Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
2~71 16~
PATENT APPLICATION
09007/9619
APPARATU5 AND METHOD FOR
MAGNETICALLY CONFINING MOLTEN METAL
Backqround Of The Invention
The present invention relates generally to
apparatuses and methods for magnetically confining
molten metal and more particularly to an apparatus
and method for preventing the escape of molten metal
through the open side of a vertically extending gap
between two horizontally spaced members and within
which the molten metal is located.
An example of an environment in which the
present invention is intended to operate is an
arrangement for continuously casting molten metal
directly into strip, e.g. steel strip. Such an
apparatus typically comprises a pair of horizontally
spaced rolls mounted for rotation in opposite
rotational senses about respective horizontal axes.
The two rolls define a horizontally disposed,
vertically extending gap therebetween for receiving
the molten metal. The gap defined by the rolls
tapers in a downward direction. The rolls are
cooled, and in turn cool the molten metal as the
molten metal descends through the gap.
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The gap has horizontally spaced, open opposite
ends adjacent the ends of the two rolls. The molten
metal is unconfined by the rolls at the open ends of
the gap. To prevent molten metal from escaping
outwardly through the open ends of the gap,
mechanical dams or seals have been employed.
Mechanical dams have drawbacks because the dam
is in physical contact with both the rotating rolls
and the molten metal. As a result, the dam is
subject to wear, leaking and breakage and can cause
freezing and large thermal gradients in the molten
metal. Moreover, contact between the mechanical dam
and the solidifying metal can cause irregularities
along the edges of metal strip cast in this manner,
thereby offsetting the advantages of continuous
casting over the conventional method of rolling
metal strip from a thicker, solid entity.
The advantages obtained from the continuous
casting of metal strip, and the disadvantages
arising from the use of mechanical dams or seals are
described in more detail in Praeg U.S. Patent No.
4,936,374 and in Lari et al U.S. Patent No.
4,974,661, and the disclosures of each of these
patents are incorporated herein by reference.
To overcome the disadvantages inherent in the
employment of mechanical dams or seals, efforts have
been made to contain the molten metal at the open
end of the gap between the rolls by employing an
electromagnet having a core encircled by a
conductive coil through which an alternating
electric current flows and having a pair of magnet
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PATENT APPLICATION
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poles located adjacent the open end of the gap. The
magnet is energized by the flow of alternating
current through the coil, and the magnet generates
an alternating or time-varying magnetic field
S extending across the open end of the gap between the
poles of the magnet. The magnetic field can be
either horizontally disposed or vertically disposed,
depending upon the disposition of the poles of the
magnet. Examples of magnets which produce a
horizontal field are descr bed in the aforementioned
Praeg U.S. Patent No. 4,936,374; and examples of
magnets which produce a vertical magnetic field are
described in the aforementioned Lari et al U.S.
Patent No. 4,974,661.
The alternating magnetic field induces eddy
currents in the molten metal adjacent the open end
of the gap, creating a repulsive force which urges
the molten metal away from the magnetic field
generated by the magnet and thus away from the open
end of the gap.
The static pressure force urging the molten
metal outwardly through the open end of the gap
between the rolls increases with increased depth of
the molten metal, and the magnetic pressure exerted
by the alternating magnetic field must be sufficient
to counter the maximum outward pressure exerted on
the molten metal. A more detailed discussion of the
considerations described in the preceding sentence
and of the various parameters involved in those
considerations are contained in the aforementioned
Praeg and Lari et al. U.S. Patents.
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Another expedient for containing molten metal
at the open end of a gap between a pair of members
is to locate, adjacent the open end of the gap, a
coil through which an alternating current flows.
This causes the coil to generate a magnetic field
which induces eddy currents in the molten metal
adjacent the open end of the gap resulting in a
repulsive force similar to that described above in
connection with the magnetic field generated by an
electromagnet. Embodiments of this type of
expedient are described in Olsson U.S. Patent No.
4,020,890, and the disclosure therein is
incorporated herein by reference.
The use of a coil to directly generate the
magnetic field adjacent the open end of the gap is
more efficient than the use of an electromagnet
because, when employing an electromagnet, the coil
is used to energize the core of a magnet through
which magnetic flux must travel to the magnet poles
which then generate a magnetic field adjacent the
open end of the gap. As a result, there is so-
called "core loss" when a coil is employed to
energize an electromagnet; but core loss is not a
significant factor when the coil is employed to
directly generate the magnetic field at the open end
of the gap.
