Note: Descriptions are shown in the official language in which they were submitted.
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The present invention concerns a process for the
continuous casting of steel, wherein mo]ten metal is poured
into a mould, the resultant strand having a liquid core is
guided and supported in a roller aprou, and turbulent flow
is induced in the liquid core by at least two electromagnetic
travelling fields.
The structure of a slab produced by continuous casting
is dependent upon the composition of the material and upon the
casting temperature. At casting temperatures of only a few
degrees Centigrade above the melting point, a globulitic, non-
directional structure predominates, and at casting temperatures
of 15 C and more above the melting temperature, a colummar
directional structure, having a pronounced central positive
segregation of the accompanying elements, predominates. Because
of the good working properties, particularly during rolling,
ingots having a globulitic structure are preferred. For reasons
of casting technique, for example because of the difficulty, in
the case of a large, of accurately maintaining a temperature
uniform to within a few degrees, during the entire casting
time, and for the purpose of preventing partial solidification
of the molten metal already in the ladle, it is necessary in
practice to carry out casting at temperatures of more than 20C
above the liquidus (hereinafter also referred to as superheat
temperatures). Many efforts have therefore been made also to
obtain a slab having a preponderantly globu]itic, non-directional
structure and no central segregation by continuous casting at
superheat temperature.
It is known in the continuous casting of steel to improve
the quality of the cast material by means of a relatively
pronounced turbulent flow achieved by magnetically stirring
the molten material in the liquid core. These improvements
have been achieved by various methods for applying thrust
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forces to the molten materia]. In many cases travelling fields
have been used for generating the thrust forces.
Alloying elements and accompanying elements, such as
C, Si, Mn, P, S etc., are present in steel, and these elements
can lead to segregations, particularly central segregations,
when the steel solidifies. Such segregations, as well as the
crystal structure, are known to be dependent upon the level of
the superheat temperature among other things. The electro-
magnetic stirring, i.e. the turbulent flow that is set up, in
intended to prevent such segregations. The intention is to
influence the solidification structure in such a way that the
largest possible zone of dense non-directional crystal structure
is obtained. It has been found, however, that the solidification
front is so influenced by the local vigorous movement of the
molten metal that what are called white bands are formed.
These white bands are negative segregations which may have a
deleterious effect upon quality.
In a known process, thrust forces are produced in the
direction of the longitudinal axis of the strand by means of
an electromagnetic travelling field, and the magnets, extending
around the strand, are arranged between the roller-pairs, from
the mould to theendof the pool. The flow induced along the
pool produces the required zone of non-columnar structure and
prevents the creation of critical segregations, particularly
central segregations and white bands. ~ecause of the large
number of magnets used, such a system requires too much space
and interferes with the supporting and cooling of the strand,
and this reduces the efficiency of the installation. Further-
more, this system is much too costly.
In another known process for producing slabs, efforts
are made to eliminate these white bands by applying uni-
directional thrust force to the ïiquid steel, which forces are
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generated by two magnets located opposite e~ch other on the
long sides. These thrust forces are intended to act trans-
versely of the ]ongitudinal axis of the strand in such a way
that flowing metal strikes the solidified wall in a gentle
manner, so that the deflected flow is kept within a limited
zone. This limited zone of action results in an inadequate
zone of dense non-directional crystal structure. ~urthermore,
it has been found that with this process the white bands can be
eliminated only partially, so that because of these disadvantages
a product of the best possible quality cannot be obtained, and
this can have a negative effect on the quality of the rolled
product, for example. A difficulty as regards the fitting of
the magnets results from the fact that it is necessary to
arrange them in the immediate vicinity of the surface of the
strand, and because of the absence of rollers and cooling
means this promotes bulging.
The object of the present invention is to provide a
process which results in an adequate zone of dense non-direc-
tional crystal structure. It is intended that the cast material
should contain very little segregation, particularly central
segregation and white bands. ~urthermore,an aim of the inven-
tion is to limit the space required for producing the magnetic
stirring effect.
According to the invention, this object is achieved in
that the turbulent flow is produced by thrust forces of differ-
ing magniture, induced by travelling fields and acting on the
liquid steel, and by the material influence of the flows
caused by the differing thrust forces of different magnitude,
produced by the travelling fields and acting on the liquid
steel in the core, and the resultant differing reciprocal flow-
effects, a turbulent flow is produced such that practically no
negative segragations, i.e. no white bands, can be seen in the
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microphotograph~ Despite a higher superheat temperature, the
required zone of non-columnar dense structure is obtained, and
in particular central porosity can be inhibited so that improved
products can be obtained for rolling. The space required for
producing the trave]ling fields is sma]l in relation to the
effective zone.
