Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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TRANSLATION (HM-566PCT-original):
WO 03/029,507 Al
PCT/EP02/10,741
METHOD FOR HOT DIP COATING
The invention concerns a method for coating the surface of
a product, especially a strip-shaped product, for example,
nonferrous metal strip or steel strip, with at least one metal
coating by passing the product through at least one molten metal
bath space that contains the molten coating material. The
invention also concerns a device for carrying out the method.
In conventional hot dip coating of strip (referred to here
as Method 1) with Zn, Zn-A1, A1, or A1-Si alloys, the strip runs
in the coating section from an annealing furnace under
conditions of air exclusion into the molten metal and is
deflected vertically and stabilized by various arrangements of
nondriven rollers (see Figure 1). This applies to all of the
specified coating metals/alloys used in hot dip coating.
A disadvantage of Method 1 is that the rollers and the
bearings of the rollers are located within the molten material,
and all parts are exposed to chemical attack by the molten
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material. The service life of the parts that are used within
the molten material is limited. In addition, a large volume of
molten material with a correspondingly large dip bath is
necessary to accommodate the rollers and all of the bath
equipment. 200 to 400 t of molten zinc are customary in hot dip
galvanizing. Due to this large volume, rapid regulation of the
temperature and alloy composition of the melt is not possible.
Large fluctuations of the specified parameters must be accepted
and sometimes result in loss of quality, since measures related
to the production of the alloy and those related to influencing
the strip quality are carried out in the same tank and thus
affect one another.
Another disadvantage is that the production speed cannot be
increased to realize an economical plant output (about 180
m/min), especially in the case of thin strip < 0.5 mm. One
reason for this is that relative motion can develop between the
rollers located in the bath and the strip. If the tension is
increased in an effort to avoid this problem, there is the risk
of strip breakage. This results in scrap and prolonged plant
shutdowns.
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The jet stripping system located above the zinc hot dip
bath further limits the maximum strip advance speed of a hot dip
galvanizing installation (see Figure 1). The coating thickness
is adjusted by air or nitrogen, and the minimum coating
thickness that can be produced increases with increasing strip
speed. This means that thin coatings cannot be produced at high
strip speeds. However, certain demanding applications require
thin coatings
(< 25 g/mz on one side in hot dip galvanized sheet).
So-called vertical hot dip galvanizing is well known as an
advanced method for the hot dip coating of ferritic steel strip
made of soft unalloyed steels and is described in various
patents, such as EP 0 630 421 B1, EP 0 630 420 B1, and EP 0 673
444 B1.
In this method (referred to here as Method 2), the strip
passes from bottom to top through a working tank filled with
molten metal composed of zinc and/or A1 alloys after it has been
subjected to a heat treatment. The strip enters the molten bath
under conditions of air exclusion. The volume of molten metal
(about 2-5 t of molten zinc) is much smaller than in Method 1.
The qualitative problems described above also do not occur,
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since the measures related to the production of the alloy are
carried out in a reservoir located alongside the line, while
measures to influence the strip quality are carried out
separately in the working tank.
The working tank and the furnace chamber located below it
are connected by a gastight ceramic duct, which is about 800 mm
high and has a passage width for the strip of only a maximum of
20 mm. The working tank is sealed at the bottom to prevent
molten metal from flowing down into the furnace chamber by means
of a seal produced within this duct by two inductors arranged at
the side of the duct or strip. These inductors induce an
electromagnetic traveling field, which produces an upwardly
directed force that prevents the molten metal from flowing down.
This inductive system acts like a pump, so that exchange of the
melt in the duct is ensured.
Method 2 is characterized by the fact that, at least in the
coating area up to the hot dip bath, significantly higher strip
speeds on the order of 300 m/min can be realized even with thin
steel strip, since there are no rollers in the coating tank.
After the strip has passed through the coating unit from
bottom to top at a temperature, e.g., in the case of hot dip
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galvanizing, of about 460°C, the desired thickness of the metal
coating is adjusted a short distance above the hot dip bath by
the jet stripping process, as in Method 1. This process is
comparable to the process used in Method 1 and involves the
blowing of compressed air or nitrogen.
