Note: Descriptions are shown in the official language in which they were submitted.
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METHOD AND DEVICE FOR CONTROLLING THE TEMPERATURE OF STEEL
BETWEEN THE MENISCUS OF A CONTINUOUS CASTING MACHINE
AND THE TAPPING OF THE FURNACE
The invention pertains to a method and a device for
controlling the temperature of the steel in a steel production
process between the meniscus of a continuous casting mold and the
tapping of the furnace and thus over the entire course of
secondary metallurgical processing with a final temperature-
determining process stage such as a ladle furnace.
For the successful casting of strands in any type of
continuous casting machine with or without oscillation and
preferably for strands of steel, the temperature of the steel at
the meniscus is an essential factor in determining the quality of
the steel (both surface quality and internal quality) and also
the reliability of the casting operation.
Controlling the temperature of the steel at the meniscus of
the continuous casting mold is especially important when the
casting speeds are increased to 10 or 12 m/min. There are also
many other important factors, however, which are important for
obtaining the desired casting result, which will be discussed
below.
The invention is based on the task of creating a method and
a device which make it possible, regardless of these numerous
factors, to plan and to control on-line the change in temperature
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of the melt between the temperature of the steel at the meniscus
and the furnace tap.
An unexpected solution which lies outside standard technical
understanding is described in the claims.
Figures 1-4 present the invention by way of example and in
highly schematic fashion:
-- Figure 1 shows the process and production chain extending
between the furnace tap and the meniscus in the continuous
casting mold;
-- Figure 2 shows the change in the temperature campaign or
the temperature curve of the steel over the course of processing
in the processing chain, starting from the meniscus in the mold
and proceeding via the ladle furnace to the furnace tap; and
-- Figure 3 shows an example of a melt plan in the form of
the two Figures 3a and 3b.
It is assumed, as illustrated in Figure 1, that, depending
on:
~ the casting speed,
~ the casting width,
~ the casting thickness,
~ the casting format (round, profiled, etc.),
~ the casting output,
~ the tundish design,
~ the degree to which the tundish is filled,
~ the lining of the tundish,
~ the technology of the tundish, and
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~ the condition of the ladle,
regardless of whether casting is carried out with
~ exposed casting steel and oil lubrication or with
~ an immersed outlet nozzle/immersion tube and casting flux,
the temperature of the steel at the meniscus (2) in the mold (1)
must be controlled and kept constant. In all casting situations,
this steel temperature (2.1) at the meniscus of the melt in the
mold (2) must be:
TML = T1; + X°C (X = 5-15°C) , Tli - f (grade of steel)
(2.1)
at a defined point which is symmetric to the shape of the mold.
In addition, it should be mentioned here that this condition
of a controlled meniscus temperature (2.1) can be applied to any
grade of steel.
To ensure the casting condition (2.1) regardless of the
possible casting situation, exact data are collected concerning
the temperature losses of the steel, starting from the meniscus
(2) and proceeding via the tundish (3), the secondary metallurgy
(4) with the ladle furnace (4.1) and, for example, a vacuum
treatment station (4.2), all the way to the tapping (5) of a
basic oxygen converter (BOF)(5.1), for example, or of an electric
furnace (EAF)(5.2).
The loss of temperature which the steel experiences over the
course of the processing and production chain is strongly
affected by the times and temperatures of the steel and of the
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vessels with their refractory linings, specifically by the type
and condition of those linings (exposure time, wear, specific
heat transfer, etc.).
Thus, for example, the tundish (3) and the ladle (4.3) can
be considered heat exchangers, the specific thermal data of which
are determined by the type, age, or wear condition of their
linings.
It is also taken into consideration that, at the start of
casting, which is a non-steady-state phase of the process not yet
in heat flux equilibrium, the heat contents of the empty tundish
(3) and of the ladle (4.3), characterized by the heat-up time,
the heat-up temperature, and the ladle history (7), have a
significant effect on the temperature losses of the steel during
its holding time in the ladle, during the transfer from the ladle
to the tundish, and in the tundish itself.
In the case of a second ladle all the way to an n-th ladle
within a casting sequence, the tundish is in thermal equilibrium
with a constant heat loss, whereas the ladle, which can be
considered an "individual", continues to lose heat at various
rates.
In the past, these temperature losses along the route from
the tapping of the furnace (5) via the secondary metallurgy (4)
with the ladle furnace (4.1) as the interface to the tundish (3)
and finally to the area of the continuous casting machine (1.1)
have been estimated on the basis of practical experience and
taken qualitatively into account in the process.