For producing slabs and larger blooms, the turbulent
flow is advantageously induced by travelling fields acting from
one side of the strand.
In a further advantageous application, the turbulent
flow is induced by uni-directiona] thrust forces produced by
travelling fields and acting on the liquid stee].
In accordance with a further feature of the invention
and for the purpose of producing the differing thrust forces,
the travelling fie]ds can be energized using differing current-
strengths. The resultant force transverse of the thrust force
acting on the liquid steel causes a more effective turbulent
flow which produces the required cast structure. Advantageously,
the magnet of one of the travelling fields is loaded with a
current which is 10 ~ to 25 % higher than that applied to the
magnet of the other travelling field.
The differing thrust forces are advantageously produced
transversely of the longitudinal axis of the strand. When the
turbu]ent flow impinges upon the solidified side wall of the
strand, vortices are created which move in large numbers in the
direction of the mould. In this way, the turbulent flow causes
the molten metal to rotate not only in the plane of the cross-
section of the strand, but over a large zone in the longitudinal
direction of the strand, and this permits an advantageous
replacement of the mo]ten metal in the immediate zone of action
of the magnets by stee] that has just flowed from the mould, so
that the temperature in the entire molten metal can ke equalized.
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Despite the small space required for accommodating the magnets
in the longitudinal direction of~ the stra~d, the li~uid steel
is stirred in this large zone.
In accordance with a further feature, the mixing of
cold steel, located below the magnets, with hot steel flowing
towards it can be improved in that a lower thrust force in the
travelling field facing the mould can be produced in the
travelling field facing away from the mould. In accordance with
an additional feature, this exchange of cold and hot steel can
be obtained over a still larger area by producing the differing
thrust forces in the direction opposite that in which the strand
moves, but this requires a somewhat larger space for accommo-
dating magnets.
In accordance with yet another feature of the invention
and for the purpose of adapting the turbulence more effectively
to suit the particular casting paramete~s that obtain, the
turbulent flow is produced by differently acting thrust forces
within at least one of the travelling fields, In this arrange-
ment, the winding of one phase of the magnet producing the
differing thrust forces is loaded with different current
strengths compared with that applied to the winding of at least
one other phase.
In this connexion, it is further preferred to produce
the differently actin~g thrust forces within a travelling field
alternately in the travelling fields.
In accordance with the broad concept of thç invention,
there is provided and claimed herein a process for the conti-
nuous casting of steel, wherein molten metal is poured into a
mould, the resultant strand having a li~uid core is guided and
supported in a roller apron, and turbulent flow is induced in
the liquid core by at least two electroma~netic travelling
fields, characterized in that the turbulent flow is produced
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by thrust forces of differing magnituder induced by travelling
fields and acting on the liquid steel, and by the material
influence of the flows caused by the differing thrust forces.
The invention will now be described by reference to
two arrangements illustrated in the attached drawings, in which:
Fig. 1 shows the arrangement of the magnets for
carrying out the process in an arcuate installation, using
travelling fields acting transversely of the strand, and
Fig. 2 illustrates an arrangement of the ma~nets in a
vertical installation using travelling fields acting in the
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direction of movement of the strand.
Referring to Fig. 1, the numeral 1 designates a cooled
arcuate reciprocating mould for casting a slab, which mould
is supplied with liquid steel from a casting vessel, not
illustrated, by way of a casting tube extending into the mou]d
1. The strand 2 produced in the mould 1 and having a liquid
core 3 is guided and supported with the aid of rollers 5 in an
arcuate roller aprou 4 which is located downstream of the
mould 1 and has a radius of 10 metres. Between the rollers 5
are arranged spray nozzles 6 for additionaly cooling the strand
2. The strand is e~tracted and straightened by a withdrawal
and straightening unit 7.