As in Method 1, the jet stripping process in Method 2 also
limits the maximum possible strip speed when thin coatings are
being applied. However, Method 2 offers greater degrees of
freedom for the galvanizing parameters of melt temperature and
viscosity and alloy composition, which likewise affect the
coating thickness. For this reason, it is to be expected that a
higher strip speed can be selected in Method 2 than in Method 1
for the same coating thickness. In contrast to Method 1, Method
2 has not yet been tested on the industrial scale. So far only
pilot plant trials with narrow strip have been conducted. These
trials were successful.
However, an obstacle to an increase in speed is presented
by the fact that the strip subsequently must be cooled below
300°C in the upwardly traveling strand before the first
deflection. If the temperature is higher, there is the danger
that metallic particles will grow on the first contact roller or
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deflecting roller in the cooling tower and cause irreparable
surface defects on the material.
The cooling is usually produced by several successive air
cooling lines. However, the cooling effect or, more precisely,
the cooling rate, is limited by the medium and cannot be
increased at will on a fixed length of line (e.g., two times 15
m) with the use of air as the cooling medium. With increasing
strip speed or with increasing mass throughput, the cooling
lines must be lengthened. However, it then becomes necessary to
raise the upper deflecting roller in the cooling tower of a hot
dip coating installation.
In installations that are operated by Method 1, the height
of the upper deflecting roller is usually 30-60 m. In the case
of Method 2, it would be necessary, at high strip speeds, to
lengthen the cooling lines accordingly, and the height of the
cooling tower would have to be increased to about 80-90 m. This
requires higher capital expenditures for buildings and
foundations.
The free-running, unstabilized strip length in the tower
would thus increase, and the strip flow would be destabilized,
so that vibrations may occur, and the product quality may be
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adversely affected. The use of other cooling media in the
upwardly traveling strand is problematic, and so far this
problem has not been solved on the industrial scale.
Another problem, which concerns the electromagnetic seal
used in Method 2, is that the forces that act on the liquid melt
also act on the ferritic strip. Undesired contact of the strip
with the duct due to the magnetic forces of the sealing
inductors is possible only by additional expensive measures.
This requires additional stabilizing coils and expensive
automatic control technology.
The objective of the present invention is to avoid the
specified disadvantages of Methods 1 and 2 and to create a high-
speed hot dip coating installation without a cooling tower,
which combines the least possible construction expense with
optimized capital investment costs and high plant output with
the best production quality.
This objective is achieved with a method of the type
described in the introductory clause of Claim 1 by sealing the
molten metal bath space by means of rotating permanent magnets.
The sealing of the molten metal bath space by rotating permanent
magnets is considerably more reliable and less expensive than an
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electromagnetic solution, and significantly less power is needed
for the rotation than for an electromagnetic seal, which is an
advantage especially in the event of a power failure.
Refinements of the method are described in the dependent
claims. A device and refinements of this device for carrying
out the method of the invention are the objects of additional
claims.
The invention is described below with reference to several
embodiments shown schematically in the drawings.
-- Figure 1 shows a conventional strip coating method.
-- Figure 2 shows an advanced coating method in accordance
with the state of the art.
-- Figure 3 shows the coating method of the invention and a
correspondingly designed high-speed hot dip coating installation
in operation.
-- Figure 4 shows the installation in Figure 3 in a start-
up situation.
-- Figure 5 shows the installation in Figure 3 during
shutdown after operation.
In accordance with Figure 3, after a deflection in the
furnace under conditions of air exclusion, strip 1 runs
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vertically downward into a molten metal bath space that contains
the hot dip bath. This hot dip bath is sealed towards the
bottom. This requires forces, but these forces are not
electromagnetic in nature, but rather are produced by rotating
permanent magnets. The sealing of the melt with permanent
magnets is well known in itself, but the prior art involved the
use of rectangular ducts. A duct shape like this cannot be
changed with respect to clearance and shape.
