Note: Descriptions are shown in the official language in which they were submitted.
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CONTINUOUS CASTING CHILL WITH A COOLANT DUCT
The invention relates to a continuous casting chill with a coolant duct, which
is
formed by a chill inner wall facing the molten metal, as the hot side, a chill
outer
wall as the cold side and a right and a left side wall.
From DE 198 26 522 Al a chill wall of a continuous casting chill is known,
which
consists of a chill inner plate and a water compartment connected to the chill
inner plate by screw connections, with the chill inner plate having on its
side
facing the water compartment webs with grooves running therebetween, in
which fillers are arranged. The grooves serve here as cooling ducts for a
coolant, generally water. The fillers serve to reduce the duct cross-section,
so
that the speed of flow of the coolant in the cooling duct is increased.
DE 198 42 674 Al describes similar fillers.
Continuous casting chills with cooling ducts are additionally known from the
documents DE 101 22 618 Al, DE 100 35 737 Al and DE 101 38 988 C2.
From DE 102 53 735 Al a chill is known for the continuous casting of molten
metals, particularly of steel, with cooling ducts such as cooling grooves,
cooling
slits or cooling bores in the contact surface lying opposite the chill hot
side. The
heat transmission of the chill is improved in that the geometric shapes of the
heat-transmitting surfaces of a cooling duct or of a group of cooling ducts is
adapted in form, cross-sectional area, circumference, boundary surface
quality,
orientation to the contact surface, arrangement and/or arrangement density
with
respect to the contact surface of the local formation of heat flow density
and/or
temperature of the contact surface in the casting operation, and in particular
in
the region of the casting level.
During continuous casting, the fluid melt flows out from a continuous casting
distributor through an immersion tube into an oscillating, water-cooled copper
chill. As a result of the heat dissipation, the melt temperature falls below
the
solidus temperature and a thin strand shell is formed which is withdrawn in
the
casting direction. With increasing cooling, the thickness of the strand shell
increases until the strand is completely solidified. Depending on the format
and
number of strands, casting speeds of 6 m/min and above are nowadays
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achieved. Typical local heat flow densities lie in the order of up to 12
MW/square metre.
The heat flow which is carried off by the coolant is, inter alia, dependent on
the
geometry of the coolant ducts, the roughness of the walls and the through-flow
speed and hence also on the degree of turbulence. The higher the degree of
turbulence on the coolant side, the more intensive is the intermixture and all
the
more heat is carried off. The heat-transmitting area can in fact be increased,
but
close limits are set for this increase. Particularly in the case of very high
heat
flow densities, a contamination frequently takes place of the heat-
transmitting
surfaces by deposits, which is known as fouling. As the deposits have a very
low thermal conductivity, fouling in the case of chill cooling leads to an
intensive
increase in the copper temperature and hence to a reduced service life of the
chill.
Conventional continuous casting chills are formed with rectangular coolant
ducts which are flowed through at speeds of flow of approximately 10 m/s. In
these coolant ducts, with Reynolds numbers of approximately 250,000 a
turbulent flow forms with a main component in the axial direction. The basic
turbulence leads to an increased exchange or mass, impulse and energy
between the individual coolant layers. Close to the wall, flow- and
temperature
boundary layers form which can be described by so-called logarithmic wall
laws.
The turbulence is attenuated with increasing proximity to the wall. The main
disadvantage of conventional cooling lies in the directed turbulence with
predominant components in the axial flow direction and lower components in
the radial flow direction.
The invention is based on the problem of providing a continuous casting chill
in
which the recrystallization process of the chill material or the material of
the
walls of the coolant duct which is dependent on the operating temperature and
the duration of operation, is decelerated, the service life of the chill and
the
turbulence are increased and a homogeneous intermixture of the coolant is
achieved.
This object is achieved according to the invention in that in a continuous
casting
chill with a coolant duct which is formed by a chill inner wall facing the
molten
metal as the hot side, a chill outer wall as the cold side and a right and a
left
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side wall, the coolant duct is formed with elements which generate turbulence.
Through the introduction of turbulence-generating elements, generally a more
intensive intermixture of the coolant is achieved. At the same time, the
turbulence-generating elements increase the heat-transmitting area of the
coolant duct or of the chill walls. The cooperation of the two measures, i.e.
turbulence generation and increase of the heat-transmitting area, improves the
local heat transmission from the walls of the coolant duct or from its walls
to the
coolant, which then carries off the heat.
