Note: Descriptions are shown in the official language in which they were submitted.
2~1837~
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RETAINING ANDJOR POURING MEANS
FOR TANKS FOR METAL MELTING BATHS
The invention relates to a means for retaining
contaminations contained in a melting bath, i.e.
containing molten metal, in a vessel, e.g. a tank, a
ladle, a distribution means, a tundish and the like.
The invention particularly relates to a
pouring means for tundishes for receiving molten metal
(steel) and for passing it on into ingot moulds or into
the mould of a continuous casting plant but is not
limited thereto. In a continuous casting process into
water-cooled moulds, the steel is passed from a ladle
into a tundish, which has outlets in the bottom
corresponding to the positions of the moulds. For
regulating the steel flow to the moulds the tundish is
either provided with slide gate nozzles from below or
with plugs from above, or, in tha case of free-runs,
with outlet nozzles only. In accordance with the
pouring systems used, the outlet areas are heated
before use by means of burners, from below or from
above, in order to avoid chilling during the start-up
of casting. The first portion of the steel reaching
the tundish and distributing there up to the outlets,
is coined by low temperatures and contaminations, which
result e.g. from refractory particles from the lining,
from sand and from the oxidation of the steel during
the start-up of the casting.
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In case of insufficient heating the following
disadvantages occur in particular with sliding gate
nozzles and free-runs:
l. Chilling of the steel in the outlet.
2. Addition of contaminations to the
first portion of the continuous steel
which leads to a devaluation or
scrapping of said first portion.
3. The contaminations present in the
initial steel can partly also lead to
a clogging of the dip tubes disposed
below the tundish.
In the past there were e.g. dams built in for
avoiding these problems, which dams were intended to
effect a rise of the contaminations due to the damming
up of the initial steel and which were also intended to
raise the initial casting temperatures due to the high
steel volume by mixing the subsequent hot steel with
the initially cold steel. This, however, could not
prevent the contaminations floating on the surface from
reaching the outlets during the starting phase, thus
leading, as already described, to a devaluation of the
initial steel strand or to difficulties in the starting
procedure of casting. Also, filters proposed for use
in the outlet were not able to solve these problems due
to blockings or difficulties in chilling.
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In the further course of development, starting
tubes were used with sliding gate nozzles,
independently from the damming and filtering above the
outlets, which starting tubes with their cylindrical
shape and with a diameter corresponding to the outlet
sprue, caused a temporary bulkheading off of the outlet
which, though raising the starting temperature, were
incapable of preventing the contaminations floating on
the surface, when reaching the top of the starting tube
or the overflow openings provided in the starting tube,
to be the first to enter the outlet.
The present invention is based on the problem
of providing means of the initially mentioned kind in a
manner as to ensure that the cast strand does not
contain contaminations.
This is achieved by means of the features as
indicated in the claims.
The gist of the invention is an annular
buoyancy body in the form of a protective ring which is
either conducted on the starting tube or the plug or,
in the case of free-runs, on a suitable means in a
manner that it prevents slag from reaching the outlet.
The protective ring serves as an inhibit member and
projects with its upper part from the surface of the
molten bath level to such an extent that no slag
particles or the like can pass, across its upper rim,
to the inner space of the protective ring. The
projection above the bath level can be achieved by
selecting the shape of the protective ring and its
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specific weight in correspondence to the particles
floating on the surface of the melting bath or of the
melting bath itself, independently from the actual
level of the bath.
In the case of use of a starting tube,
mobility of the protective ring in upward direction can
be restricted by a stopper. This prevents the
protective ring from disengaging fron the starting
tube.
The protective ring can also be fixed at a
specific position with respect to the starting tube or
its overflow openings. This can counteract the
unwanted Vortex-effect, namely, the level in the
tundish dropping on the occasion of an exchange of
ladles.
