Method of controlling flow of molten steel in mold

A water-cooled mold for use in continuous steel casting process has at least two vertically-spaced coils arranged in the wall structure of the mold so as to surround molten steel in the mold or in a solidification shell within the mold and such that a jet of molten steel from an immersion nozzle of a tundish in the molten steel collides with the mold wall at a level between the coils. During supplying the molten steel from the tundish into the mold, the coils are supplied with DC currents of opposite directions so as to generate cusp fields in the mold, thereby suppressing the movement of the jet of the molten steel, as well as ascending and descending flows of the molten steel after collision with the mold wall.

BACKGROUND OF THE INVENTION 
Hitherto, an attempt has been made for controlling state of flow of a 
molten steel in a mold by applying a static magnetic field for the purpose 
of reducing any local deviation or uneven distribution of flow of the 
molten steel which tends to occur when the molten steel is poured into the 
mold. In such a method relying upon application of a static magnetic 
field, it is necessary that a path is formed to enable free flowing of an 
induction current which is generated as a result of interference between 
the static magnetic field and the flowing molten steel, corresponding to 
the outer product U .times.B of the flowing velocity U of the molten steel 
and the intensity B of the magnetic field. For instance, in a method shown 
in FIG. 6 in which a static magnetic field is applied substantially 
uniformly, an induction current 6 (see FIG. 7) tends to be generated due 
to interaction between the static magnetic field and the flow of molten 
steel. The induction current, however, cannot flow unless a path for 
circulation of such a current is provided. Consequently, it is necessary 
to form a bypass current which passes through the region near the wall 
where the magnetic field intensity is low. In order to obtain the bypass 
current, it is necessary to use an electromotive force large enough to 
produce such a current. 
FIG. 8 illustrates the distribution of the electric potential .phi. which 
provides the electromotive force for the production of the bypass current. 
The bypass current (J.sub.1 =-.sigma.grad .phi.) tends to flow from a 
region where the potential .phi. is high to the region where the potential 
.phi. is low. The actual current J is the sum of the induction current 
J.sub.2 (.sigma.U.times.B) and the current J.sub.1 produced by the 
electromotive force. Thus, the actual current J is expressed as J=J.sub.1 
+J.sub.2 =.sigma.(U.times.B -grad .phi.). In consequence, although the 
bypass current generated by the electromotive force flows in the region 
near the wall where the magnetic field intensity is low, a potential 
gradation (grad .phi.) which serves to suppress the induction current 
J.sub.2 is formed in the region around the discharge flow of the molten 
steel, so that the actual current J is reduced in such a region. As a 
consequence, a reduction is caused in the efficiency of the 
electromagnetic brake (Lorenz force corresponding to the outer product 
J.times.B of the current J and the magnetic field intensity B). This 
reduction is generally 50% or greater. In order to obtain the desired 
electromagnetic force, therefore, it is necessary to apply a larger 
magnetic force. 
In the field of single-crystal growth process in which a single crystal is 
made to grow and be lifted in accordance with a Czochralski process, it 
has been proposed to brake a natural convection generated in a melt, as 
well as forced convection caused by rotation of the crystal or of a 
crucible, by applying a cusp field as shown in FIG. 9. This art is shown 
in JP-A-58-217493 and JP-A-61-222984. In contrast to the discharge flow of 
molten steel in a continuous casting mold, the flow of the melt in the 
single-crystal growth process occurs in the regions near the walls of the 
container which has an axisymmetrical configuration with respect to the 
axis. This cusp field is generated radially and axisymmetrically, by 
placing upper and lower electromagnets which oppose each other with the 
same poles, namely with reverse polarity, so as to surround the 
single-crystal lifting furnace. It is reported that the cusp field 
provides a high braking efficiency because it acts perpendicularly to the 
flow of the melt in the region near the wall so as to enable the induction 
current to flow circumferentially. 
The behavior of the melt in the single-crystal lifting process in which 
convection is caused by heat from the wall and shear stress generated in 
the boundary between the melt and the wall is entirely different from the 
behavior of the melt in the continuous casting of steel in which the melt 
is jetted and supplied from a immersion nozzle into a mold. Therefore, the 
manner of application of a magnetic field in the single-crystal lifting 
process cannot give any hint to the manner of application of a magnetic 
field to the melt in continuous casting process. 