By contrast, the present invention proposes two adjacent
rotors 5, 5'. The rotors are tubes 6, 6' made of materials that
are resistant to heat and molten metal, preferably ceramic
materials. Rollers, on whose cylindrical surface permanent
magnets 4 are mounted, rotate inside these tubes 6, 6', whose
diameters may be freely selected. The rotors 5, 5' can be
adjusted to the melt or to the strip. It is also possible to
close the gap 7 when the installation is shut down or is being
started up.
Permanent magnets are significantly less expensive than
electromagnetic sealing by means of coils or inductors, and much
less power is required for the rotation than for an
electromagnetic seal, which is an advantage especially in the
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event of a power failure.
In addition, much higher field strengths can be produced
with permanent magnets (by a factor of 3) than by the
electromagnetic method. These high field strengths and the
resulting higher forces are needed for the stripping process for
adjusting the desired coating thickness on the steel strip. In
the previously known methods, this adjustment must be
accomplished by additional stripping jets.
Additional measures within the magnetic seal and stripping
are no longer required in the method of the invention, since the
region of the narrowest passage of the strip 1 through the
sealing unit is now only a few millimeters. Furthermore, the
strip can be supported at much shorter lengths than in the
previously known Methods 1 and 2, since the strip 1 can be
immediately cooled and deflected into a water bath 9 directly
below the sealing unit. The support length in the present
invention is preferably only about 5,000 mm, whereas in Method 1
it is about 8-10 times greater, and in Method 2 it is greater
still.
Another advantage of the method of the invention is that
the surface of the molten metal, preferably the molten zinc, in
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the coating area is within a protective gas atmosphere, which
preferably consists of a nitrogen/hydrogen mixture, so that
interfering oxidation of the molten zinc cannot occur. In the
previously known Methods 1 and 2, this can be accomplished only
with considerable additional expense. Furthermore, in the
previous methods, it is necessary for the surface of the zinc
bath to be accessible for certain types of manual work. In the
present invention, access to the surface of the hot dip bath for
the purpose of removing particles of oxidized metal is
unnecessary.
In the embodiment in Figure 3, the installation for the hot
dip coating of a nonferrous metal strip or a steel strip 1 is
shown in continuous operation.
The incoming strip 1 to be coated passes through a tension
roller 17 and then through a lock 18, which hermetically seals
the protective gas atmosphere prevailing inside the hot dip
coating installation from the ambient, oxygen-containing
atmosphere.
In the galvanizing chamber 14 which follows, the strip 1 is
vertically deflected by guide rollers 13 towards the coating
section 19. Upon entering the coating station 19, the strip 1
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passes vertically from top to bottom through the bath of molten
metal 3 maintained in the gap 7 between the rotors 5, 5' and
thus receives the desired coating.
At the lower end of this hot dip bath 3, in a gap formed
between spaced rotors 5, 5', the molten metal is prevented from
running out at the bottom by magnetic forces of magnetic fields
or traveling magnetic fields of the rotating permanent magnets
4. The rotors 5, 5' are located inside the tubes 6, 6' that
surround them. The coating station 19 is surrounded on the
outside by a duct-like housing and holds the rotors 5, 5, which
are spaced a variable distance apart. They are surrounded by
the tubes 6, 6', which are made of materials that are resistant
to heat and molten metal, especially nonmagnetic materials and
preferably ceramic materials.
The permanent magnets 4 rotate inside these tubes 6, 6'.
The molten metal required for coating, which must be
continuously replenished, is conveyed in controlled amounts by a
metal pump 12 from a reservoir 8, in which it is conditioned,
into the gap 7 between the rotors 5, 5'. The strip 1, which is
coated in the gap 7, passes through the gap at the lower end and
then passes in succession through an arrangement 15 for air
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stabilization and an arrangement 16 for water cooling.
After it has passed through the water bath 9 and a tension
roller 20, it is removed from the installation for further use
or treatment.
The additional Figures 4 and 5 show the method of the
invention
(a) in a start-up situation, and
(b) during shutdown after operation.
(a) Start-U~ Situation:
-- strip not running
-- rotors rotate
-- gap between rotors closed
-- melt is supplied
-- furnace chamber closed.
(b) During Shutdown after Operation:
-- return of melt
-- rotors rotate
-- gap closed
-- furnace chamber opened.
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