Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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The present invention relates to an apparatus for
electromagnetic stirring of the liquid core of slabs in a
continuous casting installation and to a method of continuously
casting strands with the aid of electromagnetic stirrers for
producing traveling electromagnetic fields which generate
turbulent flow in the cast strand.
It is already known to the art to alternately cast
at the same continuous casting installation either a slab or
strands having smaller dimensions, for instance in twin pours
two blooms. This has the advantage, through the possibility
of varying the sectional shape of the cast product, to variably
structure the product pallets of a continuous casting instal-
lation. Upon casting smaller strand widths the twin pour
enables increasing the casting output of the continuous casting
installation with lesser costs, while shortening the casting
time.
It is also known in the art to produce, with the aid
of electromagnetic stirrers, traveling fields which produce
turbulent flows, in order to thereby imporve the internal
properties of the cast product in the liquid core or sump of
the continuously cast strand. These turbulent flow conditions
cause break-off of the dendrite tips at the solid~liquid
interface or solidification front, produce an intensified
seed formation and therefore favor globulitic solidification.
According to a heretofore know arrangement electro-
magnetic stirring is accomplished with the aid of electro-
magnetic, multi-phase stirrers arranged lengthwise of the cast
strand. Such electromagnetic stirrers are located externally
! of the strand guide arrangement or roller apron such that
between the stirrers and the strand surface there are arranged
one or a number of guide rolls formed of non-magnetic material.
This
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design is not suitable for stirring at continuous casting
installations which are capable of alternate casting modes.
Furthermore, there is known to the art equipment
wherein at least two stirrers are successively arranged at a
respective side of the strand near to the strand surface
between the guide rolls and in the lengthwise direction of the
strand. These stirrers induce traveling magnetic fields within
the strand which are effective either oppositc ~o or in the
casting direction. Also this equipmellt is not suitable for
stirring the molten metal with alternate mode oE operation of
the continuous casting installation.
Therefore, with the foregoing in mind it is a primary
object of the present invention to provide a new and improved
apparatus and method for the electromagnetic stirring of molten
metal in a continuous casting installation, in a manner not
afflicted with the aforementioned drawbacks and limitations of
the prior art proposals.
Another and more specific object of the present in-
vention is to devise for an alternatively operable continuous
casting installation, in other words a casting installation
which can be converted either for casting a slab or strands
of smaller dimensions, with the aid of very sim)1e constructional
means equipment for stirring slabs or blooms in a highly
effective manner.
Another important object of the present invention
is to devise apparatus which is capable of electromagnetic
stirring the molten metal while fulfilling the prerequisites
for a multiplicity of different casting programs intended to
be performed at the continuous casting installation.
Yet a further significant object of the present
invention aims at devising apparatus for performing different
stirring methods which are accommodated to related casting
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programs which are to be performed at a continuous casting
installation.
According to the present invention there is provided
an apparatus for electromagnetlc stirring of the liquid core
of slabs in a continuous casting installation, comprising:
a plurality of electromagnetic stirrers for producing travel~
ing electromagnetic fields which produce turbulent flow in the
continuously cast slab, at least two of said stirrers are sym-
metrically arranged to the longitudinal axis of the slab,
means for displaceably mounting at least one of said stirrers
for movement transversely with respec~ to the longitudinal
axis of the sla~. The electromagnetic stirrers are directed
either conjointly to act upon a predetermined path of travel
of a cast slab or individually to act upon respective ones
of two predetermined paths of travel of a respective bloom.
With t~he individual stirring of two blooms there are
required two stirrers which operate independently of one
another. In consideration of the stirring of a slab the
provision of two smaller stirrers, instead of a single large
stirrer, with at least the same stirring effect, doest not
bring about any appreciable increase in the costs of the
installation. Yet, two stirrers have the advantage that
with different casting parameters it is possible, with inten-
tional selection of marginal conditions, to adjust a multipli-
city of stirring techniques with different metallurgical
effects.