SUMMARY OF THE INVENTION 
In continuous steel casting process, suppression of the flow of the molten 
steel in the mold and reduction in the local deviation and non-uniformity 
of the molten steel, as well as oscillation of the molten steel surface, 
are quite important factors in order to attain a stable casting by 
avoiding trapping of powder into the molten steel and concentration of 
alumina-type inclusions to the slab. The control of flow of the molten 
steel in a mold requires a high magnetic field intensity or alternatively, 
a compact construction of the device for applying the magnetic field. The 
present invention has been achieved to give a solution to these problems. 
Accordingly, an object of the present invention is to provide a method of 
controlling the flow of molten steel in a mold used in continuous casting 
of steel, which can suppress flow of the molten steel in the mold and 
reduce local deviation or lack of uniformity of flow of the molten steel, 
as well as oscillation of the free surface of the molten steel and which 
can prevent mixing of concentrations of components when different steels 
of different compositions are cast consecutively. 
To these ends, according to the present invention, there is provided a 
method of controlling the flow of a molten steel in a continuous steel 
casting process, the method comprising: preparing a water-cooled mold 
having at least two vertically-spaced coils each having a plurality of 
turns arranged in the wall of the mold so as to surround the molten steel 
in the mold or in a solidification shell within the mold and such that a 
jet of molten steel from a immersion nozzle collides with the mold wall at 
a level between the coils; and supplying, during the jetting of the molten 
steel, the coils with DC currents of opposite directions so as to generate 
cusp fields in the mold, thereby suppressing the movement of the jet of 
the molten steel, as well as ascending and descending flows of the molten 
steel after collision with the mold wall. 
According to this method, the flow of the molten steel is effectively 
braked so that the oscillation of the free surface at the meniscus, so 
that trapping of inclusions and bubbles into the slab is suppressed, thus 
preventing mixing of compositions when different steels with different 
compositions are cast consecutively. 
The cusp fields generated by the upper and lower horizontal coils which are 
supplied with DC currents of opposite directions have all lines of 
magnetic force which have only horizontal components directed towards the 
center at the plane midst between the upper and lower coils. The cusp 
fields act perpendicularly to the jet of the molten steel from the 
immersion nozzle and the flow components of the molten steel deflected by 
the mold wall. Induction currents generated by the cusp fields flow in the 
directions perpendicular to the magnetic lines of force and the molten 
steel, i.e., circumferentially through a horizontal plane. The induction 
current therefore can freely flow without requiring any specific path. 
Consequently, a highly efficient electromagnetic braking effect is 
produced by the interaction between the applied magnetic field and the 
induction current. 
Two or more coils for generating cusp fields may be arranged at levels 
above and below the level at which the jet of the molten steel collides 
with the mold wall. The effect of suppression of the flow of molten steel 
and, hence, the advantages of the invention, are enhanced when a 
multiplicity of coils are used to generate multiple stages of cusp fields 
under suitable conditions. 
The arrangement may be such that each of the coils are divided into 
segments and the vertically aligned segments of the coils are connected 
through connecting portions so as to form independent DC current loops in 
the respective combinations of the segments, thereby generating at least 
one cusp magnetic field. Such an arrangement enables the method of the 
invention to be applied to a variable-width casting operation. 
The above and other objects, features and advantages of the present 
invention will become clear from the following description of the 
preferred embodiments when the same is read in conjunction with the 
accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 is a schematic perspective view of a water-cooled mold 1 having 
coils arranged in two stages: namely, an upper coil and a lower coil. The 
water-cooled mold 1 is adapted to receive a molten steel discharged from 
an immersion nozzle 5 of a tundish which has a pair of nozzle ports 5a, 
5a. The molten steel discharged form the nozzle ports 5a, 5a collides with 
the narrow side walls 1a, 1a of the mold 1, as will be seen from FIG. 3a. 
Horizontal upper and lower coils 2 and 3 are installed in the wall 
structure of the water cooled mold over the entire circumference thereof. 
These coils are positioned at levels which are above and below the level 
at which the molten steel collides with the mold walls la, la. The coils 2 
and 3 ar supplied with D.C. currents which flow in opposite directions 
each other so that they produce a cusp field as shown in FIGS. 2a and 2b. 