At a distance of approximately 5 metres below the end
of the mould and at the inner side of the arc of the strand is
arranged a housing lO having a group of two travelling field
magnets ll, 12 of known construction. It is also possible to
provide this arrangement at another side of the strand. The
travelling fields 11, 12 produce uni-directional differing
thrust forces acting transversely of the strand, which forces
are applied to the liquid steel in the core 3. An unocuppied
space is present between the two magnets 11 and 12. ~y altering
this gap and depending upon dimensions, particularly the ]iquid-
to-solid area ratio, and according to the required zone of
turbulent flow, it is possible to obtain a material influence
of the flows caused by the thrust forces. However, the gap
must be less than the size of the two adjacent flow cells
produced by the magnets. Rollers 5' made of an anti-magnetic
material, for example stainless steel, are fitted between the
magnets 11 and 12, o~ the one hand, and the surface of the
strand 2, on the other.
For a slab having dimensions of 1500 mm x 250 mm, the
windingsof the magnet 11, facing the mould 1, are lo`caded with
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a current of lnnO A~ and the windings of the magnet 12, facing
away from the mould 1, are loaded with a current of 850 A.
; The frequency for the two magnets is 2 Hz. However, the
frequencies may be different, for example 2 Hz and 1.5 Hz,
and this likewisc in~luences the turt)uLence. The uni-direct:iorla]
thrust forces of different magnitude, produced by the two
travelling fields acting transversely of the longitudinal axis
of the strand, cause differing flow velocities, so that complete-
ly effective turbulence is generated in the reciprocal zone of
action of the two flows. When the turbulent flow strikes the
side wall, additional vortices are formed. Because of the
greater flow velocity produced by the travelling field facing
the mould as compared with that resulting from the travelling
field facing away from the mould, rising of the liquid steel
along the side wall towards the mould is promotend.
In order to adapt the turbulence to suit the casting
parameters that differ from one installation to another, and
also to suit the changing casting parameters within one ins-
tallation, use can be made of the following method. 1000 A
can be applied to the two windings of the magnet 11, so that
thrust forces which are similar within its travelling field
act on the liquid ste`e]. In this example, the magnet 12 is
likewise of two-phase design.
However, the two magnets may have more than two phases.
The windings of the first phase are supplied with 900 A and
those of the second with 800 A, so that differing thrust forces
are produced within the travelling field of the magnet 12. In
the zone of influence of the flows produced by the magnets 11
and 12, other turbulence occurs, due to the previously mentioned
transverse forces, compared with the turbulence which occurs
with like thrust forces within the two travelling fields.
However, the two magnets 1l and 12 may also produce differently
acting thrust forces within the corresponding travelling fields.
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Adaptation to suit the differing casting parameters can
be further facilitated if the magnet 12 is loaded with 1000 A,
and the two phases of the magnet 11 with 900 A and 800 A on an
alternating basis. This alternation may take place for example
every 10 seconds.
In Fig. 2, the mould is again designated by the numeral
1 and the strand by thenumeral 2. The rollers 5 guide the
strand. Two travelling field magnets 21 and 22 are arranged
in the longitudinal direction of the strand in a housing 20.
For smaller dimensions of product, such as billets and smaller
blooms, there in insufficient space for arranging the magnets
side by side. In such cases, one of the magnets acts on liquid
steel from another side of the strand, and causes the flows to
be influenced on a reciprocal basis. The windings of the two
magnets are loaded with differing current-strengths so as to
produce differing thrust forces in the direction opposite to
that in which the strand moves. In this way turbulent flow,
as indicated by the arrows 23 and 24, is created. the cold
steel located below the magnets is thereby carried towards
the mould where it mixes with the inflowing hot steel.
In the two examples, the method is carried out using
two travelling field magnets arranged side by side and forming
a group. It is however possible to carry out the process with
three magnets, the two outer magnets being advantageously loaded
with current of the same strength. It is likewise possible to
carry out the process using more that three magnets.
It will also be understood that the magnetic travelling
fields used for producing turbulent flow do not necessarily have
to be arranged transversely of or parallel with the longitudinal
direction of the strand, but that they may just as readily form
some angle with this direction. Nor do the travelling fields
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need to be produced by electromagnets that are combined to
form a singie structural group. It may éven be advantageous to
arrange the groups independently of each other and to design
them to be movable. Such an arrangement enables the magnets
to be displaced relatively to each other both in the longitudinal
and the transverse direction of the strand and permits them to
be turned independently of each other.
A further possible method of producing differently
~ acting thrust forces consists in the use of two travelling
field magnets of unlike constructlon. In the case of strands
having long liquid cores, several groups of travelling fields
may be effective in the longitudinal direction of the strand.
The process in accordance with the invention can be used for all
types of continuous-casting installations having open-ended
moulds.
