Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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The invention relates to coating of metal strip on one side with a
molten metal, in particular but not exclusively for hot galvanizing steel
strip material.
For some years specialists have been endeavouring to develop suît-
able processes and devices for coating a metal strip on one side with molten
metal. The aim of these attempts is to obtain an even and precisely adjust-
able layer thickness. Subsequently, a process is required in which molten
metal does not reach the rear side of the strip during coating.
In a known process of this type the metal strip is conveyed in
close proximity with the level of the melting bath, for example, by means
of a supporting and deflection roller. Due to the surface tension of the
molten metal, a meniscus of the molten metal between the surface of the
bath and the metal strip once formed is maintained due to the short distance
between metal strip and bath surface. In this way the moving strip is con-
tinuously wetted by the molten metal. Control of the thickness of the coat-
ing layer is achieved by stripping excess metal while still liquid by means
of a connected jet knife (German Offenlegungsschrift 2,712,003). ~low-
ever, the required layer thicknesses are only obtained with this adjustment
means when the preceding layer applied is even over the entire width of the
strip. Even coating over the entire width of the strip can only be achieved
when the distance between the level of the bath and the strip does not fluc-
tuate. This requirement is difficult to achieve with a very short set dis-
tance, e.g.~ 10 mm. When the distance between the strip material and the
level of the bath fluctuates, e.g. due to changes in the level of the bath,
partial or complete breakup of the meniscus can result, or the molten metal
can flow over the edges or the strip thus causing undesired coating of the
rear side of the strip. There are several causes of fluctuations in the
level of the bath: consumption of the molten metal by the coating process,
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refilling of the molten metal, blasting of the bath surface by the blast ~et
of the nozzle stripper. All these difficulties can hardly be eliminated even
with precise guidance of the strip and the use of a regulation plant to con-
trol the level of the melting bath.
In another known process the strip to be coated is deflected by two
deflection rollers by 90 in each case and conveyed by them approximately
parallel to the level of the melting bath at a considerable distance therefrom.
The coating material is applied by means of a scoop roller immersed in the melt-
ing bath onto the strip in the unsupported area between the two deflection
rollers. The desired thickness of layer is set by means of a squeeze-off roll
running on the scoop roller (United States patent 3,228,788).
It is indeed possible with this process and device to coat the strip
on one side on its entire width without there being any danger of molten metal
reaching the other side of the strip, but adjustability of the thickness of
layer is not exact enough in spite of the additional squeeze-off roll. Even
slight uneveness in the strip or movement of the strip radially of the scoop
roller (shimmying) results in irregular contact of the strip and the scoop
roller and thus in uneven coating.
The object of the invention is to provide a process and apparatus for
coating a metal strip with molten metal which can achieve even coating of the
metal strip on one side and exact adjustability within wide ranges.
The invention provides a process for coating one side of a metal
strip with a molten metal from a melting bath, comprising the steps of
continuously conveying a previously cleaned and heated metal strip
through a protective gas atmosphere partially around a supporting roller and
above the level of the melting bath of the molten metal for bringing the metal
strip into contact with the molten metal,
supporting the metal strip on a rear or reverse side thereof on the
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supporting roller in the area of contact with the molten metal, and
conveying the molten metal from the melting bath towards the supported
area of the strip with a scoop roller immersed in the melting bath by more than
half its surface area but only by part, which scoop roller together with the
supporting roller forms an adjustable slot for the passage therebetween of the
metal strip.
Exact geometric relationships are produced at the point of coating,
where the strip maintains its position and shape and the coating material is
conveyed in sufficient supply to the same point essentially independently of
fluctuations in the level of the bath. Although no additional means is pro-
vided for stripping off the excess molten metal, a precisely adjustable layer
thickness can be obtained even when the molten metal is conveyed in excess,
since the metal strip squeezes off excess material due to the forced guidance
of the molten metal. The layer thickness can be set exactly within wide limits
depending on the forced guidance of the molten metal due to the scoop roller
and the support of the metal strip, and this is not only the case where large
amounts are to be applied, but due to the continuing good contact between the
surface of the strip and the scoop roller is the case even with small amounts
to be applied. The thickness of layer can be set by adjusting the pressure
between the scoop roller and the support roller.