According to a preferred embodiment of the
invention, the starting tube can also be provided with
openings in its lower region too. However, said
openings are at first closed by pieces of sheet
material so that at first they do not have any function
when casting starts up. Under the action of the high
temperatures of the melting bath these sheets are
eventually melted so that now steel can flow through
said openings into the outlet. In the normal melting
procedure this actually is not necessary, since the
overflow openings in the upper part of the starting
tube meet this function. At the end of the sequence,
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however, these lower openings take over the function
that the steel can flow out of the tundish so that only
a small residue will result (so-called button).
The invention is further described by way of
example only with reference to the accompanying
drawings in which:
Figure 1 shows a vertical section through a
pouring means according to the invention in use with a
sliding gate nozzle in a tundish;
Figure 2 shows a similar view to Figure 1 with
a modified starting tube; and
Figure 3 shows a vertical section through a
pouring means according to the invention in use with a
plug closure.
In Figures 1 and 2, a tundish 13 is provided
with a lining 13A which can be replaced upon wear.
Underneath said lining 13 there is a permanent lining
14 and an insulating lining 15. 16 refers to the
casing of the tundish itself.
The tundish has an outlet or pouring opening 4
in its base and the entrance to the outlet is
surrounded inside the tundish by a starting tube 1.
Starting tube 1 has openings 3 towards its upper end.
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A protective ring 2 surrounds starting tube 1
and rests on the base of the tundish when the latter is
empty. During filling with molten steel, the surface
of the molten steel is covered with a slag layer 11 and
a covering insulating layer 10 both of which represent
a source of contaminating particles for the steel to be
poured through outlet 4.
The starting tube of Figure 1 is open-topped
whereas in the embodiment shown in Figure 2, starting
tube 1 has a lid 23 of greater diameter than tube 1 so
that it projects beyond the tube at 21.
In Figure 3 is shown a tundish 24 having an
outlet 4 in its base that is closed by a plug 6. A
protective ring 2 surrounds plug 6 and sits on the
floor of the tundish when it is empty or just starting
to fill with molten steel 12. The operation of
protective ring 2 is very similar to that used with the
starting tube 1 of Figures 1 and 2. Indeed, for
convenience, since the conditions relating to use of
the present invention for the individual pouring means,
namely with a sliding gate nozzle 17 or with a plug 6
or as free-run are rather similar, the invention is
subsequently described and explained principally in
connection with Figures 1 and 2.
Starting tube 1 as shown in Figure 1 has an
inner diameter corresponding to the diameter of the
outlet and is attached to the tundish base surrounding
outlet 4 and fastened and sealed so that during the
initial phase steel cannot reach outlet 4 directly and
2~18376
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that tube 1 cannot float up. Said tube 1 is open at
the top and has slots 3 towards its upper end at a
predetermined distance from the tundish base which
slots are dimensioned such that the amount of steel
that can flow through is larger than that corresponding
to the predetermined diameter of the outlet. Around
the tube 1 there is provided a protective ring 2, the
goemetrical shape and buoyancy of which is such that it
floats up due to the buoyancy caused by the steel 12
and assumes a floating position partially submerged in
the steel 12 and partly protruding above slag 11 and
insulating layer 10. Due to the rise of the steel
during the initial casting in the tundish, ring 2 is
moved upwardly along starting tube 1, thus preventing
an overflow of slag through slots 3. This is shown
dotted in Figure 1 where ring 2 is shown closing off
opening 3 while the slag and insulating layers 11 and
10 pass opening 3. As protective ring 2 rises further,
openings 3 are exposed when beneath the surface of the
steel, which latter can then flow through the openings
to the outlet in clean, uncontaminated condition.
With the filling of the tundish to the final
steel level, ring 2 floats up completely and then is of
no further use. The tube 1 inhibits, as long as it
remains in place, and even when ladles are exchanged,
characterised by a drop in the steel level, the so-
called Vortex-effect, i.e. the drawing-in of slag into
the outlet. In order to avoid, at the end of the
casting sequence, too large a residual amount of steel
in the tundish, which is marked by the flow-in level of
20183~6
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slots 3, tube 1 is designed in a manner that at the end
of the sequence it will leave its position and float up
or that it is mechanically forced to do so.