A drawback to the latter expedient is that the
coil must be placed quite close to the open end of
the gap in order to generate a magnetic field which
will contain the molten metal there. In the
expedient employing an electromagnet, the coil can
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be relatively remote from the open end of the gap.
The closer the coil is to the molten steel, the more
severe the thermal conditions to which the coil is
subjected. Another drawback to the expedient
employing a coil for directly generating the
magnetic field at the open end of the gap is that
part of the magnetic field is radiated in a
direction away from the open end of the gap, thereby
decreasing the efficiency of the coil. The problem
described in the preceding sentence can also be a
problem when employing any electromagnet.
Summary of the Invention
The drawbacks and deficiencies of the prior art
expedients described above are eliminated by an
apparatus and method in accordance with the present
invention.
A magnetic confining method and apparatus in
accordance with the present invention employs the
proximity effect to directly generate, adjacent the
open side of the gap, a horizontal magnetic field
which extends through the open side of the gap to
the molten metal in the gap, and the magnetic field
is confined substantially to the open side of the
gap. The horizontal magnetic field is directly
generated by a coil located adjacent the open side
of the gap, with a surface portion of the coil
facing the open side of the gap. Typically,
alternating current is conducted through the coil to
generate the horizontal magnetic field which extends
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PATENT APPLICATION
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from the facing surface portion of the coil, through
the open side of the gap, to the molten metal.
Employment of the proximity effect requires
that the coil be located sufficiently close to the
open side of the gap so that the strength (H) of the
magnetic field, at the open side of the gap, is
sufficient to offset the pressures which urge the
molten metal outwardly through the open side of the
gap. The strength of the magnetic field generated
by the coil decreases with increasing distance of
the coil from the open side of the gap. The
electromagnetic pressure between two conducting
surfaces (in this case the coil and the molten
metal) is directly proportional to the square of the
magnetic field strength (H2).
The coil and its associated structure are
located sufficiently close to the open side of the
gap to contain the molten metal within the gap, and
the possible adverse effects of such close proximity
are offset by the employment of structure, to be
described below in detail, which protects the coil.
Dissipation of the magnetic field in a
direction away from the open side of the gap is
prevented by restricting the magnetic field
generated by the coil substantially to the open side
of the gap. This is accomplished, in part, by
providing a non-magnetic electrical conductor (1)
which is in electrically conductive relation with
the coil (2) which faces the open side of the gap,
and (3) which is sufficiently proximate to the open
side of the gap to confine the magnetic field
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PATENT APPLICATION
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substantially to the open side of the gap. The coil
has upper and lower portions, and the conductor
occupies substantially the entire area between the
coil's upper and lower portions. In addition, there
is structure, composed of magnetic material, which
(a) concentrates the flow of electric current in the
surface portion of the coil which faces the open
side of the gap and (b) provides a low reluctance
return path for the directly generated magnetic
field which extends through the open side of the
gap.
The non-magnetic conductor is configured to
conform to the tapered shape of the gap so as to
increase the magnetic pressure against the molten
metal, in accordance with increasing static pressure
(i.e. depth) of the molten metal in the gap. In
some embodiments, the conductor and the surface
portion of the coil facing the open side of the gap
coincide, i.e. they are one and the same.
Other features and advantages are inherent in
the method and apparatus claimed and disclosed or
will become apparent to those skilled in the art
from the following detailed description in
conjunction with the accompanying drawings.
Brief Description of the Drawings
Fig. 1 is a plan view showing an embodiment of
an apparatus in accordance with the present
invention, associated with a pair of rolls of a
continuous strip caster;
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PATENT APPLICATION
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Fig. 2 is an end view of the apparatus and
rolls of Fig. l;
Fig. 3 is a side view of the apparatus and
rolls,
Fig. 4 is an exploded perspective of the
apparatus;
Fig. 5 is a perspective of the apparatus with
all the components thereof assembled together;
Fig. 6 is a front end view of a single-turn
coil constituting one component of the apparatus;
Fig. 7 is a side view of the coil of Fig. 6;
Fig. 8 is a plan view of a magnetic cover
constituting another component of the apparatus;
Fig. 9 is a front end view of the magnetic
cover of Fig. 8;
Fig. 10 is a plan view of a conductive shield
constituting still another component of the
apparatus;
Fig. 11 is a front end view of the conductive
shield of Fig. 10;
Fig. 12 is an enlarged front end view of the
apparatus;
Fig. 13 is an enlarged plan view of the
apparatus;
Fig. 14 is an enlarged, fragmentary, sectional
view, taken along line 14-14 in Fig. 12, showing the
magnetic field generated by the apparatus, near the
top of the gap between the rolls;
Fig. 15 is a sectional view, taken along line
30 15-15 in Fig. 12, showing the magnetic field
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PATENT APPLICATION
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_ g _
generated by the apparatus near the bottom of the
gap;
Fig. 16 is a front end view of another
embodiment of apparatus in accordance with the
present invention;
Fig. 17 is a sectional view taken along line
17--17 in Fig. 16;
Fig. 18 is a sectional view taken along line
18--18 in Fig. 16;
Fig. 19 is a front end view of one component of
the embodiment of Fig. 16;
Fig. 20 is a plan view of the component of Fig.