The process according to the invention is particularly suitable for
producing so-called galvanealed material (steel strip material, in which
the molten zinc forms an alloy with the strip material), when according to
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one arrangement using zinc as the molten metal and steel as the strip mate-
rial, the molten metal has a temperature of 450C to 480C and the strip
temperature is from 530C to 650C, in particular 570, and quantity of
molten zinc applied is restricted during application to the amount necessary
for an alloy with the strip material. In addition, the danger of overflowing
the edges of the strip is avoided by maintaining these ranges in temperature
and thereby also maintaining a specific range of viscosity of the molten
metal.
In this process the increase in temperature of the coating material
necessary for the formation of the alloy is achieved by the heat of the strip
material. ~his is directly possible with the necessary low amounts of coat-
ing material to be applied. An advantage over known processes is that the
molten coating material does not need any ceramic container~ due to its low
temperature. Since the strip material is not directed through the melting
bath but comes into contact with the melting bath only indirectly, and since
the low application amount of molten coating material scooped is practically
completely applied, i.e. no excess passes back into the melting bath, the
melting bath is practically not heated further. Neither is an additional
serially connected furnace needed, as was the case previously, to heat the
costed strip material to the temperature necessary for formation of the alloy.
The strip materia] reaches the necessary temperature with the necessary
annealing prior to the coating procedure. Then cooling is less intense than
previously. With æinc as the coating material and steel as the strip mate-
rlal, temperatures of 450 C to 480 C have proved favourab]e for the melting
bath and 530C to 650 C for the strip material.
Preferably the scoop roller is immersed in the melting bath over
more than half of its periphery. It is possible with this embodiment to
convey lar~er application amounts into the nip without the scoop roller
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having its o~m drive means, since the liquid metal adhering to the scoop rol-
ler exerts considerably lower restoring moments on the scoop roller than with
a scoop roller which is only slightly immersed in the bath. Furthermore, in
the case of a scoop roller which is more than half immersed in the bath there
is less tendency for molten metal to flow back into the bath than in the case
of a scoop roller which is only slightly im~ersed.
It has been found that with a scoop roller which is immersed too
deeply in the bath there is danger of the molten metal flowing over the
edges of the strip. In order to both eliminate this danger and apply the
molten metal optimumly the scoop roller should pro~ect 10 to 45 mm, in
particular 10 to 15 mm, out of the melting bath.
Preferably the diameter of the supporting and deflection roller
should be ~ubstantially larger than the diameter of the scoop roller, in
particular about double the size thereof. It is here advantageous when the
diameter of the scoop roller is relatively small, in particular 150 to 350
mm, preferably about 250 mm.
Both the supporting and deflection roller and the scoop roller
can be mounted to be freely rotatable. In this case, they are rotated by the
strip running between them. In order to eliminate the danger of the roller
coming to a standstill, the strip can be directed round the supporting and
de~lection roller over a large angle of contact.
If a particularly thick layer is required, not only is a cor-
respondingly wide passage slot or nip set between the scoop roller and the
cupporting and deflection roller, but also the scoop roller is immersed to a
correcponding depth in the melting bath. It can be of advantage here if
the surface of the scoop roiler also has a high bonding strength for the
molten metal due to roughening or suchlike. Further, the supply of the
molten metal into the nip can be influenced by the speed of the surface of
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the scoop roller being altered by means oP a drive means. If the speed of
the surface of the scoop roller is greater than that of the strip, more
molten metal is conveyed into the nip, if the speed of the surface of the
scoop roller, however, is less than that of the strip, less molten metal is
conveyed into the nip.
A preferred embodiment of the invention is explained in more
detail below by means Or the drawing which represents the embodiment
schematically.