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For controlled casting, therefore, the starting point is the
desired steel temperature TML = T11 + X°C (2.1), and on this basis
the desired steel temperature in the tundish (3.1) is determined
for a planned casting speed and a planned casting format. The
tundish temperature (3.1) is adjusted appropriately on the basis
of the knowledge of the temperature loss (4.1.1) which occurs
between the ladle furnace and the tundish. A melt plan is shown
in Figure 3; it starts with the temperature of the steel at the
meniscus (2.1), expressed as the equivalent liquidus temperature
in the tundish T°li (3.1) with auxiliary temperatures of +5, 10,
15, 20°C (3.1.1), as a function of the casting speed (6) --
Figure 3a -- up to the delivery temperature from, for example,
the ladle furnace LF-ex (4.1) in the area of the secondary
metallurgy (4). The delivery temperature of the steel from the
ladle furnace is determined by the ladle history (7) with its two
essential factors:
~ the holding time of the steel in the ladle (7.1) between
the time it is tapped from the furnace and its delivery from the
ladle furnace for the start of casting, i.e., the "ladle full"
time, and
~ the period of the time when the ladle is not holding any
steel (7.2), i.e., the "ladle empty" time,
and by the tundish history (3.2) with its important factors:
~ the tundish preheating temperature (3.2.1),
~ the tundish preheating time (3.2.2), and
~ the tundish turnaround time (3.2.3) with its specific
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preparation steps.
Figure 4 shows a suitable ladle identification system (8)
based on the encoding of the natural radiation of the ladle wall
by a perforated metal plate (8.1.1) and the detection of that
radiation by a radiation sensor (receiver) (8.1.2).
Figure 1 shows the entire process and production chain,
which extends between the tapping of the furnace (5) and the
continuous casting mold (1). So that the temperature of the
melt, starting from the desired temperature of the steel at the
meniscus (2.1), can be planned and controlled on-line, first for
the tundish and then for the ladle furnace LF-ex, it is necessary
to know:
~ the casting parameters, which here are:
~ the dependence of the temperature of the steel in the
tundish on the casting speed (6), described in the TT/VC system
(10) (see Figure 2), for a given casting output or holding time
of the steel in the tundish;
~ the history (3.2) of the tundish, comprising its
preheating temperature (3.2.1), preheating time (3.2.2), and the
tundish turnaround time (3.2.3) with the various possible
preparation steps such as relining, spraying, drying, etc.; and
~ the history of the ladle (7), comprising the holding time
of the steel in the ladle (7.1) and the remaining ladle time
(7.2), during which no steel is in the ladle, and the condition
(7.3) of the ladle, which is determined by:
-- the age of the ladle lining (wear) and
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-- the type of ladle lining.
This history of the ladle (7) thus consists of:
~ the "ladle full" time, i.e., the time between the start of
the tap and the delivery from the ladle furnace with a maximum
duration until the start of casting of, for example, 25 minutes
(7.1);
~ the "ladle empty" time (7.2), and
~ the ladle condition (7.3),
where the
~ "ladle full" time (7.1) and
~ "ladle empty" time (7.2)
constitute the standard ladle cycle time (7.4).
During the "ladle empty" time (7.2), the ladle passes
through various treatments such as cleaning (7.2.1) and the
preparation of the slide valve for the next use (7.2.2). In
addition, the ladle can also be taken out of the ladle cycle so
that it can be heated on a heating stand (7.5) during long empty
periods or placed on a ladle lining stand (7.6) so that it can be
given a new lining.
To record all the numerous factors which act on the ladle
and thus ultimately determine the temperature loss (4.1.1)
between the ladle furnace and the tundish, the ladle
identification system (8) is installed in the ladle cycle (7.4)
at the locations relevant to the determination of the ladle
history (7). The detection system works preferably by means of
the encoding of the ladle radiation (8.1) and is based on the
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natural radiation of the ladle. A measuring system of this type
can record the movement and the differentiated time sequences in
the "ladle full" (7.1) and "ladle empty" (7.2) phases, so that
the temperature losses (4.1.1) between the LF and the tundish can
be put into functional relationships with each other on-line on
the basis of the data thus acquired.
A procedure similar to that for the ladle is also
appropriate in the area of tundish management (3.2), where
special attention should be paid to the start of casting of a
casting sequence consisting of several ladles. Here the history
of the tundish (3.2) is superimposed additionally on the
temperature loss (4.1.1) between the LF (4.1) and the tundish
(3); this history consists of:
~ the preheating temperature of the tundish (3.2.1),
~ the tundish preheating time (3.2.2),
~ the tundish turnaround time (3.2.3),
~ the tundish drying time and temperature (3.2.4), and
~ the tundish lining (3.2.5).
Figure 2 shows the change in the temperature of the melt as
it travels from the electric furnace (5.2) or BOF (4.1) via the
ladle furnace LF-ex (4.1) to the meniscus (2) in the mold (1).
The desired temperature of the steel at the meniscus (2.1) is the
basis on which the temperatures in the tundish and in the ladle
furnace LF-ex (4.1) are planned.
The discontinuity in temperature between the meniscus and
the tundish (9), i.e., the decrease in the temperature of the
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steel between the tundish and the meniscus, is determined
essentially by the casting speed (6) and the casting output, that
is, the solidification thickness and casting width.
The discontinuity in the temperature between the tundish and
the ladle furnace (4.1.1), however, is determined essentially by
the ladle history (7) and the tundish history (3.2).