19;
Fig. 21 is a front end view of another
component of the embodiment of Fig. 16;
Fig. 22 is a plan view of the component of Fig.
21;
Fig. 23 is a sectional view taken along line
23--23 in Fig. 16;
Fig. 24 is a perspective of the embodiment of
Fig. 1 in association with bus bars and cooling
conduits;
Fig. 25 is an exploded perspective of a further
embodiment of an apparatus in accordance with the
present invention; and
Fig. 26 is a perspective of the apparatus with
the components thereof assembled together.
Detailed Description
Referring initially to Figs. 1-3, 12 and 13,
indicated generally at 30 is a magnetic confining
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PATENT APPLICATION
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-- 10 --
apparatus constructed in accordance with an
embodiment of the present invention. Apparatus 30
employs the proximity effect to prevent the escape
of molten metal through the open side 36 of a
vertically extending gap 35 located between two
horizontally spaced, metal rolls 31, 32 in a
continuous strip caster. Rolls 31, 32 rotate in
respective opposite, rotative senses about
respective axes 33, 34. Molten metal is normally
contained in gap 35. Rolls 31, 32 are cooled, in a
conventional manner not disclosed here, and as
molten metal descends vertically through gap 35, the
metal is cooled and solidified into a metal strip 37
(Fig. 12) descending downwardly from the narrowest
part of gap 35.
But for apparatus 30, molten metal in gap 35
would escape through open side 36 of gap 35.
Although only one open side of gap 35, and one
apparatus 30 is shown in the figures, it should be
understood that there is an open side at each open
end of gap 35 and an apparatus 30 at each open end.
Apparatus 30 comprises a current-conducting
coil 40 located adjacent open side 36 of gap 35 and
having a coil surface portion facing open side 36.
Alternating current is conducted through coil 40, in
a manner to be subsequently described, and this
directly generates a horizontal magnetic field
which, because of the proximity of coil 40 to open
side 36, is caused to extend from the facing side of
the coil, through open side 36 of gap 35, to the
molten metal in the gap. Coil 40 is sufficiently
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PATENT APPLICATION
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-- 11 --
proximate open side 36 so that the directly
generated horizontal magnetic field has a strength
sufficient to exert a confining pressure against the
molten metal in gap 35.
Apparatus 30 comprises structure, to be
described in detail later, for preventing the
magnetic field from dissipating in a direction away
from open side 36 of gap 35. This structure
confines the magnetic field generated by the coil
substantially to the open side 36 of the gap.
Referring now to Figs. 4-5, coil 40 comprises a
single turn which faces the open side 36 of gap 35.
Coil 40 comprises a pair of half coils 41, 42
separated by a narrow vertical space 44 and
conductively joined adjacent an end of each by a
connecting element 43 located at the bottom of coil
40. Each half coil 41, 42 is vertically disposed
and has a respective vertically disposed front wall
45, 46 facing open side 36 of gap 35. The two front
walls 45, 46 together constitute a non-magnetic,
electrical conductor which (a) is in electr~cally
conductive relation with coil 40 and (b) faces open
side 36 of gap 35 and (c) is sufficiently proximate
to open side 36 to confine the magnetic field,
generated by coil 40, substantially to open side 36.
As shown in Fig. 6, the conductor defined by front
walls 45, 46 occupies substantially the entire area
between top and bottom portions 113, 114 of coil 40,
except for narrow vertical space 44.
