MAGNETIC BRAKE FOR CONTINUOUS CASTING MOULDS

FREIN MAGNETIQUE POUR COQUILLE DE COULEE CONTINUE

English Abstract

The invention relates to a continuous casting mould, in particular a thin slab
mould, whereby the flow of liquid metal is influenced by means of permanent
magnets arranged on the mould. The permanent magnets give a varying field
strength by means of differing magnet strengths over the width and/or height
thereof, or varying separations thereof. In order to vary the magnetic field
strength the permanent magnets can be differently adjusted in groups to alter
the field strength distribution.

French Abstract

L'invention concerne une coquille de coulée continue, en particulier une coquille à brames minces, dans laquelle un champ magnétique, produit au moyen d'aimants permanents placés sur la coquille, influe sur l'écoulement du métal liquide dans la coquille, ces aimants permanents présentant, sur la largeur et/ou la hauteur, des intensités d'aimant différentes ou des écartements différents les uns par rapport aux autres pour une intensité de champ différente. L'objectif de cette invention est de maintenir une variation de l'intensité du champ magnétique. A cet effet, ces aimants permanents peuvent être placés en groupes de façon diverse pour une répartition différente des intensités de champ.

Claims

CLAIMS:
1. A continuous casting mold, in particular a thin slab mold in which
permanent magnets are mounted on a mold for influencing the flow of a
liquid metal in the mold by a generated magnetic field, wherein the
permanent magnets have, over the width and/or height thereof, different
magnetic strengths or are spaced from each other by different distances
for a different field strength and for a different field strength distribu-
tion, are differently displaceable in groups over the mold, and wherein
the permanent magnets are arranged in a water box of the continuous
casting mold and are placed so that they are directly abut the mold,
characterized in that
the permanent magnets are displaceable on pivotable adjusting means
on the mold for adaptation of field strength.
2. A continuous casting mold according to claim 1,
characterized in that
adjusting means for the permanent magnets is formed as rotating
devices or rotating spindles.

3. A continuous casting mold according to claim 1 or 2,
characterized in that
the adjusting means for the permanent magnets is formed as rotating
devices, hydraulic cylinders, or rotating spindles.
4. A continuous casting mold according to claim 1, 2, or 3,
characterized in that
between the magnets and a copper plate, an iron core is arranged.
5. A continuous casting mold according to claim 1, 2, or 3,
characterized in that
the permanent magnets are arranged in a water box of the continuous
casting hold and for directly abutting the mold plate.
6. A continuous casting mold according to one or several of claims 1
through 4,
characterized in that
an iron core, as a pass-through body of the water box, fills space
between the copper plate and a permanent magnet.
11

7. A continuous casting mold according to claim 6
characterized in that
between the pass-through body and the adjustable permanent magnets,
a separation plate, which is formed, preferably, of non-ferromagnetic
material or plastic material, is inserted.
8. A continuous casting mold according to one or several of claims 1
through 7,
characterized in that
the permanent magnets consist of a plurality of small separate
magnets which are arranged on a large-surface carrier of a
ferromagnetic material and are operatively corrected in several layers
to form a large surface magnets.
12

Description

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MAGNETIC BRAKE FOR
CONTINUOUS CASTING MOLD
The invention relates to a continuous casting mold, in particular a thin slab
mold in which permanent magnets are mounted on a mold for influencing the
flow of a liquid metal in the mold by a generated magnetic field, wherein the
permanent magnets have, over the width andlor height thereof, different
magnetic strengths or are spaced from each other by different distances for a
different field strength and for a different field strength distribution, are
differently displaceable in groups over the mold, and wherein the permanent
magnets are arranged in a water box of the continuous casting mold and are
placed so that they are directly abut the mold.
The use of magnetic means for braking and homogenizing the liquid metal
flow is a known technique and is described in numerous technical documents.
The installation components, which are described in the documents, have all
large masses which make difficult the oscillation of the mold that is
necessary
for the operation.
The document EP O 880 417 B describes a magnetic brake for casting metal
in a mold and which consists of a magnetic core and a coil supplied with
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permanent current or low-frequency alternating current. There is further
provided a return line for closing the magnetic circuit.
The progress in the development in the field of permanent magnets (hard
ferrites, rare-earth magnets) opened, meantime, new uses for possible field
la