Figure 3 is divided into parts 3a and 3b. Figure 3a shows
the tundish temperature (3.1) TT versus the casting speed (6) VC
for the equivalent liquidus temperature of the steel (3.1.1) in
the tundish T°~i with the auxiliary temperatures (3.1.1.1') T°li
+
5, 10, 15, 20°C.
If it is planned to cast at a rate of, for example, 5 m/min,
then a tundish temperature (10) of 1,560°C is obtained, which is
to be considered the ideal steel temperature (10.1)
TT = Tli + 10°C (VC = 5 m/min)
in the tundish for a grade with Tli = 1,530°C
To arrive at the ideal temperature (10.1) of, for example,
1,560°C, the discontinuity in the temperature (4.1.1) of the
steel between the tundish and the ladle furnace LF-ex (4.1) is
determined as shown in Figure 3b.
Here the temperature loss (4.1.1) is shown versus the time
factor "steel in the ladle (7.1)" for the start of casting in the
case of
~ a tundish preheating temperature of 1,100°C (3.2.1.1),
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~ a tundish preheating temperature of 1,200°C (3.2.1.2),
and also for a 2nd and an n-th ladle (3.2.1.3) of a sequence.
The figure makes it clear how the temperature losses change
as a function of the holding time of the steel in the ladle (7.1)
and the tundish preheating temperature (3.2.1), both for the
first ladle of a sequence and for all the subsequent ladles.
These temperature losses also depend on the remaining part of the
ladle history (7) and can be determined by means of the on-line
method described here.
Figure 4 shows the ladle identification system (8), which
works with a perforated metal plate (8.1.1) to encode the natural
radiation (8.1). The encoded radiation is then detected by one
or more radiation sensors (8.1) located at strategic points in
the ladle cycle (7.4).
These descriptive process-related and device-related
features lead to controlled casting, which is characterized by:
~ good, maximized product quality,
~ high casting reliability with
~ high casting speeds and thus
~ high productivity.
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List of Reference Numbers
1 Continuous Casting Mold
1.1 continuous casting machine
1.2 measuring means
2 Meniscus in the Mold
2.1 planned temperature window of steel at the meniscus,
TML = Tl~ + x°C (x = 5-15°C)
3 Tundish
3.1 temperature of the steel in the tundish, TT
3.1.1 equivalent liquidus temperature in the tundish, T°~i
3.1.1.1 auxiliary temperatures T°1~ + 5, 10, 15, 20°C
3.2 tundish history, tundish management
3.2.1 tundish preheating temperature
3.2.1.1 tundish preheating temperature of 1,100°C
3.2.1.2 tundish preheating temperature of 1,200°C
3.2.1.3 2nd to n-th ladle of a sequence
3.2.2 tundish preheating time
3.2.2.1 tundish preheating stand
3.2.3 tundish turnaround time
3.2.4 tundish drying time and temperature
3.2.5 tundish brick lining
3.2.6 tundish spray lining
3.n measuring means
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4. Secondary Metallurcty
4.1 final temperature-determining process stage
such as a ladle furnace, LF-ex
4.1.1 temperature loss or jump between ladle furnace and
tundish
4.2 vacuum treatment stand
4.3 ladle
4.4 computer
Tappincr from Furnace to Ladle
5.1. converter, BOF
5.1.1 oxygen lance
5.2 electric furnace
6 Castinct Speed, VC in m/min
7 Ladle History
7.1 holding time of the eel in the ladle from the tapping
st
of the furnace t o the delivery of the ladle furnace
(LF-ex) or until the opening" of the ladle (start
" of
casting), "ladle full" time
7.2. time during whic h the ladle contains no steel, "ladle
empty" time
7.2.1 cleaning of the ladle,bottom, nozzle bricks, slide
valve
7.2.2 preparation of the slide valve, inspection, packing the
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slide valve sand, etc.
7.3 ladle condition, age and type of lining
7.4 ladle cycle time
7.4.1 ladle cycle, ladle turnaround
7.5 ladle heating status
7.6. ladle brick lining status, new spray lining
8 Ladle Identification System
8.1 individual ladle with encoded radiation system, based
on the natural radiation of the ladle body, individual
ladle
8.1.1 coded metal plate
8.1.2. radiation sensor
9 Temperature Jump
steel at the meniscus/steel in the tundish or
temperature loss of the steel between the tundish and
the meniscus, function of the casting speed (VC) (6),
tundish design, etc.
Distributor Temperature, TT in °C
10.1 ideal target temperature TT-ideal = T°1~ (3.1.1) + 10°C
10.1.1 ideal target temperature TT-ideal = T°li (3.1.1) + 10°C
for VC = 5 mlmin
10.2 real temperature window TT-real = T°1~ (3.1.1) + 10°C ~
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5°C
10.2.1 real temperature window TT-real = T°li (3.1.1) + 10°C ~
5°C for VC = m/min
11 Rolling Mill
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