In order to maintain the functions of the
total system over a complete sequence, tube 1 may be
designed in a manner that floating ring 2 is prevented
from floating up further at the upper part of tube 1
during the casting procedure. This embodiment is shown
in Figure 2.
As before, in its upper region the starting
tube 1 is provided with overflow openings 3, which have
a specific height and which are distributed over the
circumference of starting tube 1. Overflow openings 3
have a total surface which is adjusted to the inlet
opening of outlet 4. At its upper end, tube 1 is
provided with a cover 23, which may be integral or a
separate lid.
Numeral 21 designates a projection of cover 23
which prevents the floating protective ring from
disengaging from starting tube 1 in the upward
direction - as shown in dotted form at the right hand
side.
In this embodiment in the lower region of
starting tube 1 a further opening is shown at 22,
though a plurality of such openings can be provided
distributed over the circumference of starting tube 1.
Said opening 22 is closed by the sheet element 22A so
that these openings, at first, cannot perform any
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function. However, when the sheet 22A melts under the
action of the temperature of the molten steel 12, steel
can flow through opening 22. Since openings 22 are
positioned rather far down the starting tube it can be
achieved that the tundish empties to a very large
extent at the end of the sequence. Here there is no
risk either that slag 11 or insulating material 10 will
reach the casting strand, since with falling bath level
protective ring 2 covers said openings 22 in the same
manner as openings 3. In comparison to common practice
the use of such starting tubes is of advantage since in
the processes used so far, starting tube 1 was removed
after the actual starting procedure. This gives rise
to the danger that through pouring from the tundishes
contaminations get into the casting strand.
Thus, the consequence is that when the steel
level drops to slot 3 level within a sequence or at the
end of a sequence due to an exchange of ladles,
floating ring 2 inhibits again the flow of slag and
powder into tube 1.
The tube is also designed such that the
pouring system can be preheated from below, which
nowadays is a common measure. When doing so, care must
be taken that the waste gases resulting from the
burners can escape through slots 3 in the inner body,
without problems occurring. The heating of the pouring
system thus is more intensive so that the heating
period or the energy density of the burner can be
reduced. There is no reason either why a common
tundish should not be heated from above, as both
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.
systems - heating from below and heating from above -
can work independently from each other. Due to the
large heat reservoir after the heating it is possible
to prolong the interruption period between heating and
use of the tundish while equally good preheating is
maintained.
(By means of Figure 1 there can also be
described the conditions as valid for a free-run. The
guidance provided for the protective ring 2, according
to the invention, can have a cylindrical shape corres-
ponding to the shape of the starting tube 1 as shown in
Figure 1, which is also open at the top.)
In cold start free-runs, appropriate
temporary closure of the nozzles and different levels
of slots 3 in tubes 1 of the individual strands, there
can be achieved a timely delayed automatic start of the
individual strands. The system car. be used for
applying, immediately after the heating and before the
feeding of the steel to the tundish, the intended
covering agents for insulation onto the tundish floor,
since floating-up ring 2 around tube 1 prevents the
corresponding slag or the still loose covering agent
from flowing into the pouring system. ~his avoids the
introduction of oxygen at the steel surface occurring
hitherto and simultaneously reduces heat radiation
and/or reduction in the steel casting temperature in
the tundish during the first few minutes after the
start.
20~8376
~ FS 1427
In the case of plug-controlled tundishes
(Figure 3), floating ring 2 is put around the plug
which floats up with the flowing-in steel and which,
during casting, inhibits an increased erosion of the
plug in the slag or the contaminated steel area. The
partly occurring clod formation of the basic covering
agents, together with the slag, which influences the
plug function, can thus be avoided. By means of a slag
agent and/or covering agent applied between plug 6 and
ring 2 an oxidation possibly occurring in gap 7 between
ring 2 and plug 6 can be avoided. Said agents can be a
component of ring 2. The advantage of adding slag
formers and/or covering agents without an impediment of
the plug is obvious. The covering - and/or slag-
forming agents, however, can also be applied separately
between ring 2 and plug 6. Apart from this, when used
together with plug 6, ring 2 effects an appropriate
heat balance in the plug tip during the start-up period
of casting.