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strengths of permanent magnets, which permanent magnets appears to be a
suitable alternative for use instead of the above-described electrical magnet.
It has already been proposed to replace the electromechanical brake (EMBr)
equipment, which was used up to the present for generating the magnetic field
(field coils, electrical control, outer yoke for conducting the magnetic flux,
etc.), with permanent magnets which are directly mounted on the mold.
The document EP 0568 579 describes a method of controlling the flow of the
molten metal in a non-solidif ed metal region of a casting mold, wherein the
mold is supplied with at least one primary flow of the molten metal and a cast
strand is formed, and wherein at least one static magnetic f eld is generated
by
poles which are arranged adjacent to the mold and consist of permanent
magnets. The magnetic field seines for breaking the primary flow of the molten
metal flowing in the mold and for splitting the primary flow and for
controlling
the produced secondary flow. The magnetic field is so arranged that it acts
over
the entire width of the strand formed in the mold. The magnetic field should
extend in a plane extending perpendicular to the cast direction and at level
at
which the magnetic field strength reaches its maximal value and can be varied
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within a range of fiom 60% to 100% of the maximal value, while
simultaneously the f eld strength has a maximum value of 500 Gauss at a Ievel
with the highest outer surface/meniscus of the moltezi metal. The magnetic
f eId is controlled and distributed by providing displaceable magnetic poles
and/or adjustable core members.
The document EP 00 40 383 Bl describes a method of stirring the non-
solidified region of a cast strand, wherein the strand is formed in a mold,
and
the cast steel flows tlwough a pouring spout or directly into the mold. There,
where the cast steel penetrates the melt already amassed in the mold, at least
one static magnetic field is generated that brakes the cast or pouring steel
and so
splits it that its momentum is weakened or absorbed. The device, which is
provided to this end, can be formed of one or several permanent magnets.
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Document JP 08155610 discloses a rectangular mold in four corners of which
permanent magnets are arranged for generating South and North magnetic
fields.
Document EP 0 568 579 A discloses a continuous casting mold in which
permanent magnets are arranged on the mold for influencing the flow of a
liquid metal, and wherein the magnets have different magnetic field strengths
and are adjustable with insertable magnetic and non-magnetic core elements.
Permanent magnets have a substantially smaller configuration at the same
magnetic induction field strength and, therefore, a significantly reduced
mass.
They do not require any additional means for conducting a magnetic flux in
form of an outside yoke. When necessary, it is sufficient to use ferromagnetic
materials, which are available in the mold frame, for closing the magnetic
flux
circuit.
However, use of permanent magnets requires other special procedures. In the
state of the art, permanent magnets are used as possible sources of static
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magnetic fields but only as equipment for the case when the magnetic field is
generated by current coils with direct current DC or low-frequency alternating
current, as discussed above, but not, however, for permanent magnets.
Because permanent magnets have no switch for turning on and off, they
require special safety measures for installation and monitoring of the
equipment. In distinction from the alternating current drive, special methods
of equipment are necessary for operating a continuous casting machine.
With a magnetic brake, there are provided, on both sides of the mold opposite
each other, permanent magnets for generating a magnetic field. The induction
field strength at this arrangement follows, at a spacing between the permanent
magnets in the intermediate space, an equation:
~~ [z-a~
B(z) = 2 ~ Bo ~ cosh
h
wherein Bo is the induction field strength of one of the permanent magnets, z-
distance from one of the magnets, d-distance between the magnets and h-
operating height of the magnets. The operating height is determined by
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measurement. His the number Pi (=3.14...), and cos is a hyperbolic cosine
(see Fig. 1 ).
The object of the invention is to provide, on a continuous casting mold, means
for varying the magnetic field strength of permanent magnets.
According to the invention, this object is achieved by differently adjusting
the
permanent magnets in groups for a different distribution of the field
strength.
According to an advantageous embodiment of the invention, the permanent
