Method of and apparatus for electromagnetic mixing of metal during continuous casting

A molten metal is poured into the upper end of a vertically tubular continuous-casting mold and is withdrawn as a hardened casting from the lower end of the mold. An electromagnetic field is displaced upwardly counter-current to the descending metal so as to mix the molten part of the metal and displace non-magnetic inclusions away from the hardening skin of the metal. The mold is operated in accordance with the following formula: EQU B.sup.2 .times.L=(1/.gamma.v)(16d.sup.2 +120d) wherein B equals effective strength of magnetic field in tesla, L equals overall vertical length of field in meters, .gamma. equals electrical conductivity of metal being cast in ohms.sup.-1 .times.meter.sup.-1, v equals vertical travel speed of field in meters/second, and d equals desired surface depth of non-metallic inclusions in millimeters i.e., the distance from the outer surface of the casting in a direction normal to this surface. As a rule the overall length L is varied to space the non-metallic inclusions the desired depth below the surface of the thus-produced casting.

CROSS-REFERENCE TO RELATED APPLICATIONS 
This application is related to our commonly assigned and jointly filed 
application Ser. No. 815,417 as well as to commonly assigned patent 
applications Ser. Nos. 723,194 now U.S. Pat. No. 4,042,008; 723,647 now 
U.S. Pat. No. 4,040,467 and 763,971, now U.S. Pat. No. 4,067,378 all of 
whose disclosures are herewith fully incorporated by reference. 
BACKGROUND OF THE INVENTION 
The present invention relates to a method of and apparatus for the 
continuous casting of a metal. More particularly this invention concerns 
the electromagnetic mixing of a metal as it is continuously cast. 
In a continuous casting operation such as described in the above-mentioned 
copending applications as well as in French patent applications Ser. Nos. 
72/20544 now French Pat. No. 2,187,465 and 76/15178 filed 5, 19, 1976, and 
U.S. Pat. No. 3,941,183, all of whose teachings are also herewith 
incorporated by reference, a metal is poured in molten condition into the 
top of a vertically tubular mold. This mold is cooled so that at least the 
outer portion of the descending column of a metal in the mold is hardened. 
Thus a hard casting exits continuously from the bottom of the mold. In 
such arrangements it has been found extremely advantageous to 
electromagnetically mix the molten metal, normally steel, in the mold. 
Such mixing is achieved by forming a relatively powerful magnetic field and 
causing it to travel vertically along the mold countercurrent to the 
descending metal. Since the molten metal is normally introduced into the 
top of the mold by a so-called dip tube at the center of the mold, the 
molten metal in the mold forms an inverting toroid that descends in the 
center of the mold and rises along the periphery. Such mixing is extremely 
advantageous in that it not only speeds the hardening of the metal by 
increasing the circulation and heat dissipation in the mass and between 
the mass and the mold, but it also brings impurities that would form 
inclusions to the surface of the mold where many oxidize and are lost, or 
where they even can be caught in a thin slag-like layer on top of the 
column of descending molten metal in the mold. Such electromagnetic mixing 
eliminates the necessity of flame scarfing which can waste as much as 4% 
of the casting. 
Nonetheless the known methods, although they do considerably reduce the 
overall quantity of inclusions and do tend to distribute these 
non-metallic inclusions more toward the center of the casting, do not give 
reliably even results. Furthermore the known methods do not allow for an 
adjustment of the subsurface depth of the non-metallic inclusions. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide an improved 
method of and apparatus for continuous casting. 
Another object is to provide an improved electromagnetic mixing method and 
arrangement for use in a continuous-casting system which allows the exact 
establishment of the subsurface depth of the non-metallic inclusions in 
the casting. 
These objects are attained according to the present invention in a 
continuous-casting method carried out substantially in accordance with the 
following formula: 
EQU B.sup.2 .times.L=(1/.gamma.v)(16d.sup.2 +120d) 
wherein B equals effective strength of magnetic field in tesla, L equals 
overall vertical length of field in meters, .gamma. equals electrical 
conductivity of metal being cast in ohms.sup.-1 .times.meter.sup.-1, v 
equals vertical travel speed of field in meters/second, and d equals 
desired subsurface depth of non-metallic inclusions in millimeters. Only 
numerical values are to be given weight in this and the following 
equations; once the parameters are set in the proper units such units are 
to be disregarded. 
According to further features of this invention the establishment of the 
preselected subsurface depth for the non-metallic inclusions is determined 
mainly by varying the length L over which the magnetic field is effective 
on the molten metal in the mold. This is achieved by adding or subtracting 
coils from the coil stack at the mold which serves for the electromagnetic 
mixing. The lowermost coils can either by physically removed or simply 
electrically disconnected in order to shorten the length and extra coils 
can be bolted on or connected up in order to increase the length. 
To this end in accordance with yet further features of this invention the 
coil that serves for electromagnetic mixing is constituted as a fixed 
upper stack of coils received in a first housing through which a coolant, 
normally water, is circulated for hardening the column of descending 
molten metal, and a second lower stack of coils below this upper stack and 
removable or disconnectable in the manner described above. 
