Note: Descriptions are shown in the official language in which they were submitted.
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WE/wa 93165C~
23.01.1995
CASTING CHILL MOULD
The invention relates to a casting chill mould for metal melts,
which is formed by electrically conductive wall elements
enclosing the metal melt.
Such casting chill moulds are used, for example, in the
continuous casting of steel. The melt solidifies in the chill
mould, while at the same time the solidified steel is
continuously drawn off from the chill mould. The formation of
structure during solidification can be positively influenced by
subjecting the melt to an inductive stirring process. To this
end the melt is exposed to a magnetic field which changes in time
and space, via which a magnetic rotary field is generated in the
melt. This exerts on the melt a torque via which the melt is
"stirred".
The operation of such inductive stirrers is comparable with the
operation of an asynchronous motor. The interconnected values
(magnetic field and electric current) determining the torque
generated and transmitted by a transformer in accordance with the
law of induction are transmitted in a contactless manner via a
magnetic rotary field through the chill mould enclosing the melt
to the liquid metal.
One proble~ of inductive stirring is that the magnetic rotary
field generated by the stirring device acts not only on the melt,
but also on all other metal members disposed in its range of
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action. The consequence is that at first only the chill mould
enclosing the melt is subjected to the full effect of the
magnetic induction. Since due to its electric conductivity the
chill mould forms a closed conductor circuit, as soon as the
chill mould is exposed to a magnetic rotary field, an induction
current flows in the mould and again produces a magnetic field.
This second opposing magnetic field, directed oppositely to the
primary magnetic field generated by the stirring device,
partially cancels out the effect of the primary field, so that
finally only a fraction of the magnetic induction originally
generated operates in the metal melt. The casting mould
enclosing the melt therefore screens the melt against the
required action of the magnetic field generated by the device.
The screening effect of the chill mould is particularly heavy in
the stirring of a steel melt during the continuous casting of
steel, since the chill mould used in continuous casting is
generally made of copper and has a particularly high
conductivity.
It is known that the screening effect of the chill mould depends
mainly on the frequency of the rotary field, the geometry of the
chill mould and the electric conductivity of the material of the
chill mould. Since the two last-mentioned values as a rule are
fixed, in practice attempts are made to increase the depth of
penetration of the magnetic rotary field by reducing its
frequency, thereby lessening the screening effect of the chill
mould. However, the disadvantage of that step is that a
reduction in the frequency of the rotary field also reduces the
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required movement in the melt. The consequence is, for example,
a reduction in the quality improvement of the steel thus treated.
Another possibility is to increase the strength of the magnetic
field, but this involves a high energy cost and the accompanying
high operating costs.
It is an object of the invention to provide a chill mould which
enables the metal melt to be effectively stirred by induction at
a low energy cost.
This problem is solved according to the invention by the feature
that each wall element is electrically insulated from the
adjoining wall elements. In this way the chill mould enclosing
the melt no longer forms a closed conductor, so that in the
device according to the invention induction current can no longer
flow in the chill mould. In contrast with the prior art, this
feature results in a considerable reduction of the opposing
magnetic field generated in the chill mould, so that when the
chill mould according to the invention is used in an inductive
stirring device, apart from small losses, the primary magnetic
field generated by the stirring device acts with its full power
directly on the melt.
Since the chill mould now has substantially no influence on the
effect of the primary magnetic field, the frequency and therefore
the speed of rotation of the rotary field generated by the
stirring device can be increased without the risk of additional
losses. As a result, the effectiveness of the inductive stirring
is enhanced.
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Moreover, as a result of the reduced losses, the energy required
for a stirring device equipped with the casting chill mould
according to the invention is less than that required by
conventionally equipped devices. As a result, transformers and
frequency converters which are of lower power and therefore cheap
can be used for supplying the device operated in conjunction with
the chill mould according to the invention, while the geometry of
the mould remains unchanged. In a precisely similar manner, the
cross-sections of the cables used can be reduced due to the low
electric power required, the result being a reduced requirement
for material and therefore a saving in costs. The reduction of
the power density in the stirring coil of the stirring device
accompanying a reduction in the electric power required also has
positive effects on the device's service life.
On the other hand, due to the low screening effect of the casting
chill mould, its wall thicknesses can be increased while the
power of the stirring device remains unchanged. This results in
a lengthening of the service life of the casting chill mould.
Lastly, with an inductive stirrer operated in conjunction with
the casting chill mould according to the invention it is no
longer necessary to adapt the stirring current to the state of
wear of the wall elements, since the thickness of the wall
elements of the mould no longer influence their screening effect.
In this way it is possible to obviate errors caused in the prior
art by a delayed readjustment of the magnetic field generated by
the stirrer. This produces appreciable advantages in securing a
constant satisfactory quality of the melt treated.
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The chill mould according to the invention is more particularly
suitable for use as a continuous casting chill mould.
An embodiment of the invention shown in the drawing will now be
explained in greater detail.
The drawing shows in perspective a water-cooled casting chill
mould 1 for the continuous casting of steel. The mould is
enclosed by a stirrer generating an electromagnetic field acting
on the melt. The chill casting mould is formed by four
positively assembled plate-shaped wall elements 2 which enclose
the steel melt cast into the chill mould. The wall elements 2, 3
are insulated electrically from one another by insulating paper 4
which is inserted in joints 5 between the wall elements 2. This
prevents the occurrence of an opposing field to the primary
magnetic field generated by the stirrer. The primary magnetic
field can therefore act with substantially full strength on the
melt during its solidification.