The fundamental principle of all turbulence-generating elements is based on
the
turbulence-induced transportation of mass, impulse and energy. The thermal
transmission in the coolant duct of continuous casting chills is improved in
accordance with the invention. As a result of the more intensive intermixture,
the turbulence generators lead to higher local heat flow densities, i.e. the
heat
which is carried off per unit of area is increased. The turbulence, both in
the
vicinity of the wall and also in the region of the core flow is increased and
a
homogeneous intermixture is achieved. Through the turbulence-generating
elements, a better intermixture of the cooling water is achieved and the
temperature level in the copper is reduced, with the recrystallization process
of
the chill material or of the material of the walls of the coolant duct, which
is
dependent on the operating temperature and duration, being decelerated. This
leads to an increase in the service life of the chill. The material of the
chill or of
the chill walls is, for example, copper, partially copper or another material.
In
addition, the contamination and the tendency to deposits are reduced by the
increased turbulence and the greater shear forces on the hot side of the
cooling
duct.
On the rear edge of the turbulence-generating elements, the water flow breaks
off and a non-steady and eddied, ie. turbulent recirculation area forms. A
first
embodiment of turbulence-generating elements consists of horizontal stages in
the coolant which are formed for example by rectangular profiles which extend
over the entire width or partial regions of the coolant duct. A second and
third
embodiment of turbulence-generating elements has the form of tetrahedra and
winglets. In these forms, inwardly turning vortex trains are induced which
lead to
an even more intensive intermixture of the coolant. Vortex trains can be seen
for example at the end of an airfoil or behind motor vehicles, where they are
basically undesired. The turbulence-generating elements are arranged on the
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hot side for example staggered one behind the other, with the spacing being
determined applicably by the spatial extent of the recirculation area lying
upstream. Alternatively, the turbulence-generating elements can also be
installed on the cold side, because the effect of the recirculation extends up
to
the hot side. A combination of tetrahedra on the cold side and horizontally
arranged stages on the hot side of the coolant duct is also possible.
Likewise, it
is conceivable to install the turbulence-generating elements only in the inlet
of a
coolant duct or only at the height of the casting level, in order to keep the
expenditure as regards manufacturing technology within limits. In addition to
the
above-mentioned effects with regard to flow technology, the heat-transmitting
area is increased somewhat by the turbulence elements, by approximately 6 %
with the described tetrahedra. In this way, the local heat flow density is
also
increased. The pressure loss can be kept low through the dimensions of the
turbulence elements which are not selected to be too great.
The basic mode of operation of the coolant duct according to the invention can
be verified by means of numerical flow simulations (CFD - Computational Fluid
Dynamics).
Example embodiments of the invention are described in further detail by means
of very diagrammatic drawings.
Fig. 1 a part of a continuous casting chill in three-dimensional illustration;
Fig. 2 the continuous casting chill in front view in section with turbulence-
generating elements according to a first embodiment;
Fig. 3 the continuous casting chill in front view in section with turbulence-
generating elements according to a second embodiment;
Fig. 4 the continuous casting chill in front view in section with turbulence-
generating elements according to a third embodiment; and
Fig. 5 the continuous casting chill in side view in section with turbulence
generating elements.
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Figure 1 shows in three-dimensional illustration a part of a continuous
casting
chill 1 with a coolant duct 2, which is formed by a chill inner wall 3 facing
the
molten metal as the hot side, a chill outer wall 4 as the cold side and right
side
wall 5 and a left side wall 6. Turbulence-generating elements 7, 9 and 10 are
arranged in the direction of flow 8 on the chill inner wall 3, the hot side,
and
project into the coolant duct 2.
Figure 2 shows in a front view in section the coolant duct 2, in which
turbulence-
generating elements 7 in the form of tetrahedra are arranged in two rows 11 on
the chill inner wall 3. The tetrahedra point with their tip in opposition to
the
direction of flow 8. Through such an arrangement, an increasing resistance is
produced. The coolant behaves in a turbulent manner behind the tetrahedron.
The tetrahedra can also be arranged so as to be staggered.
In Figure 3, turbulence-generating elements 9 are illustrated in the form of
horizontal stages. The horizontal stages are formed for example by a
rectangular bar (see Figure 5) which extends over the entire width of the
coolant
duct 2.
A further form of the turbulence-generating elements 10 is illustrated in
Figure
4. These turbulence-generating elements 10 have the form of winglets. These
winglets, known for example from aeroplane wings, are either fastened on the
chill inner wall 3 aligned in rows 11 one behind the other, or are fastened
distributed on the chill inner wall, as indicated by the lowermost winglet.
All the turbulence-generating elements 7, 9 and 10 project from the chill
inner
wall 3 into the coolant duct 2 or vice-versa and influence the coolant when it
flows in the flow direction 8 through the coolant duct 2.
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List of reference numbers
1 continuous casting chill
2 coolant duct
3 chill inner wall
4 chill outer wall
right side wall
6 left side wall
7 tetrahedron
8 flow direction
9 horizontal stage
winglet
11 row
