Continuous casting mold stirring

During continuous casting, the stream of molten metal poured into the mold is acted on by a static magnetic field so as to split the stream into smaller streams stirring the unsolidified metal in the mold.

BACKGROUND OF THE INVENTION 
Molten metal, particularly steel, is continuously cast by being poured 
through the nozzle of either a ladle or an intervening tundish into the 
top of an open topped continuous casting mold having cooled side walls and 
containing a body of previously poured molten metal, the metal descending 
through the mold while solidifying against the mold's side walls so as to 
form a solidified skin containing unsolidified metal and producing a 
continuously descending cast strand, the mold having an open bottom 
through which the strand travels downwardly with its skin still containing 
some of the unsolidified metal until at some distance below the mold the 
strand completely solidifies throughout. Thereafter, the strand is cut to 
lengths which are inspected for surface defects which must be removed by 
chipping, milling, etc., as required for reheating and rolling of the 
lengths. 
The skin solidifies initially in the upper portion of the mold and 
gradually increases in thickness with downward movement of the strand 
forming in the mold, thus forming the strand with an interior representing 
a sump containing molten metal until at the point where the strand 
completely solidifies throughout this sump is terminated by a resulting 
solid front. 
The metal poured from the nozzle of either the ladle or the intervening 
tundish unavoidably contains particles of slag. If the top surface of the 
molten metal body within the mold is relatively static, it possibly cools 
so it solidifies enough to form particles of solidified metal. The molten 
metal necessarily poured from a height above the mold's top is in the form 
of a stream having at the mold's top portion a velocity typically in the 
order of from 1 to 1.5 m/sec. The result is that the stream has enough 
momentum to penetrate the body of molten metal in the mold for substantial 
distances before losing velocity to a degree where the stream blends into 
the body of molten metal. 
If the molten metal enters the body of unsolidified metal in the mold 
centrally in the form of a vertical stream, it can possibly penetrate as 
an internal stream not only the unsolidified metal body in the mold itself 
but also the unsolidified metal in the sump below the mold where the 
skin's walls are converging towards each other. The molten metal may be 
poured from a tundish via a pipe having a lower end submerged in the body 
of unsolidified metal in the mold and having an open bottom so that the 
metal is in effect injected as a vertical downward stream, particularly 
when the mold is contoured to cast a strand of billet or bloom cross 
section of generally square shape. In the case of a mold having a slab 
cross section with a width very substantially greater than the thickness, 
this casting pipe may have a closed bottom and oppositely positioned 
outlets pointing towards the narrow side walls of the mold, in which case 
the stream is split into two separate streams traveling towards those 
narrow walls internally within the body of unsolidified metal in the mold. 
It follows that the particles of slag can be via the stream carried to the 
skin forming within the mold, or possibly adjacently below the mold, so 
that the particles are entrapped by the solidifying skin-forming metal 
where the particles remain after the strand completely solidifies so as to 
form the solid front. Particularly when the two streams are formed by the 
casting pipe having the opposite side openings, the particles can be 
driven more or less directly towards and into the solidifying skin inside 
of the mold. Characteristically the bottom of the casting pipe is 
positioned not very far below the level of the body of molten metal in the 
mold so the particles are carried by the streams into the portion of the 
skin where it is just beginning to form by sollidification and is 
therefore relatively thin, thus causing the particles to be contained by 
the finally solidified strand on or near its surfaces. Particles of metal 
inadvertently solidified at the surface of the molten body in the mold may 
be drawn downwardly into the forming skin in the mold. 
Semi-finished products cut from a solidified strand having a surface 
containing such particles as surface defects requires processing by 
undesirably extensive chipping, milling, etc., to remove the defects prior 
to reheating and rolling. This undesirably adds to the cost of making the 
final product. 
Below the mold and above the solid front of the strand, it is possible to 
stir the unsolidified metal in the strand by using a multi-phase AC 
magnetic stirrer positioned outside of the strand so as to induce a 
traveling multi-phase field in the unsolidified metal which stirs the 
metal and distributes slag and possibly other particles uniformly so they 
do not concentrate at any location. It is also possible to use such a 
stirrer on the outside of the mold itself to in this way stir the molten 
metal inside of the mold so as to prevent the particles from becoming 
entrapped by the solidifying skin in the mold. However, a continuous 
casting mold must have thick water-cooled walls made of heavy copper 
plates so as to remove the heat from the molten metal and solidify a skin 
of adequate thickness before the forming strand leaves the mold. The mold 
walls may characteristically have a thickness of up to 80 mm, and although 
not solid, these walls make it very difficult for a multi-phase AC 
traveling field to penetrate them so as to be effective inside of the 
mold. For example, the effective penetrating field from a typical 
multi-phase AC stirrer operating even at the low frequency of 1.5 Hz is 
only from 50-60 mm through solid copper. 