When the above-described molding apparatus is used for the production of 
rods or the like of relatively small cross-sectional size, the coils are 
all annular and completely surround the descending column of molten metal. 
When, however, a slab or relatively large billet or the like is to be 
produced having an elongated cross-sectional shape, each of the coils is 
formed as a pair of coil parts that are elongated in the direction of 
elongation of the mold cross-section and flank the mold. Thus the major 
faces of the billet will be subjected to electromagnetic mixing as 
described above whereas the edge surfaces will not. In such an arrangement 
the lower coils may be provided inside of the rollers that directly 
contact the wide faces of the slab and pull it from the lower end of the 
molds. 
Thus in accordance with the present invention it is possible exactly to 
control the depth at which the non-metallic inclusions will lie. Normally 
these inclusions lie immediately beneath the surface or even at the 
surface of the casting, so that they greatly interfere with subsequent 
rolling and similar operations. With the system according to the present 
invention, however, it is possible exactly and without experimentation to 
establish the subsurface depth of these non-metallic inclusions. 
It is possible to regulate the effective intensity (B) of the field simply 
by varying the wattage or current flowing through the inductor. The 
effective length (L) of the inductor is regulated as described above by 
either disconnecting some of the coils or physically shortening the 
inductor altogether. It is essential that the magnetic field not start at 
a level lower than that where the casting is completely solid. Furthermore 
it should not extend higher than the upper surface of the liquid in the 
mold. In this context it is noted that it is not possible to determine 
exactly where the liquid mass or crater inside the body being cast 
terminates. Normally the liquid metal extends down well below the mold, 
however. For this reason the upper end of the field is normally placed 
directly at the upper surface in the mold in order to insure that the 
lower end of the field does not extend past the crater of the body. 
It is also, of course, possible to adjust the depth by maintaining the 
field strength and length the same and varying the speed (v) at which the 
field is moved upwardly countercurrent to the descending column of metal. 
This is done by changing the frequency (N) in herz of the electricity 
energizing the conductor. Thus the speed is directly proportional to twice 
the frequency multiplied by the spacing (.tau.) between regions of 
opposite polarity along the inductor, or: 
EQU v=2.tau.N 
Since the walls of the mold are typically formed of an electrically 
conductive material, normally copper or an alloy of copper, the 
electromagnetic field passing through these walls is weakened 
disproportionately as its frequency increases. Thus for a given mold there 
is a maximum frequency beyond which the field strength drops off so 
greatly that this maximum frequency in effect constitutes the upper limit 
of the energization frequency. Thus the travel or propagation speed is 
normally considered to be a parameter that is fixed at a predetermined 
value corresponding to the optimal frequency for the energization current. 
The novel features which are considered as characteristic for the invention 
are set forth in the appended claims. The invention itself, however, both 
as to its construction and its method of operation, together with 
additional objects and advantages thereof, will be best understood from 
the following description of specific embodiments when read in connection 
with the accompanying drawing.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
As shown in FIG. 1 a mold according to this invention basically comprises 
an upwardly open mold tube 1 surrounded by a housing defining an annular 
cooling chamber 2 surrounding the inner wall of the mold tube 1 and having 
an inlet chamber 3 and an outlet chamber 4 for vertical circulation of 
cold water along the outer wall of the mold tube 1. 
Liquid metal is introduced into the top of the mold 1 through the passage 7 
of a dip tube 6 whose lower end lies below the upper surface 12 of the 
crater 5 formed by the liquid metal. Since the walls of the mold 1 are 
cooled a hard skin of increasing thickness is formed around this crater 5 
and the interface between them is shown at 9. 
An inductor 10 formed of a fixed upper part 11 and an adjustable lower part 
13 is constituted as a stack of nine like coils 14, six in the upper part 
11 and three in the lower part 13. All of these coils 14 are connected to 
an alternating-current power supply 27. The lower coils 14 are connected 
via openable switches 28 to the power supply 27 so that the effective 
length L of the inductor 10 can be reduced. The power supply 27 energizes 
the coils 14 sequentially in such a manner that the field effectively 
travels upwardly at a velocity v equal to twice the frequency N of the 
power supply multiplied by the spacing .tau. between adjacent coils 14 of 
like polarity. 
The overall length L can be changed by disconnecting some of the coils 14 
of the lower stack 13 by opening of the switches 28, or by unbolting the 
coils 14 of the lower stack 13 from each other at bolt connections 16 or 
from the upper stack 11 at bolt connections 15. 
Thus as the metal is introduced in molten form into the mold via the dip 
tube 6 it will harden on the outer portion 8, but the crater 5 inside this 
outer portion 8 will form an inverting toroid, descending in the center 
and rising along the edges. This will automatically bring to the surface 
non-metallic inclusions where they can either remain in a slag-like layer 
or oxidize. The casting is continuously drawn off downwardly in the 
direction D indicated by the arrow in FIG. 1 at such a rate that the upper 
level 12 remains generally level with the upper end of the inductor 10. 