According to a preferred feature of the invention
the electromagnetic stirrers are effective from a wide side
of the roller apron or strand guide arrangement. In this
way, without any additional equipment expenditure, upon
changing the cas-ting program, there is possible alternate
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1 15'j630
stirring of either a slab or two blooms~
If at a continuous casting installation there is only
cast, in each case, either a slab sectional shape or a bloom
sectional shape, then for the purpose of realizing a good
stirring action also at the blooms it is adyantageous to ins- .
tall the electromagnetic stirrers such that during the casting
of two blooms the stirrer-central axes, extending in the
lêngthwise direction of the continuously cast strand, coincide
wit~ the related lengthwise axis of the cast bloom.
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In this way, whenever there is a change in the sectional shape
of the cast product, there is beneficially dispensed with the
need for long equipment change-over times.
According to a further feature of the invention the
electromagnetic stirrers may ~e rotatably arran~ed with respect
to the lengthwise axis of the continuously cast slab, for the
purpose of changing the direction of their traveling fields.
Depending upon the strived for metallurgical effect and in
consideration of the relevant strand format or sectional shape
there are desired variable effective directions of the produced
traveling fields. Due to the rotatable arrangement of the
electromagnetic stirrers it is possible to adjust any desired
angle between the primary direction of movement of the traveling
fields and the strand withdrawal direction.
Advantageously, the electromagnetic stirrers are
arranged at essentially the same elevational position. During
electromagnetic stirring of a slab, if the electromagnetic
stirrers a~re dispositioned at the same level or height, and if
they are effective for instance in the lengthwise direction of
the cast strand, it is possible to produce at the strand core
or pool flow currents which flow symmetrically with respect
to the lengthwise axis of the cast strand. In this way there
is beneficially produced a compensated temperature profile at the
liquid pool or core which, in turn, insures for a uniform advance
of the solidification front with respect to the center of the
strand, and thus there are extensively avoided deformations of-
the strand.
According to a further aspect of the invention it is
advantageous when stirring blooms if the electromagnetic stirrer
is effective at the outer, unsupported side of the cast strand.
This is particularly then desirable if, owing to a larger slab
sectional shape, there is required for the support of the strand
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a larger diameter of the guide rolls, which, in turn, requires
a greater spacing from the strand surface, and therefore, a
lower degree of effectiveness of the electromagnetic stirrer.
With an arrangement where the electromagnetic stirrer is
effective from the wide side of the roller apron or strand
guide arrangement, transversely with respect to the withdrawal
direction of the bloom, it is therefore not always possible to
obtain an adequate stirring action owing to the short effective
length. A stirrer which is effective at the unsupported strand
side directly near to the region of the strand surface has
appreciably less losses in its stirring action, so that there
also can be obtained in the transverse direction the desired
stirring movement~
For the same reason it is also advantageous, when
casting smaller slabs, to have the electromagnetic stirrers
effectivQ at the narrow sides of the slab.
Due to the fact of having at least one of the
electromagnetic stirrers arranged to be displaceable transver-
sely with respect to the lengthwise direction of the strand,
it is possible when casting a slab to exactly adjust the degree
of the turbulence in the moved molten metal by moving closer
the traveling electromagnetic fields until there is an inter-
section of both flows, and to thereby compensate the turbulence
to the relevant casting parameters. When casting two blooms it
is easily possible, owing to the displaceablility of the stir-
rers, to bring into coincidence the lengthwise axes of the
blooms in the event of changes in sectional shape. In this way
there can be beneficailly avoided limiting the field of applica-
tion of the equipment for only use with slab or bloom sectional
shapes. All of the strand sectional shapes or formats which
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can be cast at the related continuous casting installation
equally can be effectively stirred without any difficulty.
According to the present invention there is also
provided in a method oE continuously casting strands wlth the
aid of electromagnetic stirrers for producing traveling elec-
tromagnetic fields which generate turbulent flow in the cast
strand, the improvement comprising the steps of: providing
a plurality oE electromagnetic stirrers which are conjointly
directed at the cast strand when casting a slab or indivi-
dually at a related one of related blooms; and generating
turbulent flows by the traveling electromagnetic fields which
have the same effective direction.