It is apparent that the continuous casting art needs some more effective 
means for splitting up or stirring within the mold the stream or streams 
of molten metal injected into the body of molten metal to maintain its 
volume while it with its forming skin is descending through and from the 
mold. 
SUMMARY OF THE INVENTION 
According to the present invention, such a more effective means is provided 
by projecting a stationary magnetic field of constant direction through 
the mold and into the body of molten metal in the steel and transversely 
through each stream of molten metal fed into that body to keep it supplied 
for the formation of the strand leaving the mold's bottom. The field may 
be supplied by permanent magnetic or electromagnetic means. 
In the above way the velocity and momentum of each stream is abruptly 
reduced, the effect being similar to that of an eddy current brake. With 
its velocity suddenly reduced, each stream as it pushes against its slowed 
portion splits or breaks up and stirs into the body of unsolidified metal 
in the mold so that any particles are distributed substantially uniformly 
and are not entrapped by the solidifying skin-forming metal which will 
ultimately become the surface of the finely solidified strand. Below the 
mold multi-phase AC stirring can be used to continue the stirring. 
Preferably the field is formed with an elongated cross section such as in 
the form of an oblong and which is oriented to form an acute angle with 
the stream of supply molten metal. This causes the dispersions of the 
stream to form upwardly towards the upper level of the molten metal in the 
mold, carrying heat to this area. 
Such a field can be projected through the mold walls by DC electromagnetic 
stirrers positioned on opposite sides of the mold's outside. Such a 
stirrer can be made somewhat like a multiphase AC coil wound stirrer, but 
with a core forming two pole pieces and wound for DC operation and, of 
course, powered by DC. Particularly when the casting pipe has the closed 
bottom and oppositely pointing side outlets as used for casting a slab 
strand, two such stirrers can be used on opposite sides of the mold with 
their pole pieces aligned with each other, the mutually opposite pole 
pieces of the two stirrers having opposite polarities. With the stirrers 
appropriately positioned, the two fields are intersected by the two 
streams of molten metal leaving such a casting pipe. By making the pole 
pieces with horizontally oriented oblong contours, and because such a 
casting pipe is normally made to eject the two streams at an angle from 
the horizontal plane, the result is that the streams flow at acute angles 
with respect to the resulting two horizontally elongated fields. Because 
there is no periodic reversal of polarity as in the case of the 
conventional multi-phase AC field provided by the conventional 
electromagnetic stirrer, substantially no losses occur by passage of the 
flux through the copper walls of the continuous castng mold. The mold wall 
thickness only represents an air gap or gaps insofar as their penetration 
by the flux field of constant direction. Such a field is sometimes called 
a static magnetic field but with the present invention it may be desirable 
under some circumstances to further break up and distribute the stream of 
supply metal by periodically varying the strength of the flux field but, 
of course, without changing its direction and with an adequately low 
frequency as required to preserve the advantages of using a static field. 
For the eddy current brake action, the magnetic field must be stationary 
and capable of carrying the reaction required to slow the velocity and 
reduce the momentum of the stream in the body of metal in the mold. This 
is made possible by rigidly positioning the source or sources of the 
static flux field on the outside of the mold as by anchoring the DC 
stirrers previously mentioned just as it required in the case of 
multi-phase AC stirrers. The degree of stirring obtained depends in each 
instance on the movement of the stream through the static or non-reversing 
magnetic field. Therefore, the field should be positioned as close as 
possible to the casting pipe outlet or other source of the stream where 
the stream's velocity is at its maximum.

DETAILED DESCRIPTION OF THE INVENTION 
In FIG. 1 the broken line 10 indicates how without the practice of this 
invention the supply stream of molten metal leaving the downwardly angled 
outlet 11 of the vertical casting pipe 11' is injected into molten metal 
in the mold directly towards the narrow or edge side of the continuous 
casting mold M of slab cross section and how at this narrow side any slag 
particles or other particles are driven into the just forming skin S of 
the solidifying metal, the abrupt stop at the mold side splitting up the 
stream with minor portions going downwardly and to some extent looping 
upwardly and around to rejoin the stream moving at high velocity from the 
outlet 11. 
The static magnetic field of the present invention is indicated at 12 
positioned immediately at the casting pipe's outlet 11 and of oblong cross 
section with its long axis extending horizontally and therefore forming an 
acute angle with respect to the normal downwardly angling direction of the 
stream as indicated by the broken line 10. In other words, the stream is 
ejected by the nozzle opening 11 at its highest velocity diagonally with 
respect to the elongated field 12. The field is shown as being located as 
close as possible to the outlet 11 of the casting pipe, the result being 
that as the moving stream goes through the static magnetic field the eddy 
current brake action is effected, the sudden or relatively abrupt 
reduction in the velocity of the stream causing the stream to break up 
into a number of upwardly directed smaller streams 13. The action is one 
of stirring within the mold itself and because of the acute angle or 
diagonal relationship between the flowing direction of the stream 10 of 
the field 12 the stirring action is mainly upwardly. This upward stirring 
has the advantage of carrying heat to the upper level L of the molten 
metal bottom which must be maintained within the mold as metal leaves the 
mold via the cast strand (not shown). 
