205/154 ;
204/274 |
Current CPC
Class: |
C25D 5/505 20130101;
Y02P 10/253 20151101; C23C 2/40 20130101; C23C 2/405 20130101; C23C
2/08 20130101; C23C 2/285 20130101; C23C 2/28 20130101; Y02P 10/25
20151101; H05B 6/104 20130101; C21D 9/60 20130101 |
Class at
Publication: |
205/154 ;
204/274 |
International
Class: |
C25D 5/50 20060101
C25D005/50 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 23, 2012 |
DE |
10 2012 100 509.5 |
Claims
1. Method for improving a metal coating on a steel strip or steel
sheet, wherein the coating is melted by inductive heating, with at
least one induction coil, to a maximum temperature above the
melting temperature of the material of the coating, and is
subsequently cooled, in a cooling device, to a quenching
temperature below the melting temperature, wherein the coating is
held during a holding time at a temperature above the melting
temperature and that the holding time is adapted to the maximum
temperature and the thickness of the coating by moving at least one
of the induction coils relative to the cooling device so as to
completely melt the coating over its entire thickness down to the
boundary layer with the steel strip.
2. Method according to the preamble of claim 1, wherein the maximum
temperature is higher than 310.degree. C. and that the coating is
completely melted over its entire thickness down to the boundary
layer with the steel strip.
3. Method according to claim 1, wherein the maximum temperature is
between 310.degree. C. and 360.degree. C., and preferably between
320.degree. C. and 350.degree. C.
4. Method according to claim 1, wherein the heating rate of the
inductive heating is between 600 K/s and 1300 K/s, and preferably
between 900 K/s and 1100 K/s.
5. Method according to claim 1, wherein the coated steel strip is
moved at a strip speed relative to the induction coil.
6. Method according to claim 1, wherein the distance of the
induction coil to the cooling device can be adjusted continuously,
so as to set the holding time at a desired value.
7. Method according to claim 1, wherein the holding time is between
0.1 s and 1.0 s, and preferably between 0.2 s and 0.3 s.
8. Method according to claim 1, wherein a thin alloy layer, which
essentially consists of iron atoms and atoms of the coating
material, is formed on the boundary layer between the coating and
the steel strip.
9. Method according to claim 7, wherein the alloy layer is thinner
than 1.3 g/m.sup.2, and preferably thinner than 1.0 g/m.sup.2.
10. Apparatus for the application of a metal coating on a steel
strip, in particular, a strip tin-plating unit, in which a
continuous steel strip is moved at a strip speed in a movement
direction of the strip and is electrolytically provided by a
coating device with a metal coating, wherein a melting device
follows the coating device in the movement direction of the strip,
and in the melting device, the coating is melted by inductive
heating at a maximum temperature above the melting temperature of
the material of the coating, and a cooling device follows the
melting device, and in the cooling device, the coated steel strip
is quenched to a quenching temperature that is below the melting
temperature, wherein the melting device can be moved relative to
the cooling device so as to set the distance between the melting
device and the cooling device in the movement direction of the
strip.
11. Apparatus according to claim 10, wherein the melting device
contains at least one induction coil arranged so it can move in the
movement direction of the strip.
12. Apparatus according to claim 11, wherein the melting device
contains a plurality of induction coils arranged one behind the
other in the movement direction of the strip, wherein at least the
last induction coil, which is closest to the cooling device, can
move relative to the cooling device.
13. Apparatus according to claim 10, wherein the cooling device
comprises a quenching tank filled with a cooling liquid.
Description
[0001] 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.
[0002] 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.
[0003] 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.
[0004] 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.
[0005] 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 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.
[0006] 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.
[0007] 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.degree. C.).
[0008] 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 frequency range (50 kHz to 1
MHz), wherein frequencies are preferably around 150 kHz to attain
penetration depths in the range 10 to 100 .mu.m.
[0009] 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.degree. C. so as to melt the coating over the holding
time. The range from 310.degree. C. to 360.degree. C. has proved to
be particularly advantageous, and the range from 320.degree. to
350.degree. is particularly preferred for the maximum temperature.
With a heating to temperatures above 360.degree. 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.
[0010] By comparative experiments, it was surprisingly possible to
show that in maintaining a maximum temperature of more than
310.degree. 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
(FeSn.sub.2) is formed, for example, on the boundary layer of the
tin coating with the steel.
[0011] 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.degree. 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.
[0012] 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 g/m.sup.2 or less. With regard to the
corrosion stability and the formability, alloy layers that are
thinner than 1.0 g/m.sup.2 have proved to be particularly suitable,
and alloy layers with a thickness in the range of 0.05 to 0.6
g/m.sup.2 have proved to be particularly preferred. With thicker
alloy layers, corresponding to a coating of more than 1.3
g/m.sup.2, the formability of the coated steel sheet worsens, for
example, for the production of cans for beverages or food.
[0013] 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/m.sup.2 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.degree.
C. and 360.degree. C. and the holding time, between 0.1 s and 1.0
s, and preferably between 0.2 s and 0.3 s.
[0014] 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.
[0015] 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.
[0016] 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.
[0017] 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.
[0018] 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:
[0019] FIG. 1: schematic representation of an apparatus for the
application of a metal coating on a steel strip;
[0020] FIG. 2: schematic representation of the melting device and
the cooling device of the apparatus of FIG. 1;
[0021] FIG. 3: perspective representation of the movable melting
device of the apparatus of FIG. 1.
[0022] The apparatus shown schematically in FIG. 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 v.sub.B. 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.
[0023] The strip tin-plating unit for the electrolytic tin-plating,
shown schematically in FIG. 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. 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
(v.sub.B) 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.degree. C.), so as to melt the deposited
coating. The melting device 5 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).
[0024] 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 (v.sub.B). 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.
[0025] 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 .mu.S/cm.
[0026] 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/m.sup.2 and 5.6 g/m.sup.2. However, a coating of the steel strip
with thinner or with thicker tin layers is also possible.
[0027] 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 FIGS. 2 and 3.
[0028] FIG. 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
FIG. 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 (v.sub.B). 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 T.sub.s; this
is 232.degree. 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.degree. C. are to be
preferred, and are preferably in the range between 320.degree. C.
and 350.degree. 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.
[0029] With the induction furnace, the coated steel strip can be
heated to temperatures above the melting temperature T.sub.s 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.
[0030] 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.
[0031] 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 FIG. 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.sub.s 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
(v.sub.B), and is calculated as t.sub.H=D/v.sub.B. This time period
t.sub.H is designated below as the holding time.
[0032] 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.
[0033] In accordance with the invention, provision is then made so
that the entire melting device 5 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 FIG. 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.
[0034] FIG. 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.
[0035] 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 FeSn.sub.2
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.
[0036] 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.degree.
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.
[0037] 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/m.sup.2 were
treated in accordance with the invention, wherein in one embodiment
example, a heating rate of 963.degree. C./s and a maximum
temperature (PMT) of 330.degree. 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/m.sup.2. 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 values of 0.12 .mu.A/cm.sup.2 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 .mu.A/cm.sup.2. 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 .mu.A/cm.sup.2.
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.degree. C.
* * * * *