Depending upon the arrangement of the stirrers,
such has decisive effects upon the nature and configuration
of the individual flows in the strand pool or core. If when
stirring a slab the electromagnetic stirrers are arranged such
that the partial flows produced thereby augment one another,
then such affects the intensity and magnitude of the total
flow. From the shape and magnitude of the flow or the flows
in the strand core or pool it is however possible to realize
a multiplicity of metallurgical effects, and it is intentio-
nally possible to take into account different easting para-
meters.
According to a further feature of the invention
the turbulent flows may be produced by traveling fields whieh
are effective in the strand lengthwise direction, and advan-
tageously, by traveling fields which are effective in a
direction opposite to the strand withdrawal direction. By
virtue of the thus realized transport of cooler melt from the
lower situated region of the metal pool in the direction of
the continuous casting mold and the therewith associated
,
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transport of inflowing hot melt in the opposite direction
there is realized at the prevailing counterflow at the region
of action of the stirrer, a temperature compensati'on in the
liquid pool of the continuously cast strand. At the lower
region of the pool there prevails an increase of the tempe-
rature gradient directly at the solidification front or solid-
liquid interface associated with a reduction in the solidi-
fication rate and there is obtained a reduction of the
heterogeneous layer, so that there is favored
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the strived for globulitic solidification.
For many fields of application it can be advantageous
if the turbulent flows are produced by traveling fields which
are effective perpendicular to the strand withdrawal direction.
Additionally, by virtue of symmetrical powering of the phases
of the coils of both electromagnetic stirrers the turbulent
flows may be produced by traveling fields which generate equal
magnitude thrust forces. Upon occurrence of the turbulent
flows at the solidified side wall of the continuously cast
strand there are produced vortices. In this way it is possible
to circulate, by means of the turbulent flow, the melt not
only in the plane of the strand cross-section, but also over
a larger region in the lengthwise direction of the strand.
This produces an advantageous exchange of the melt from the
effective region of the electromagnets of the stirrers with
fresh molten steel which is inflowing from the continuous
- casting mold. Hence, in this way there is possible temperature
compensati~on throughout the entire melt. Notwithstanding the
low space requirements of the electromagnets in the strand
lengthwise direction the stirring effect is realized throughout
a large region of the liquid steel. ~raveling fields producing
equal magnitude thrust forces at both stirrers effective upon
a slab require, related to the slab center, to the left and
right thereof the same flow magnitudes, and therefore, a uniform,
defined solidification behavior at the corresponding affected
part of the slab.
A further advantageous effect of the method is
realized when stirring slabs if the turbulent flows are mutually
effected by the thrust forces produced by the traveling
fields. In this way there is realized a different turbulence
in the entire effective region of the traveling field, which
turbulence is favorable in terms of reducing segregation
phenomenon.
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An intensification of this eELect is advantageously
realized in that, during stirring oE slabs with different elec-
trical powering of each stirrer, there is produced within the
strand traveling electromagnetic fields of different thrust
force. Owing to the therewith associated different flow
velocities of both flows caused by the traveling Eields, an
additional turbulent flow or vortex exists in the layer of
mutual effect, which causes an intensification of the uniform
distribution of elements tending towards segregation.
In accordance with a further aspect of the invention it
is possible by asymmetrically electrically powering the phase
coils of at least one stirrer to produce within the traveling
field differently effective thrust forces. When stirring a
slab it is ~thus possible to additionally intensify the agitation
or stirring effects. Surprisingly it has been found that by
virtue of these measures there can be reproducibly prevented
negative segregation effects, for instance in the form of so-
called white bands.
According to a further aspect oE the inventive
method, while utilizing the apparatus of the invention for
stirring oE slabs, it can be advantageous if the turbulent
flows are generated by traveling ields having opposite
effective direction. For wide slabs it can be necessary for
producing the desired metallurgical effect to produce a single
circulating flow instead of two circulating partial flows.
By means of the traveling fields effective towards one another
in the strand lengthwise direction this is possible without
any additional equipment expenditure.
Furthermore, it is advantageous if there are produced
independently of one another in both strands, by asymmetrically
powering the phases of both stirrers, turbulent flows by the
action of thrust forces which are difEerently efective in the
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traveling fields.