The field 12 can be projected through the wide side of the slab-forming 
mold M by means of one or more permanent magnets on the outside of the 
mold. Preferably electromagnets are used as shown by FIGS. 2 and 3 where 
the static fields B are shown as being diagonally intercepted by the two 
streams 16 and 17 which are injected into the mold's metal, by 
electromagnets having cores with pole pieces 15 positioned on opposite 
sides of the mold M and energized by the DC powered coils or windings 16'. 
The arrangement should be such that the oppositely positioned pole pieces 
are of opposite polarity and the pole pieces should have the oblong or 
horizontally elongated contours required to provide on opposite sides of 
the casting pipe 11' the horizontal oblong fields of which one is shown at 
12 in FIG. 1. The two pole pieces of each core of each electromagnet are, 
of course, inherently of opposite polarity as indicated by FIG. 2 where 
the field intersecting the stream 16 is towards the observer while that 
intersecting the stream 17 is away from the observer. 
In the event the single field 12, used on both sides of the casting pipe 
11' at its two outlets, is not enough to produce the stirring or 
stream-splitting action required to prevent the stream from reaching the 
forming skin S, successive fields of the same kind may be used further 
along the direction of the stream as indicated at 12' and 14 in FIG. 1, 
thus producing successively additional stream retardations as indicated by 
the arrows 13' with possibly some slight downward splitting as indicated 
by the arrow 13". Such additional fields may be used to control the 
stirring effected. 
The sudden reduction in the velocity of the stream causes the stream to 
split mainly upwardly towards the molten body's upper level L and away 
from the skin S. Any slag particles are continuously stirred uniformly 
throughout the body of molten metal in the mold, while the upper molten 
metal level L receives heat to prevent its premature solidification 
possibly producing solid metal particles. It is important that the first 
field 12 be positioned close to the outlet 11 of the casting pipe because 
it is here that the velocity of the stream is at its maximum. The eddy 
current braking action depends on the velocity of the stream traveling 
through the static magnetic flux stationarily positioned because the 
stirrers shown by FIG. 3 are, of course, rigidly mounted to accept the 
reaction of the braking action. If the successive fields 12' and 14 are 
used, they should preferably also be horizontally elongated and all of the 
fields 12, 12' and 14 should be parallel to each other and, therefore, 
diagonally oriented with respect to the downward angularity of the stream 
10. If the direction of the stream is diverted by the action of the first 
field, following fields should be positioned to intersect the diverted 
stream. For emphasis, it is repeated that the first and possibly only flux 
field used should be positioned almost immediately or as close as possible 
to the outlet 11 of the casting pipe, this applying, of course, also to 
the other side of the casting pipe where the conditions are the mirror 
image of those shown by FIG. 1. 
With the mold walls water-cooled and made of copper plates as usual, the 
walls only act as air gaps insofar as their penetration by the static 
magnetic fields of constant or non-reversing direction are concerned. With 
the mold of slab contour as shown by FIG. 3, namely having wide sides and 
narrow edge walls, the static magnetic fields do not have to penetrate a 
great thickness of the non-solidified metal in the mold or the solidifying 
skins on the wide sides. Magnetic fields of high intensity are possible. 
Under some circumstances it may be desirable to periodically vary the 
strength of the static fields as by varying the voltage applied to the 
coils 16' in FIG. 3. If this is done at too high a frequency, the 
strengths of the fields may be reduced by the copper walls of the mold, 
but this effect can be avoided if the strength variations are of 
adequately low frequency. This low frequency will depend on the manner in 
which the mold is constructed and its dimemsions. These factors must be 
considered when determining cross section area and intensity of each 
magnetic field required for stirring effective to avoid each injected 
stream of supply molten metal from being carried to the narrow sides of a 
mold of slab contour. 
It is particularly in the casting of slab strands that the problem dealt 
with by this invention is involved. With the wide and relatively thin slab 
contour a casting pipe having the closed bottom and angularly pointing 
side outlets is used, making the principles of the present invention of 
particular value. However, in the case of billets and blooms, the mold 
cross section is more or less square and an open bottomed casting pipe may 
be used. This possibly involves the problem that the injected molten metal 
supply stream extends downwardly into the part of the strand immediately 
leaving the continuous casting mold's bottom where the use of multi-phase 
electromagnetic stirring may be undesirable. In such an instance, a static 
magnetic field projected through the mold so as to be intersected by the 
descending stream close or at its source will exert a sudden slowing of 
the stream's velocity with a consequent stirring action.