magnets are displaced on displaceable and/or pivotable adjusting means over
the mold for adapting the field strength.
This is effected, e.g., by changing the distance of the magnets from each
other,
advantageously, by pivoting the carrier of the permanent magnets away from
the mold. There exist further possibilities by a direct method with rotatable
spindles or hydraulic cylinders (see Fig. 2). In case of pivoting of the
magnet
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carrier away from the casting mold, the weakening of the field strength
follows the following equation:
~ - I B I I A I cos(<(B,A)),
where ~ is magnetic flux, B is magnetic field strength, A is pass-through
surface to the casting mold, and cos is cosine of an angle between the vector
of the magnetic field strength and the vector of the surface normal of the
pass-
through surface. The varying of the magnetic flux is effected over the field
weakening B according to the equation B (~) and the angle. W case of the
mechanical displacement, as changing of the distance, changing of ~ is
effected
only over the field weakening B according to the above-mentioned equation
over B(~).
The rotation facilitates, on one hand, detachment of the magnets fiom the pass-
tlwough surface, then, according to the instructions for mounting of these
permanent magnets, they are put on an edge and, thereafter, are placed on the
can-ier with a constantly diminishing angle (see Fig. 3). The magnets are not
placed directly on the can-ier from a ferromagnetic material, rather, to
facilitate
detachment for rotation or mounting, a layer of a non-ferromagnetic material
is
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provided therebetween. This can be an austenite steel, however, a plastic
sheet
with a thickness of about lmm suffice. The non-uniform distances of tile
magnets to the pass-through surface, which are associated with rotation, are
magnetically equalized by a pass-through body, the water box of the casting
mold of a fewomagnetic material.
There exist two configurations of the casting mold, a mold with a recess for a
magnetic brake advanced from outside, and a configuration with a magnetic
brake integrated into the water box. For both cases, the following equipment
is
necessary:
Casting moldings with window for a magnetic brake applied fi~om outside:
The field strength of the magnetic field, which is generated by permanent
magnets, should remain adjustable. To this end, the permanent magnets are
mounted on the teeth of a rake that engages the reinforcing ribs of the water
box
of a casting mold. A device provides for adjustment of the distance of the
teeth
to the mold by displacement. Thereby, it is possible to vary the strength of
the
magnetic field. The device can be displaced by a mechanical spindle or a
hydraulic cylinder.
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Casting molds with an integrated magnetic brake:
The electrical device, which was used for generating a magnetic field, is
removed, and then a device for holding the permanent magnets is mounted on
an uncovered ferromagnetic block (the pass-tlwough window) in the water box
This device is displaceable by rotation and, thus, the magnetic field is
varied.
The device can be displaced by a mechanical spindle or by a hydraulic
cylinder.
W addition, there exists a possibility to have this device rotate about an
axis on
the lower edge and, thereby, to provide for changing the distance between the
permanent magnets and the fen omagnetic block. This likewise provides for
adjusting the magnetic field strength.
Permanent magnets are so strong that they cannot be made as large-surface
elements. Such a magnet can explode under its own field strength, i.e., acW
ally
be destroyed. One is thus compelled to make large-surface magnets for the
width of a continuous casting mold of a plurality of separate magnets which
are
glued onto a large-surface cawier of a ferromagnetic material, in order to
combine magnetic flux densities of the plurality of separate magnets into a
large-surface magnetic flux which exercises a metallurgical effect in the
mold.
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It is also of importance that by effecting tile same alignment of the magnetic
poles, small magnets are not arbitrarily arranged tightly next to each other
and,
finally, the same poles should be repelled. Therefore, one is compelled to
form
the magnet can-ier of several layers because the still open intermediate space
of
the first layer should be covered ill the second layer by permanent magnets.
Further, with a rake (comb-shaped brake), tile magnets must not only be
located
on the teeth of the rake but rather on tile back side of the magnet carrier
(rake)
of a ferromagnetic material and, here, again of several layers, because
otheuwise
the necessary magnetic flux density in the metallurgical section of the mold
would not be reached.
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