The cusp field generate lines of magnetic force which have only horizontal 
components at the position in the middle of the gap between two coils. All 
the lines of magnetic force are directed towards the center B of the 
horizontal plane of the mold. The intensity of the magnetic field is 
highest at the point A midst of the coils and lowest at the center B. The 
relationship between the flow 10 of the molten steel and the lines 9 of 
magnetic force, supplied from the immersion nozzle 5 into the molten steel 
4, is shown in a vertical sectional view of FIG. 3a. The state of 
generation of the induction current 6 in the molten steel 4 is shown in 
FIGS. 3b and 3c which are sectional views taken along the lines b--b' and 
c--c' of FIG. 3a. The induction current 6 flows in the circumferential 
direction in a plane perpendicular to the lines of magnetic force 9 and 
the flow 10 of the molten steel, i.e., within a horizontal plane. 
Therefore, the induction current is allowed to flow circumferentially 
without requiring any bypassing path. Consequently, an electromagnetic 
braking of a high efficiency is effected on the molten steel by the 
interaction between the applied static magnetic field and the induction 
current. Specifically high braking effects are produced on the molten 
steel flowing in the regions near the portions of the mold wall 
corresponding to the lines b--b' and c--c', due to the fact that the lines 
of magnetic force perpendicularly intersect each other, as will be seen 
from FIGS. 3a, 3b and 3c. 
FIG. 4 illustrates the state of generation of cusp fields generated when 
the mold wall structure has three coils, i.e., upper, intermediate and 
lower coils. It is possible to increase the number of coils to generate 
cusp fields in a multiplicity of stages so as to increase the effect of 
suppressing molten steel flow, thus enhancing the effect produced by the 
method of the present invention. 
FIG. 5 shows another embodiment in which upper and lower coils are divided 
into segments. More specifically, the upper coil is divided into segments 
2a, 2b, 2c and 2d, while the lower coil is divided into segments 2e, 2f, 
2g and 2h. The segments 2a and 2e, 2b and 2f, 2c and 2g and 2d and 2h of 
the upper and lower coils, respectively, are connected through connecting 
portions 2i, 2j, 2k, 2l, 2m, 2n, 2o and 2p. In operation, independent 
loops of DC current are formed for the respective pairs of segments of 
upper and lower coils as indicated by arrows, thus generating a cusp 
field. 
TEST EXAMPLE 1 
A test was conducted for evaluating the effects of a cusp field under the 
operating conditions shown in the following Table 1. By way of comparison, 
a test also was conducted by the known method shown in FIG. 6, under 
operating conditions as shown in Table 2. 
It has been confirmed that the level at which the jet of the molten steel 
collides with the narrow side walls of the mold is at 500 mm from the 
meniscus, through measurement of a heat flux conducted by means of 
thermo-couples embedded in the mold wall structure. 
TABLE 1 
______________________________________ 
Operating Conditions Under Cusp Field 
______________________________________ 
Mold 1800 mm wide, 150 mm thick 
specification 
Immersion nozzle 
300 mm deep, discharge angle 20.degree. 
Casting speed 2.0 m/min. 
Coil position Upper coil: 100 mm below meniscus 
pattern A Lower coil: 500 mm below meniscus 
Coil position Upper coil: 300 mm below meniscus 
pattern B Lower coil: 700 mm below meniscus 
Coil position Upper coil: 500 mm below meniscus 
pattern C Lower coil: 900 mm below meniscus 
Current supplied 
0 to 1000 A to normal condition 
coil of 100 turns 
Maximum magnetic 
0.00, 0.05, 0.10, 0.15 Tesla 
field generated 
in mold 
______________________________________ 
TABLE 2 
______________________________________ 
Operating Conditions of Known 
Process Under Magnetic Field 
______________________________________ 
Mold 1800 mm wide, 150 mm thick 
Immersion nozzle 
300 mm deep, discharge angle 20.degree. C. 
Casting speed 2.0 m/min. 
Coil position Set at level 400 mm below meniscus 
and centered at position 450 mm 
spaced from shorter mold wall 
Maximum magnetic 
0.30 Tesla 
field generated 
in mold 
______________________________________ 
Castings were conducted under the conditions of Tables 1 and 2 and ingots 
were extracted from the mold, followed by measurement of amounts of slime 
of a lumina-type inclusion sin the inclusion accumulation zone which is 
about 1/4 level from the liquid level. The measured amounts of slime were 
normalized with the value obtained when no cusp field is applied, and the 
results are shown in Table 3. 