; A container 1 is filled with molten metal 2, e.g. zinc. A cover-
ing hood 3, which is open at the bottom, is immersed in the melting bath 2
at its lower edge. An inclined channel 4 and a vertical channel 5 lead into
said covering hood 3. The channel 4 has a sluice-type inlet 6 and the chan-
nel 5 has a sluice-type outlet 7.
A supporting and deflection roller 8 is freely rotatable above the
level of the melting bath 2 and at a short distance therefrom. A scoop
roller 9, which is substantially smaller in dia~eter than the supporting and
deflection roller 8, is arranged below the latter, the axis of the scoop
roller 9 being arranged parallel to the axis of the supporting and deflection
roller 8 and in the same vertical plane. The scoop roller 9 is immersed in
the melting bath 2 by more than half its surface area. The scoop roller 9
can be freely ~ournaled or actuated by an ad~ustable drive means.
The cleaned and heated strip material 12 to be coated runs over the
aluice-type inlet 6, the channel 4 into the area of the covering hold 3 which
is under an atmo8phere of protective gas. Here it is deflected by the sup-
porting and deflection roller 8 and passed vertically upwards through chan-
nel 5 and the sluice-type outlet 7. The strip material 12 loops the sup-
porting and deflection roller 8 over an angle of about 135C. Thereby, exact
guidance is achieved at only a short distance from the level of the bath.
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~26593
The scoop roller 9, which touches the surface of the strip material 12,
transports molten metal into the nip 10 when the strip moves. The molten
metal is applied with the desired thickness dependine on the selected dis-
tance set between the two rollers 8,9. Excess metal is thereby squeezed off
and flows back into the melting bath 2. Contrary to the known process of the
German Offenlegungsschrift 2 712 003, however, the applied layer here can be
set exactly independent of fluctuations in the level of the bath, as the
molten metal is conveyed by the scoop roller to the surface of the strip
which iB fixed in position and thus constant relationships are ensured. The
setting of the thickness of layer is also more exact than is the case with
the other known process of the United States patent 3 228 788, as contrary
thereto the application amount here is not metered before the area of
application but at the area of application itself. This means that in the
process according to the invention high quality strip material can be pro-
duced with different layer thicknesses while maintaining very close tol-
erances. Maintaining close tolerances is not only of advantage for further
processing but also means that considerable amounts of coating material are
saved.
Although the layer thickness can be set exactly with a low cost
in apparatus in the process according to the invention, ad~ustable nozzle
strippers known per se can be additionally provided. Such nozzle strippers
can be set at a height above the level of the melting bath, at a distance
from the surface Or the bath and at the angle at which the gas jet is blown
onto the bath. It has proved particularly advantageous to arrange the
~tripping nozzles at a height of lOO to 250 mm, in particular of about 150
mm, above the level of the melting bath. Since coating is carried out under
an atmosphere of protective gas, it is, of course, necessary to also operate
the nozzle strippers with a corresponding protective gas, e.g. nitrogen or
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argon.
Zinc coatings on a steel strip often show large areas of crystal
patterns (flowers of zinc), which are particularly undesirable when the
galvanized surface of the strip is placed on the visible side of a structural
part. This flower formation can be prevented very simply and effectively by
blowing a nucleus forming agent, e.g. zinc or zinc oxide dust, onto the zinc
layer when still liquid together with the gas jet of the stripping nozzles in
accordance with a preferred feature of the invention.
The molten zinc in the melting bath is covered by a floating layer
(not shown) of a substance which is insoluble in and/or not wettable by the
molten zinc. This floating layer prevents or minimizes evaporation of the
molten zinc, and may be particulate in nature, for example comprising a
single layer or several layers of graphite balls. The effect of this layer
is to improve the quality of the product. If eva~oration of the zinc is not
~revented. zinc vapor tends to condense within the covering hood 3 above the
melting bath, and may drop as zinc dust onto the metal strip being processed,
so that the reverse surface of the strip, or the strip before it is coated
becomes dirty, thus necessitating an additional cleaning operation.
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