Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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The present invention relates to a new and improved
mQthod for the continuous casting of steel, wherein the molten
metal or melt is cast into a mold, the formed strand having a
liquid core is withdrawn, guided and supported at a strand
guide arrangement or roller apron, and a turbulent flow is
produced in the liquid core by mea~s of at least one stirrer
in~ucing a traveling electromagnetic field in the strand.
During the continuous casting of steel it is known
to use electromagnetic stirring of the melt or m~lten rnetal
in the liquid core to achieve an improvement in the quality
of the cast product by means of a more or less turbulent flow.
Tnese improvements of the quality of the strand are realized
by different methods which apply thrust forces to the melt. In
order to generate these thrust forces, in many instances, there
are utilized magnetic traveling fields, also referred to as
traveling electromagnetic fields.
There are contained in steel alloying and accompany-
ing elements, such as carbon (C), silicon (Si), manganese (Mn),
phosphorus (P), sulphur (S) and so forth, which, upon solid-
ification, can lead to segregations, especially center segrega-
tions. Such segregations, and also the crystal structure, as
is well known, are dependent, among other things, upon the
magnitude of the superheating temperature or superheat of the
steel. By virtue of the electromagnetic stirring action and
the thus produced turbulent flow, it is intended to prevent
all such type of segregations. The solidified structure should
be affected in such a manner that there is obtained as large
as possible zone of compact or dense, equia~ed crystal structure.
~lowever, it has been found that due to the local pronounced
rnovement of the melt or molten metal the solidification front
or solid-liquid interface is affected such that there are
formed so-called whi-te bands. These bands are negative
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segregations which can impair the quality of the casting.
According to a heretofore known method, thrust forces
are generated, by means of a traveling electromagnetic field,
in the direction of the lengthwise axis of the strand. The
magnets extending about the strand are arranged between the
rol1 pairs up to the end of the liquid core of the strand. The
flow produced along the liquid core produces the desired region
of non-columnar structure and prevents the formation of decisive
segregations, especially the center segregation and white bands.
Su~h arrangement, however, requires a large amount of space
because of the multiplicity of employed magnets, hinders cooling
of the strand and is much too complicated
With another known method for slab sections, it has
been attempted to eliminate these white bands in that, by
means of traveling electromagnetic fields, produced by two
magnets located opposite one another at the lengthwise sides
of the strand, thrust forces are applied to the liquid steel.
These thrust forces should be effective transversely with
respect to the lengthwise axis of the strand, so that there is
obtained a gentle impact of the métal flow at the solidified
strand wall, so that the deflecte~ flow is maintained within
a limited region. This limited effective region or zone
produces an inadequate zone of de~se or compact, equiaxed
crystal structure. Furthermore, it has been found that with
this method the white bands only can be inadequately eliminated,
so that by virtue of these drawbacks there can not be obtained
any optimum product, something which has a negative effect in
terms of the quality of, for instance, the rolled product.
Therefore, with the foregoing in mind it is a primary
object of the present invention to provide a new and improved
method of continuously casting metals which is not afflicted
with the aforementioned drawbacks and shortcomings of the pr;~-
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art proposals.
Another and more specific object of the pres~ntinvention aims at providing an improved method of continuously
casting steel producing a sufficient zone of dense, equiaxed
crystal structure.
Yet a further important object is to provide a method
of continuously casting steel wherein there is low segregation
within the casting, especially in terms of the center segrega-
tion and white bands.
Another important object is to provide a continuous
castin~ method where there prevail extremely small space
re~uirements for producing the mag~etic stirring effect.
Now in order to implement these and still further
objects of the invention, which will become more readily
apparent as the description pro^eeds, the method contemplates
producing the turbulent flow by different thrust forces acting
u on the melt and which are affected by asymmetry in the phases
of the traveling electromagnetic field.
Tests carried out with a traveling magnetic field
effective transversely with resp~ct to the lengthwise direction
of the strand, wherein there are ùsed symmetrically wound
coils carrying the same current intensity in order to produce
the traveling magnetic field, have found that in the micrograplns
of the produced strands or castings there appear white bands
and a wide zone of den~rites, resulting in insufficient quality
of the cast steel. However, it has been surprisingly found that
in the presence of different thrust forces within the traveling
magnetic field and acting up~n the melt or molten metal,
wherein the traveling magnetic field imp~rtantly is generated
3~ by providing asym~etry of the phas~s, such as for instance
asymmetrical turns or windings of the coils or different
currents flowing through the phase coils, it is p~ssible to
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produce such type of a turbulent flow chat, practically no
white bands ap~ear in the micrograph and there is obtained
the desired zone of dense, equiaxed crystal structure, not-
withstanding higher superheat, an~ without any structurally
complicated stirrer equipment.
Surprisingly, according to an important teaching of
the invention, it has been found that the turbulent flow becom~s
more efficient if, a-cording to a further feature of the inven-
tion, the weaker thrust force within the traveling magnetic
field becomes effective prior to the stronger thrust force.
With the thus obtained optimum mixing of the molten steel it
is possible to realize an even better metallurgical structure
of the casting.
At the start of casting there expires a certain
time before the liquid core is placed into a rotational move-
ment. In order to be able to rapidly produce the strived for
turbulent flow, an additional facet of the invention contem-
plates a~justing the asym~etry in the phases of the traveling
electromagnetic field during the start-up phase of the casting
2~ operation from approximately null to a predetermined maximum
value. This adjustment can be accomplished by any known tech-
niques, for instance by using a potentiometer or potentiometer
circuit arrangement, variably controllable voltage sources,
and equivalent means. In this way there can be achieved the
beneficial result that also the forwardmost section or leading
region of the cast strand has the deslred metaIlurgical quality.
