Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02404331 2002-09-27
The invention relates to a method of an a device for
automatic casting in continuous casting systems, preferably, for
casting of strand thicknesses between 10 and 150mm and widths
up to 3,500 mm by using regulation of the casting level and a
closure system consisting of a slider system or, preferably, a
stopper. Continuous casting systems with an~oscillating mold were
developed in last years for high-output plant that are designed for
operating at speeds up to l Om/min. Here, in particular, thin slabs
producing plants with a casting thickness in the mold from 40 to
1 SOmm and a width of 800-3,SOOmm should be mentioned.
These plants make an automatic start more and more
important as a strand should include a reproducible and minimized
dummy piece which, as a rule, is discarded. Furthermore, the
direct connection of the continuous casting system with a rolling
process-here to be called, e.g., a compact strip production plant
(CSP-plant)-requires maximization of the steel quality and
minimization of the discarded material because the process
connection with respect to temperatures makes "cleaning" of slabs
impossible.
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In addition, a hopper mold is necessary (German Patent DE
3400220) that permits to reproduce filling up of a mold from
opening of the distributor until the removal of a slab from the mold
with a following reproduction starting strategy with a set speed of,
e.g., up to 6m/min.
During casting of thin slabs with or without a runner (with
parallel wide walls) in the mold, the starting process (from opening
of the distributor up to removal of the mold bottom with a dummy
strand or slab) should take from 10 to 20 minutes.
In order to obtain this time window, which is predetermined
by metallurgical conditions in the mold, with a starting strategy
that is based on the measurement of the casting level and the use of
the casting level regulation ( 10) during filling of the mold (Fig. 1 ),
the heat balance of steel in the ladle ( 1 ) as it travels from the
crucible furnace (2) to the continuous casting distributor (3), and in
the distributor itself is determined from kinematic and not from
thermodynamic point of new.
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The heat balance in the distributor (3) depends substantially
from the temperature TT of the distributor and the heating-up time
tT (5). If the heating-up temperature TT (4.1) amounts to, e.g.,
1,200 C instead of 1,300 C (4.2), the solidification (9) on the inner
wall (6.1 ) of the distributor, which is formed of refractory bricks
which are carried by a steel jacket (6.2), is greater during the
casting process. This solidification effect is observed also at the
stopper (7) and the stopper seat (8.1 ) that forms the entrance of a
submerged outlet (8), and leads to the distortion of a uniform steel
flow with respect to the valve characteristic that is determined by
the stopper position and the stopper seat.
A better insight is the distortion of casting at the stopper seat
(8.1 ) can be gleamed from Fig. 2. Here, the initial solidification (9)
at the stopper seat and the stopper (7) is clearly shown.
When the stopper opens, the initial solidification obstructs a
uniform steel flow corresponding to the mass flow characteristic of
a valve seat consisting of the stopper (7) and the stopper seat (8.1 )
of the submerged outlet (8).
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In addition, Fig. 2 clearly shows that the material of the
submerged outlet (8) and of the stopper (7) has a higher heat
conductance of about 10 W/K~m than a conventional refractory
material (6.1 ) of the distributor with conductance of about 3 w/mk,
whereby the initial solidification at the stopper seat is built-up at a
greater extent than in the distributor.
A shorter heating-up time tT (S) exerts also a further
increased influence on the initial solidification and, thereby, on the
distortion of the starting process, because the temperature gradient
in the distributor wall, to the moment of casting, between the hot
face (6.1.2) and the cold face (6.2.1 ) of the steel j acket (2) is
greater with a shorter heating-up time and is inversely proportional
thereto.
In addition to the influence of the initial solidification at the
stopper (7) and the stopper seat (8.1 ), naturally, the steel
temperature in the ladle, e.g., determined by the discharge
temperature, TLF ( 11 ) of steel at the end of the secondary
metallurgical range, e.g., at the crucible furnace (2), also exerts an
influence.
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In addition, the thickness of the initial solidification at the
stopper seat is influenced by shape of the distributor , distributor
volume, ladle output, and the ratio of the distributor surface to the
distributor volume. However, these influence variables could be
considered as constant system data, and they do not directly
influence an optimal process control "on-line".
This distortion of the start of casting by an uncontrolled
initial solidification which depends essential on:
- steel temperature upon discharge at the end of the
secondary metallurgical range,
- heating-up time of the distributor, and
- heating-up temperature of the distributor,
often leads to the distortion and, thereby, to the interruption
of the casting process which is often accompanied by mold
overflow with a following rupture of a strand shell.
Accordingly, an object of the invention is to provide a
method and a device which would make possible to obtain,
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independently from the thickness of the initial solidification, a
desired casting time in a range of, e.g., 10-20 sec by using casting
level regulation ( 10), start strategy ( 10.1 ), strand drivers ( 10.2) and
stopper or slide valve adjustment (10.3).