Each front wall 45, 46 of a half coil 41, 42
has a width which narrows downwardly along the
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PATENT APPLICATION
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- 12 -
vertical dimension of the half coil in conformity
with a narrowing in the width of open side 36 of gap
35 (Figs. 4, 6 and 12). In other words, the
conductor defined by front walls 45, 46 has a shape
conforming substantially to the tapering shape of
open side 36 of gap 35. The current density and
magnetic field intensity in a front wall 45, 46 is
determined by the total current across the wall
divided by ~he width of the wall. As the width
decreases, the current density and magnetic field
intensity increase. Accordingly, when current of a
given magnitude flows through coil 40, the current
density in front walls 45, 46 increases in a
downward direction with decreasing width of the
front walls. The static pressure developed by the
molten metal in gap 35 increases with increased
depth. However, increased current density produces
increased magnetic field intensity and increased
magnetic pressure. As a result, the configuration
of the conductor defined by front walls 45, 46
brings about an increase in the magnetic pressure
associated with the magnetic field generated by coil
40, thereby offsetting the increased static pressure
developed by the molten metal in gap 35.
The conductor defined by front wall 45, 46 is
shown in the figures as having an arcuately tapered,
downwardly converging shape. A triangular, straight
line, downwardly converging shape could also be
employed.
Each half coil 41, 42 has, in addition to
respective front walls 45, 46, respective outside
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PATENT APPLICATION
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walls 51, 52, respective inside walls 53, 54 and
respective rear walls 55, 56 (Fig. 14).
Associated with coil 40 are a pair of members
composed of magnetic material, cooperating to
concentrate the current which flows through coil 40
in coil front walls 45, 46 and to produce a low
reluctance return path for the magnetic field
produced by coil 40 and which extends through open
side 36 of gap 35. There is a ver~ically disposed,
substantially planar, first magnetic member 48
(Figs. 14-15) which (a) lies in a plane parallel to
the axis of rolls 31, 32 and (b) has a pair of
opposite side surfaces 71, 72 (Fig. 15). Each
vertically disposed half coil 41, 42 is located
adjacent a respective opposite side surface 71, 72
of first magnetic member 48 and is electrically
insulated therefrom by a thin layer of electrical
insulating material (not shown). First magnetic
member 48 has a front edge 49 facing open side 36 of
gap 35 in substantially the same close proximity
thereto as the front walls 45, 46 of half coils 41,
42. First magnetic member 48 also has a rear edge
60 in substantially abutting relation with rear wall
57 of a second magnetic member 50.
Second magnetic member 50 partially encloses
coil 40. More particularly, second magnetic member
50 has a rear wall 57 enclosing the rear walls 55,
56 of the two half coils 41, 42 and
electrically insulated therefrom by a thin layer of
electrical insulating material (not shown). Second
magnetic member 50 also has a pair of spaced apart
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side walls 58, 59 each enclosing and closely
following the contour of a respective outside wall
51, 52 of a respective half coil 41, 42 and
electrically insulated therefrom by a thin layer of
electrical insulating material (not shown). Each
side wall 58, 59 of second magnetic member 50 has a
front end 61, 62 (Figs. 4-5) facing a respective
rotatable roll 31, 32 adjacent a peripheral side
edge 37, 38 of the roll (Figs. 12-13).
First magnetic member 48 and second magnetic
member 50 comprise structure cooperating to provide
a low reluctance return path for the directly
generated magnetic field which extends through open
side 36 of gap 35 by coil 40.
In addition to the components described above,
apparatus 30 also comprises a shield 65 composed of
non-magnetic, conductive material. Shield 65
partially encloses second magnetic member 50, in a
manner to be described below, and prevents a
magnetic field from forming around the outside of
and behind second magnetic member 50. In other
words, shield 65 confines that part of the directly
generated magnetic field which is outside of the low
reluctance return path to substantially a space
defined on one side by the coil's front walls 45, 46
and on the other side by the molten metal in gap 35.
Shield 65 comprises a rear wall portion 66,
enclosing rear wall 57 of second magnetic member 50
from behind and electrically insulated therefrom by
a thin layer cf electrical insulating material (not
shown). Shield 65 also includes a pair of side wall
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PATENT APPLICATION
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portions 67, 68 each enclosing a respective side
wall 58, 59 of second magnetic member 50 from the
outside and electrically insulated therefrom by a
thin layer of electrical insulating material (not
shown).