With the inventive mode of operation there is real-
ized, by virtue of the prevailing laws of physics, in addition
to the force effective in the lengthwise direction of the
electromagnetic stirrer, a transverse force which pulsates
between null and a maximum value. This superimposed force, by
virtue of an addition~l intensification of the turbulence,
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then has a beneficial effect upon the turbulent flow, if,
according to a still further feature of the invention, by
adjusting the direction of movement of the traveling electro-
magnetic field, the force resulting from the asymmetry of the
phases and extending perpendicular to the surface of the
stirrer becomes effective away from the stirrer surface
confronting the strand. In this way there can be realized an
additional improvement in the strand quality, especially when
using a stirrer arrangement which is effective at one side
of the strand.
In the descriptive to follow the invention will be
more fully explained. The described results were obtained and
an electromagnetic stirrer installed at a continuous casting
installation for slabs, as the same has been disclosed in
German patent publication No. 2,720,391 to which reference may
be readily had. The slabs which were cast at this casting
instàllation had a sectional shape or format of 1550 by 270 mm.
The primary effective direction of the traveling electromagnetic
field produced by the electromagnetic stirrer extended trans-
versely with respect to the lengthwise axis of the cast strand.
There was employed a two-phase stirrer having symmetrically
wound coils.
Both phases were powered with an alternating-current
voltage having a frequency of 2 Hz, and the current intensity
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in one phase a~ounted to approximately 800 amperes and that
in the other phase to about 1000 amperes. By virtue of the
difference of the current intensities, i.e., the asymmetry of
both phases, there is predicated the basis for the discovered
favorable effect upon the turbulent flow in the strand core.
Due to this asymmetry there are formed thrust forces having
different thrust action at the melt, i.e., by virtue of the
frequent, frequency-dependent chan~e ketween the more intensive
thrust force and the weaker thrust force there is appreciably
1~ increased the turbulence within the flow. Due to the time
sequencin~ of the weaker thrust force so as to be effective
before the stronger thrust force, i.e. , the phase which is
i~pinged with 800 amperes becomes effective before that impinged
with 1000 amperes, at the effective region of the traveling
magnetic field there is realized an increased turbulence due
to the periodically pulsating thrust force.
Micrographs of castings which have been produced
during practical operation of a casting system have shown that
at the start of casting there expires an appreciably lon~er
amount of tims until reaching a qùalitatively faultless casting
structure, when, instead of operating the electromagnetic
stirrer with an approximately symmetrical mode, such is asym-
metrically operated. In particular, to obtain the desired
turbulence in the melt, initially there must be formed in the
strand core revolving or rotary flow circulations, in which
then, because of the difference in the thrust forces of both
phases with asy~notrical stirrer operation, there are integrated
additional turbulences. Viewed from the practical standpoint,
at the start of casting, until there has formed an adequate
liquid pool or sump in the strand, the stirring action is
initiated with only a slight difference in the current impinge-
ment of both phases, for instance the first phase or phase
coil is operated at 1000 amperes and the second phase or phase
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coil at approximately 1000 amperes. ~fter formation of thenecessary flow conditions, i.e., after attaining a turbulent
revolving movement in the liquid core, there is then switched
to the previously described asymmetrical impingement or phase
mode. In this way it was possibie to clearly decrease the size
of the first portion of the slab where there existed a p~orer
casting quality.
Furthermore, it has been found that also the direction
of travel of the electromagnetic field, i.e. whether it moves
in the slab at its wide side so as to be effective from the
left towards the right or vice versa, has a decisive effect
upon the quality of the casting. In the preferred case, the
transverse force resulting from the asymmetry, and governed
by the laws of physics, is effective perpendicular to the main
movement component and likewise perpendicular to the strand
withdrawal direction, towards the center of the strand and
can develop without hindrance its effect as concerns intensify-
ing the turbulence. With certain casting parameters its
effective direction can be rotated through 180, i.e., can
be directed from the center of the strand towards the strand
skin.
During co~ction of two oppositely situated electro-
magnetic stirrers, for instance when working with thick slabs,
such preferably can be connected such that their traveling
fields move towards one another, so that both the transverse
force of the one and also that of the other traveling field
are directed towards the center of the strand.
~ he above-described asymmetry of the phases producing
the differently effective or intensive thrust forces, of course
also can be obtained by other constructional m~asures well
known in the electrical arts, such as, but not limited to,
producing unequal turns of the phase coils or the coaction of
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a number of stirrer segments which have different power or
voltage applied thereto.
White 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 of the
following claims. -
In this respect and in accordance with thebroadconcept of the invention, there is provided and claimed herein,
a method for the continuous castlng of steel wherein the molten
metal is cast into a mold, the formed strand having a liquid
core is withdrawn from the mold, guided and supported in a
strand guide arrangement and by means of at least one stirrer,
inducing a traveling electromagnetic fie,ld in the strand, there
is produced a turbulent flow in the liquid core of the strand,
wherein the improvement comprises the steps of: producing
the turbulent flow by thrust forces which are differently
effective at the molten metal, said thrust forces being
affected by asymmetry in the phases of the traveling
electromagnetic field; and controlling the weaker thrust
force of the traveling electromagnetic field so as to be
effective prior to the more intensive thrust force.
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