An unexpected solution of the problem, which would not
have been obvious to one of ordinary skill in the art, is described in
the claims and would be explained in detail with reference to Figs.
1 through 4.
Fig. 1 shows an influence of actuating variables on
the thermal condition of steel, e.g., between
a crucible furnace and the start of casting
during a predetermined time period of 10-20
sec.
Fig. 2 shows an initial solidification, e.g., at a
stopper seat of a submerged outlet without
(a) and with (b) formation of the initial
solidification.
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Fig. 3 shows a functional connection, e.g., in form
of a mathematical product, between the time
of filling up of a mold and the process
actuating variable for different inner
distributor wall temperatures.
Fig. 4 shows an example for a planned filling-up
time and for a predetermined steel
temperature in a ladle and the distributor
heating-up time which provide for a desired
weight of steel in the distributor for different
distributor cold face temperatures.
Fig. 1 shows the influence of the actuating variables between
a crucible furnace (2) and a stopper seat (8.1 ) on the initial
solidification and, thereby, on the casting process in a continuous
casting mold (13).
The following parameters exert an increased influence on the
initial solidification (9) at a stopper (7) and the stopper seat (8.1 ) of
the submerged outlet (8).
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reduced steel temperature, TLF ( 11 ) in the crucible
furnace (2),
a reduced distributor temperature in the interior (4.1 )
at the hot face (6.1.2) or outside at the distributor
casing (6.2), as a cold face temperature (6.2.1 ) at the
end of a distributor heating-up time tT (5) in the
distributor heat-up condition (5.2), and
reduced distributor heating-up temperatures tT (5)
upon the use of a burner (8.2.1 ) or a furnace (8.2.2).
The weight of steel in the distributor WT ( 14) is a process
parameter that breaches the initial solidification upon opening of
the distributor and releases the position of the valve between the
stopper (7) and the stopper seat (8.1 ). The greater is the initial
solidification (9) the greater the pressure (14.1) or the steel weight
( 14) in the distributor should be at the opening of the stopper while
retaining the valve opening and maintaining the same filling-up
time.
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In Fig. 2, in a partial view a), the initial solidification (9) is
partially shown, representing an uncontrollable mass around the
stopper. In a partial view b), a condition after the formation of the
initial solidification (9), e.g., by at least one-time rapid opening and
closing of the stopper before the start of the process, is shown.
With this measure, an uncontrolled initial solidification, which
consists of crystals and melt (steel sponge filled with melt), is
provided to form a temporary valve seat that provides for a uniform
steel flow during casting and increases the reliability of the casting
process.
Fig. 3 represent a functional connection between the filling-
up time (15) and, e.g., a product of
~ a discharge temperature of steel in the crucible furnace
T~LF ( 11 ),
~ a root of the heating-up time of the distributor t~T
(5.1 ), and
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~ the weight of steel in the distributor W~T ( 14) at the
moment of opening of the distributor for different hot
face distributor temperature T~T (4.1 ) and (4.2).
This function is valid for the following boundary conditions.
~ a constant time period between the discharge of steel
at the crucible furnace (2) and opening of the ladle,
~ a constant time period between the heating-up of the
distributor (5.2) and the opening of the distributor,
~ a constant and definite brick lining of the distributor,
and
~ a predetermined distributor shape and volume.
Fig. 4 shows tables which make clear the inventive step of
the invention. The examples clearly show that at a predetermined
steel temperature T~LF ( 11 ) and the distributor heating-up time t~T
(5) immediately before the opening of the ladle, for a desired
filling-up time t~M(15) of, e.g., 14 or 10 sec, a corresponding
filling ratio expressed as the weight of steel ( 14) in the distributor
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or as a ferrostatic pressure ( 14.1 ), can be determined on-line with a
mathematical function in order to reliably establish the desired
filling-up time.
The tables make clear that at distributor hot face temperature
of 1,200 C (4.1), the casting time t~M or the filling-up time (15) of
~ 14 sec at 18.2 t, and
~ lOsecat 19.6t
or of 1,300 C (4.2), the filling-up time of
~ 14 sec at 13.8t, and
~ 10 sec at lS.St
is established.
This shown connection makes it only possible, independent
from the steel temperature ( 11 ) and the filling-up time of the
distributor (5), which vary during the operation, to control and,
thereby, completely automatize the casting process by
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determination of the corresponding weight of steel in the
distributor.
In addition to the above-mentioned actuating variables, other
energetically relevant actuating variables (21) e.g., the remaining,
after preheating in the wear layer (6.1.1 ) that, is deposited, e.g., in
form of an injection mass, on the permanent layer (6.1), residual
moisture or gases likewise quantatively influence the initial
solidification.
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