Each side wall portion 67, 68 of shield 65 has
an inner surface which (a) is in close proximate
relation to the adjacent side wall 58, 59 of second
magnetic member 50 and (b) follows the contour of
the adjacent side wall. Rear wall portion 66 of
shield 65 has an inner surface in close proximate
relation to rear wall 57 of second magnetic memher
50.
Shield 65 has a hollow interior, shown at 69,
70 in Fig. 11, defining a passage through which a
cooling fluid can be circulated through inlet and
outlet openings (not shown).
Referring now to Figs. 4-5 and 14-15, apparatus
30 further comprises a refractory member 80 covering
the front edge 49 of first magnetic member 48 and
also covering front walls 45, 46 of half coils 41,
42. Refractory member 80 has a pair of opposed side
edges 81, 82 each abutting against a respective side
wall 58, 59 of second magnetic member 50.
Refractory member 80 also has a vertically disposed
outside surface 83 which lies in substantially the
same vertical plane as front ends 61, 62 of
sidewalls 58, 59 on second magnetic member 50.
Refractory member 80 covers that part of coil
front walls 45, 46 otherwise exposed to the molten
metal in gap 35. In the illustrated embodiment,
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PATENT APPLICATION
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refractory member 80 does not cover front ends 61,
62 of side walls 58, 59 on second magnetic member
50.
As noted above, first magnetic member 48 is
electrically insulated from the two half coils 45,
46, and second magnetic member 50 is electrically
insulated from half coils 45, 46 and shield 65. To
perform the insulating function, one may employ a
commercially available, electrical insulating tape
which can be wrapped around magnetic members 48 and
50. The tape should be a temperature-resistant,
insulating film capable of withstanding temperatures
up to 177~C (350~F) with a maximum film thickness of
about 0.127 mm (0.005 in.).
Coil 40 is composed of a highly conductive
material such as copper or copper base alloy. Each
half coil 41, 42 has a hollow interior defining a
passage through which a cooling fluid may be
circulated, and this will be described subsequently
in greater detail.
As shown in Fig. 12, first magnetic member 48
has a lower portion 47 at substantially the same
vertical level as the narrowest part of open side 36
of gap 35. Lower portion 47 is composed of a
plurality of laminated, horizontally disposed,
vertically layered strips of grain oriented silicon
steel, a conventional magnetic material. The upper
portion of first magnetic member 48 may be composed
of the same material, although the layered strips of
silicon steel need not be horizontally disposed but
may be vertically disposed.
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- 17 -
Horizontally disposed silicon steel strips are
employed at the lower portion 47 of first magnetic
member 48 because they produce less core loss than
do vertically disposed strips. Neither ferrite nor
powdered iron should be used for lower portion 47 of
first magnetic member 48 because the saturation
levels of these two materials are much less than the
saturation levels of grain oriented silicon steel.
However, ferrite and powdered iron may be used at
the uppermost portion of the magnetic member where
the magnetic field density and resultant flux
density, which increase with increased depth of the
molten metal, are relatively low and can be handled
by materials having relatively low saturation
levels. Where the depth of the molten metal is at a
maximum, magnetic field density and resultant flux
density are at a maximum and require the use of a
material having a relatively high saturation level,
namely, grain oriented silicon steel.
Second magnetic member 50 may be composed of
any material heretofore conventionally employed as a
magnetic material in electromagnets. In addition to
laminated strips of silicon steel, second magnetic
member 50 may be composed of compacted ferrite
powder or compacted iron powder, for example. If
laminated strips of silicon steel are employed on
second magnetic member 50, the laminations may be
either horizontally disposed or vertically disposed~
the latter being preferable.
Refractory member 80 is composed of a ceramic
material such as boron nitride or a material known
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PATENT APPLICATION
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- 18 -
b 3000 or 3300, a low density alumina
material made by Carborundum Corp. The ceramic
material of which refractory member 80 is composed
must have sufficient temperature resistance to
protect coil 40 if there is a current failure
causing a cessation of the magnetic field. In such
a case, of course, the molten metal in gap 35 would
be urged outwardly through open side 36 of the gap
toward coil 40. Refractory member 80 protects coil
40, should that occur. ~efractory member 80 is
wedged between front ends 61, 62 of second magnetic
member 50 and is adhered to front walls 45, 46 of
half coils 41, 42 employing a high temperature epoxy
cement, for example.
RG11S 31, 32 are preferably made of a highly
conductive copper base alloy composed primarily of
oxygen free copper and ma~ contain small amounts of
silver (0.07-0.12 wt.%) and phosphorous (about 0.02
wt.%), for scratch resistance.