TABLE 3 
______________________________________ 
Amounts of Slime Extracted 
______________________________________ 
When no cusp field is applied 
1 
Conventional method 0.30 Tesla 
0.49 
Under cusp field (pattern A) 
0.10 Tesla 0.79 
0.15 Tesla 0.65 
Under cusp field (pattern B) 
0.10 Tesla 0.45 
0.15 Tesla 0.23 
Under cusp field (pattern C) 
0.10 Tesla 0.63 
0.15 Tesla 0.40 
______________________________________ 
Castings were conducted under the conditions of Tables 1 an d2 and ingots 
were extracted from the molds, followed by measurement of amounts of 
white-blot defects in the surfaces of the extracted ingots. The measured 
amounts of defects were normalized with the value obtained when no cusp 
field is applied, and the results are shown in Table 4. 
TABLE 4 
______________________________________ 
Amount of White Blot Defects 
______________________________________ 
When no cusp field is applied 
1 
Conventional method 0.30 Tesla 
0.34 
Under cusp field (pattern A) 
0.10 Tesla 1.05 
0.15 Tesla 0.90 
Under cusp field (pattern B) 
0.10 Tesla 0.42 
0.15 Tesla 0.22 
Under cusp field (pattern C) 
0.10 Tesla 0.68 
0.15 Tesla 0.32 
______________________________________ 
A test operation also was conducted under the conditions of Table 1 (only 
pattern B) and Table 2. In the test, steels of different compositions were 
cast consecutively, and the lengths of the portions of the ingots to be 
wasted due to mixing of the compositions were measured. The measuring 
results are shown in Table 5 below, in terms of value normalized with the 
value obtained when no cusp filed is applied. 
TABLE 5 
______________________________________ 
Lengths of Ingots to be Wasted 
______________________________________ 
When no cusp field is applied 
1 
Under cusp field (pattern B) 
0.10 Tesla 0.64 
0.15 Tesla 0.48 
______________________________________ 
As will be understood from the foregoing data, it was confirmed that the 
present invention offers the following advantages. 
(1) Reduction in accumulation of inclusions in the ingot thanks to the 
suppression of flow of the molten steel effected by the cusp field. 
(2) Reduction in generation of defects in the ingot surface thanks to the 
suppression of flow and oscillation of the free surface of the molten 
steel effected by the cusp field. 
(3) Prevention of mixing of compositions during consecutive casting of 
different steel compositions, thanks to the suppression of flow of the 
molten steel effected by the cusp field. 
TEST EXAMPLE 2 
Test operations for evaluation was conducted under the conditions shown in 
Table 6, using the molding apparatus of the type shown in FIG. 5. 
Castings were conducted under the conditions of Table 6 and ingots were 
extracted from the molds, followed by measurement of amounts of slime of 
alumina-type inclusion sin the inclusion accumulation zone which is about 
1/4 level from the liquid level. The measured amounts of slime were 
normalized with the value obtained when no cusp field is applied, and the 
results are shown in Table 7. 
TABLE 6 
______________________________________ 
Operating Conditions Under Cusp Field 
______________________________________ 
Mold 1800 mm wide, 150 mm thick 
specification 
Immersed nozzle 
300 mm deep, discharge angle 20.degree. 
Casting speed 2.0 m/min. 
Coil position Upper and Lower coils were divided 
pattern into four segments, respectively, 
as shown in FIG. 5. 
Upper coil: 300 mm below meniscus 
Lower coil: 700 mm below meniscus 
Current 1000 A to normal condition coil of 
supplied 100 turns (to each coil) 
Maximum magnetic 
0.15 Tesla 
field generated 
in mold 
______________________________________ 
TABLE 7 
______________________________________ 
Amounts of Slime Extracted 
______________________________________ 
When no cusp field is applied 
1 
Conventional method 0.30 Tesla 
0.49 
Under cusp field (coils not divided) 
0.23 
0.15 Tesla 
Under cusp field (Coils divided) 
0.25 
0.15 Tesla 
______________________________________ 
It is thus understood that the effect in the reduction of amounts of 
inclusions is substantially the same, regardless of whether the coils are 
divided or not. 
As will be apparent from the above, according to the present invention, 
electric currents of opposite directions are supplied to two or more coils 
arranged around a water-cooled mold used in continuous casting of steel, 
iron or non-ferrous metal, so that cusp fields are generated to 
efficiently uniformalize the flow of the molten steel in the mold, while 
suppressing oscillation of the free surface of the melt in the mold, as 
well as mixing of compositions when different types of metals are cast 
consecutively. Both ordinary conductive coils and superconductive coils 
are equally usable as coils for generating the cusp fields.