When stirring blooms it is possible, by flows caused
by a stirrer at the relevant strand pool, through asymmetrical
powering oE the stirrer, to produce such turbulent flows that
it is possible to positively prevent possible pronounced segre-
gations.
The invention will be better understood and objects
other than those set forth above, will become apparent from
the following detailed description of preferred embodiments
having reference to the annexed drawings wherein:
Figure 1 is a top plan view of a portion of a strand
guide arrangement or roller apron in a secondary cooling zone
of an alternative continuous casting installation having two
fixedly installed electromagnetic stirrers;
Figure 2 is a top plan view of an exemplary embodi-
ment using electromagnetic stirrers which are mounted to be
movable;
Figures 3, 4 and 5 show symmetrically powered elec-
tromagnetic stirrers having different effective directions
during the stirring of slabs;
Figures 6, 7 and 8 illustrate electromagnetic
stirrers which are effective opposite to the strand withdrawal
direction where the turbulent flows which are produced mutually
affect one another during the electromagnetic stirring of slabs;
Figure 9 illustrates two independent asymmetrically
powered stirrers during stirring of two blooms; and
Figure 10 illustrates an arrangement of electro-
magnetic stirrers which are effective at the outer unsupported
strand side during the stirring of two blooms.
Describing now the drawings, it is to be understood
that only enough of the structure of a continuous casting
installation has been shown in order to enable those skilled in
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the art to readily understand the underlying principles and
concepts oE the inventioil. Equally, electrolllagnetic stirrers
suitable Eor electromagnetically stirring molten metal are
well known in the art and since the invention is not concerned
with details of the construction thereof the same need not here
be further considered beyond the disclosure to follow regarding
the mode of operation oE such conventionally structured electro-
magnetic stirrers. Turning attention now to rigure 1 there is
shown in top plan view a partial section oE a wide side of a
slab 1 or, as shown in phantom lines, two blooms 2, supported
within a strand guide arrangement or roller apron 5, located
after a conventional and thereEore not further shown continuous
casting mold, of a so-called alternative or convertible continuous
casting installation for steel. According to the embodiment
under discussion there is only cast one slab size or one bloom
size. The strand guide rolls are designated by reference
character 4. Two electromagnetic stirrers 3, producing traveling
fields within the cast product, are mounted at the wide side of
the rollers 5 of the strand guide arrangement or roller apron 6.
The rollers 5 which are located directly below the electro-
magnetic stirrers 3 advantageously have a smaller diameter in
order to reduce the spacing between the electromagnetic stirrers
3 and the surfaces of the related strands 1 and 2. These rollers
5 are formed of a non-magnetic material. Reference character
1~ designates the central axis of the slab 1, whereas reference
characters 7 and 7' designate the central axes of both of the
blooms 2. The not particularly referenced arrow indicates the
strand withdrawal direction. The electromagnetic stirrers 3
which are conjointly effective at the slab 1 or individually
at each of the blooms 2 are fixedly mounted in such a manner
that following a mold change from slab to bloom sectional
shapes the stirrer-central axes, extending in the strand length-
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55630
wise direction, coincide with thc lengthwise axes 7 and 7'of the blooms 2. The stirrer sides ~, which in this embodiment
extend transversely with respect to the strand withdrawal
direction, are smaller than the bloom width ~, so that there
need not be tolerated any electrical losses. With regard to
the strand axes 7 and 7' there is produced a symmetrical flow
of the melt in the liquid strand pool or core and there is
obtained application of the generated electromagnetic forces
with as little 105s as possible. Depending upon the design of the
casting installation the electromagnctic stirrers 3 can be
arranged such that the traveling fields produced thereby within
the cast products or strands l and 2 generate turbulent flows
which extend opposite to as well as also in and also transversely
with respect to the lengthwise direction of the strand. Depend-
ing upon the arrangement of the electromagnetic stirrers 3 with
regard to the spacing from the continuous casting mold and
depending upon the magnitude of the electrical powering thereof
the resultant flows can extend up to the region of the continuous
casting mold, in order to also efect at that location the
solidification behavior of the molten metal. lf necessary,
the electromagnetic stirrers 3 also can have a rectangular,
elongate configuration instead of the square shape here shown
by way of example.