Since the skin 8 increases in thickness downwardly eventually the magnetic 
field of the inductor 10 is not sufficient to mix the molten metal 5 
through the relatively thick skin 8. At this time the inclusions will 
become trapped at the interface 9. 
The arrangement of FIG. 1 is used for making relatively small-diameter rods 
and the like and the coils 14 are all annular and completely surround the 
mold 1. Removal of the lower stack 13 sets the lower limit on the length 
L, and corresponds to a subsurface depth of inclusions equal to between 1 
and 10 mm. 
When slabs or the like of larger dimensions are to be continuously cast a 
system such as shown in FIG. 2 where like reference numerals are used 
where like structure is employed. In the arrangement of FIG. 2 the body 
being continuously cast is elongated horizontally in a direction 
perpendicular to the plane of the view. 
Here, the inductor 10 is formed as described in the above-cited U.S. patent 
application Ser. No. 723,194 now U.S. Pat. No. 4,042,008 whose entire 
disclosure, as mentioned above, is hereby fully incorporated by reference. 
Thus in this arrangement the upper inductor 10 is formed as a pair of 
stacks of soft-iron plates forming straight parallel notches 19 in which 
are received respective coil parts 18. The two sides of the upper inductor 
11 are mirror-symmetrical about a vertical plane passing through the 
center of the mold and here, once again, perpendicular to the plane of the 
view and extending in the direction D. Thus the upper inductor 11 is 
formed of two like parts each extending parallel to the respective side of 
the mold 1. 
The lower inductor 13 here is formed of rollers 21 in which are received 
cruciform-shaped plates forming horizontal stacks 22 receiving coils 23. A 
housing 25 provided with spraying nozzles 26 is provided surrounding the 
rollers 21 housing the magnets 22, 23 for cooling them and cooling the 
casting descending downwardly from the mold 1. In this arrangement it has 
been found that by appropriate angular spacing of the coils 23 in the 
rollers 21 and of the coils 18 in the upper inductor 11 it is possible to 
achieve a completely uniform electromagnetic mixing. The rotation rate of 
the rollers 21 and the coils 23 with them determines the advance speed for 
the lower inductor 13 whereas that for the upper inductor 11 is simply 
determined by the frequency and spacing as described with reference to 
FIG. 1. 
EXAMPLE 
The arrangement of FIG. 1 is employed to make steel billets of 
square-section measuring 120 mm on a side. The continuous billet is 
withdrawn downwardly in direction D at a velocity v of 2 m/minute. Thus 
the thickness of the skin 8 at the lower end of the mold 1 is 
approximately 12 mm. 
The inductor 10 is fed with three-phase current and the various coils 14 
are connected to a common phase on one side and connected together in 
series-opposition. Two consecutive coils connected to the same phase are 
separated by two other coils each connected to a respective one of the 
other phases of the current from the power supply 27. The coils have a 
polar spacing .tau. of 0.24 m. The coils are so wired that they can be 
energized at 350 A without heating excessively. This creates an electrical 
field having a strength of 0.042 tesla in the casting being formed 
immediately inside the interface 9. The frequency N of the energization 
current is fixed at 10 Hz which for the mold in question is the optimum 
value. 
In order to space all of the non-magnetic inclusions approximately 8 mm 
below the surface of the billet being formed it is necessary to adjust the 
arrangement so that 
EQU B.sup.2 L=6.6.times.10.sup.-4 Tesla.sup.2 .times.m 
The electrical conductivity .gamma. of the steel is equal to 
EQU 6.25.times.10.sup.-5 .OMEGA..sup.-1 m.sup.-1 
Removing the lower part 13 of the inductor altogether gives the arrangement 
an effective length L equal to twice the spacing .tau. between coils of 
like polarity or 0.48 m. 
Thus for these calculations it is necessary to have an effective field of 
0.037 tesla. Thus in the arrangement wherein the effective magnetic field 
strength is 0.042 tesla and the inductor has an overall length of 0.48 the 
inclusions will be at least 8 mm deep. 
Subsequently the same test was provided but with the three coils 14 of the 
lower stack 13 connected to the power supply 27 so that the overall length 
L was increased by one increment .tau.. According to the calculations it 
would only be necessary to use a field strength of 0.030 tesla to achieve 
the same results as given above, that is pushing the inclusions at least 8 
mm beneath the surface of the billet. In reality it was found that it was 
necessary to use a field strength of 0.034 tesla. Since, however, this is 
within approximately 10% of the calculated result the formula proves true. 
The invention described above can be used in any type of continuous-casting 
of a metal. In accordance with this invention it is possible therefore to 
position the non-metallic inclusions at the desired depth below the 
surface in a sure and accurate manner. Thus a later working of the 
continuously made castings is substantially eased, in particular when 
laminating or otherwise operating directly on the surface of such 
castings. 
Without further analysis, the foregoing will so fully reveal the gist of 
the present invention that others can by applying current knowledge 
readily adapt it for various applications without omitting features that, 
from the standpoint of prior art, fairly constitute essential 
characteristics of the generic or specific aspects of this invention.