In the case of a roller apron having a greater spacing
between the guide rolls the electromagnetic stirrers, for the
purpose of generating the traveling field in the strand with
as little loss as possible, can be designed such that one or
a nulllbcr of the stirrcr parts, in thc form ol cxcitation plugs
or cores, serving for applying the magnetic flux to the strand,
can extend between the guide rolls and into close proximity
to the strand surface.
It is advaJltageous if the electromagnetic stirrers 3
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are arranged at the same level or elevation. The direction
of action of their traveling fields can be equally directed or
towards one another or can be directed at an angle to one
another. Equally, the sectional shapes of the strand can also
be like that of bars or billets. The electromagnetic stirrers
are normally mounted in close proximity up to as much as
several meters below the continuous casting mold in order to
obtain desired metallurgical effects, such as compensation of
the sump temperature, separation of non-metallic inclusions
or avoiding core segregation or core pipe.
Figure 2 shows a similar sectional view like the
showing of Figure 1. Reference character 1 designates the
slab, reference character 2 shows in broken lines two blooms.
The guide rolls have been conveniently omitted in order to
improve clarity in illustration. There is provided a multipli-
city of strand sectional shapes. The electromagnetic stirrers
have been generally designated by reference character 3. In
contrast to the electromagnetic stirrers shown in Figure 1 the
stirrers 3 of this arrangement are here rotatable, as generally
schematically indicated by the stirrer rotation means symbol~
ized by arrow 10 in the right-hand stirrer of Figure 2. In
this way by selecting th~ direction of action of the traveling
fields, as indicated by the arrows 11, it is possible to take
into account different casting parameters, such for instance
casting temperature, withdrawal speed and so forth. It is
also possible to mount the electromagnetic stirrers, for
instance as indicated for the stirrer 3 at the left-hand
showing of Figure 2, within a conventional guide or guide
track means, so as to be displaceable in the direction of the
arrows 12 transversely with respect to the lengthwise axis 18
of the slab. Due to such transverse displaceability of the
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electromagnetic stirrers 3 it is possible to align the
stirrers 3 in accordance with each encounterccl strand sectional
shape so that there is realized at the related strand pool or
core an optimum stirring acti.on. As to the multiplicity of
possible stirrer positions one such stirrer position 3' has
been shown in broken lines. The electromagnetic stirrers 3
and 3' are mounted at essentially the same elevational position,
so that at equivalent locations in the strands there can be
produced similar stirring movemcnts with the same effect.
Now in Figures 3 to 9 there have been illustrated
schematically diEfcrent features oE the inventive method while
utilizing the apparatus of this development. The guide rolls
have not been shown to simpliEy the illustration. Figures 3
to 5 show two electromagnetic stirrers 3 which are effective
at a slab 1, these stirrers 3 producing by means of the generated
traveling fields turbulent flows within the liquid core or pool.
The two-phase construction of the stirrer has been generally
symbolized by the arrow 11. The electromagnetic stirrers 3
are mounted at the same spacing from one another such that the
partial flows, caused thereby within the strand 1, do not affect
one another. The phase coils of both electromagnetic stirrers
3 are equally, symmetrically electrically powered, for instance
with 1000 amperes at a frequency of 2 I-lz and with 200 volts.
This has been conveniently symbolized by arrows 11 of the same
magnitude. The traveling fields o each pair of coils have
the same effective direction. In Figure 3 the traveling fields
of both electromagnetic stirrers are effective opposite to the
strand withdrawal direction indicated by the arrow, whereas in
Figure 4 thetraveling fields are effective transversely with
respect to the strand withdrawal direction, and finally, in
Figure 5 the traveling fields are effective in the strand with-
drawal direction. However, the traveling fields also can be
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directed opposite to one another in the lengthwise direction
of the strand and produce oppositely directed turbulent
flows, so that there is formed in the liquid pool or core a
circulating flow.
Figures 6 to 8 illus-trate electromagnetic stirrers
3 effective contra to the strand withdrawal direction, as
indicated by the arrows, wherein the traveling fields thereof
or the partial flows produced thereby mutually affect one
another, as schematically symbolized by the arrows 13. Through
the showing of equal size arrows 11 in Figure 6 there is
symbolized an identical electrical powering of both electro-
magnteic stirrers 3. Figure 7 shows -two differently powered
electromagnetic stirrers 3, wherein however the phases of the
coils of the stirrers 3, 31 are symmetrical, for instance the
phase coils of the stirrer 3 are powered by 1000 amperes and
the phase coils of the other stirrer 3' by 800 amperes. The
number of windings of each of the phase coils for each phase
of each stirrer is the same, so that for each stirrer there
is formed a traveling field having the same size thrust forces
within the traveling field.
Now in Figure 8 there has been symbolized by diffe-
rent size arrows 14 and 15 an asymmetrical electrical powering
of the phase coils of at least one stirrer. Details of the
operating principles of an electromagnetic stirrer which is
asymmetrically powered have been disclosed in German
Auslegeschrift DE 29 30 281 B2 published on February 1980,
inventors Lipton, Jan, et al. Thus, one of the phase coils of
the stirrer 3 is powered with 900 amperes and the other with
800 amperes, whereas the first phase or phase coil of the
stirrer 3' is powered with 600 amperes and the second phase
coil with 500 amperes. The voltage and frequency are like that
heretofore disclosed. The second phase coil of the stirrer 3'
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1 15S63û
can however also be powered in the sarne manller as the Eirst
phase coil, as this has been schematically symbolized by
the broken line arrow. In this way there are produced within
the traveling field differently eEfective thrust Eorces, so
that different flow velocities also are formed within the
related partial Elows or Metal streams. At the mutual region
of influence of both of the partial flows of the molten metal,
as symbolized by the arrows 13, there is produced an effective
turbulence.
The asymmetry Or the electrical powerillg can however
also be accomplished by difEerences in the current intensity
and/or the Erequency of the individual phases. Also differently
designing the coil windings produces the same eEfects. The
electromagnetic stirrers also can be designecl to be more than
two-phase. In the embodiments here under discussion they have
only been considered as two-phase stirrers for the sake of
simplicity.
Figure 9 illustrates two stirrers 3 which act inde-
pendently upon a related respective bloom 2. The phase coils
of both stirrers, symbolized by the arrows 14 and 15, are
asymmetrically powered, like in the arrangement of Figure 8,
so that the above-described effects can be obtained. In this
embodiment under discussion the traveling Eields are eEfective
opposite to the strand wlthdrawal direction, but it is possible
to select any desired angle between the main eEfective direction
of the traveliny fields and the strand lengthwise axes 7 and 7'.
It is not absolutely necessary that the electromagnetic stirrers
3 be located at thc samc clevation or level.
Finally, Figùre lO illustrates the possibility of
stirring the unsupported strand sides and shows a stirrer arrange-
ment during stirriny of two blooms 2. Two electromagnetic
stirrers 3 shown in broken lines and effective upon a not par-
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ticularly illustrated slab in the strand lengthwise direction,because of too great effective losses owing to the large
spacing of the stirrers from the strand surface due to too
large roll diameter of the support guide rolls 5, are instead
mounted at the outer unsupported strand side 16, these stirrers
being indicated by reference character 3''. When using support
guide rolls as described in ~nited States Patent No. 4,071,073
there are not located at the unsupported, exterl-al strand side
any roller bearinys for the wide side rolls which get in the
way, so that the electromagnetic stirrers 3 can be brought with
their effective faces or sides 17 at the optimum spacing from
the strand surface 16 in order to obtain a good degree of
turbulence . The electromagnetic stirrers 3 are displaceable
in all directions and designed such that they also can extend
between the guide rolls up to the side surfaces of the strand.
What has been stated above is equally applicable when stirring
a slab from the narrow side thereof.
While there are shown and described present preferred
embodiments of the invention, it is to be distinctly understood
that the invention is not limited thereto, but may be otherwise
variously embodied and practiced within the scope o~ the follow-
ing claims.
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