To position coil 40 as close as possible to
open end 36 of gap 35, apparatus 30 preferably
substantially abuts against the ends of rolls 31,
32, with only a very slight space or clearance
between apparatus 30 and rolls 31, 32.
Apparatus 30 is supported in the desired
positional relationship with rolls 31, 32 by
structure, illustrated in Fig. 24, which also
functions as bus bars for conducting electric
current to coil 40 and provides conduits for
circulating cooling fluid into and out of coil 40.
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PATENT APPLICATION
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-- 19 --
As shown in Fig. 24, located above coil 40 are
a pair of metal conductive members 85, 86 connected
electrically and structurally to half coils 41, 42
respectively. At an end of each member 85, 86,
remote from coil 40, is a respective flange 89, 90
which is (a) mechanically connected to supporting
structure (not shown) and (b) electrically connected
to a source of alternating current (not shown).
Mechanically and electrically connecting member 85
to half coil 41 is a conductive metal plate 88
resting atop half coil 41. The mechanical
connection of plate 88 to half coil 41 employs
conventional metal mechanical fasteners. A plate
similar to 88 connects member 86 to half coil 42.
That plate is not shown in Fig. 24, but it is
horizontally spaced away from plate 88 which
connects member 85 to half coil 41. Members 85 and
86 are similarly horizontally spaced apart. Members
85, 86 and plate 88 may be composed of the same
material as coil 40.
Current is conducted through member 85 and
plate 88 to half coil 41, then through connecting
element 43a and half coil 42 to the plate (not
shown) atop half coil 42 and then through member 86.
Connecting element 43a in Fig. 24 is located below
coil 40 rather than to the rear of coil 40 as is
connecting element 43 in Figs. 4-7.
Member 86 is a mirror image of member 85, and
half coil 42 is a mirror image of half coil 41. The
following discussion will be in connection with
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PATENT APPLICATION
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- 20 -
member 85, but member 86 has similar features which
are mirror images of those in member 85.
Extending alongside member 85 is an integral
inlet conduit 92 which communicates with a
distributor upper portion 93 separated from a
distributor lower portion 94 by a horizontally
disposed internal partition not shown in Fig. 24.
Distributor upper portion 93 communicates with a
vertical conduit 95 which communicates with an inlet
opening 96 in the top of a half coil (Fig. 6).
Inlet opening 96 communicates with an inclined
inlet passage 97 which introduces cooling fluid into
the interior of a half coil. An inclined guide
member 98 in the interior of the half coil directs
incoming fluid initially along one side of the
interior of the half coil and then along the other
side. Cooling fluid circulates through the half
coil and is withdrawn therefrom through a vertically
disposed outlet passage 99 communicating with an
outlet opening 100 communicating with lower
distributor portion 94 which in turn communicates
with an outlet conduit 101 disposed along the side
of member 85. Although, in Fig. 6, elements 96-100
are shown in association with half coil 46, the same
elements would be present in half coil 45 as mirror
images.
Cooling fluid is introduced into inlet conduit
92 on member 85 through an inlet fitting 91,
connected to a source of cooling fluid (not shown),
and cooling fluid is withdrawn from outlet conduit
101 through an outlet fitting 102.
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PATENT APPLICATION
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The cooling fluids circulated through coil 40
should be high purity, low conductivity cooling
water, for example.
The cooling fluid circulated through connecting
element 43 on coil 40 (Figs. 2-7) is separate from
the cooling fluid circulated through each half coil
41, 42. Cooling fluid is introduced into and
withdrawn from connecting element 43 via inlet and
outlet conduits 63, 64 respectively (Figs. 1 and 3).
Similarly, the cooling fluid circulated through
connecting element 43a in the embodiment of Fig. 24
is separate from the cooling fluid circulated
through each half coil 41, 42. In the embodiment of
Fig. 24, cooling fluid is introduced into connecting
element 43a through an inlet 103 and is removed from
connecting element 43a through an outlet opening
(not shown) on the opposite side of connecting
element 43a from inlet 103.
In the embodiment of Fig. 24, current enters
and leaves half coils 41, 42 via members 85, 86
located at the top of coil 40. In an alternative
embodiment, bus bars can be located at the bottom of
each half coil 41, 42 rather than at the top. In
such an alternative embodiment, connecting element
43 or 43a would be located at the top of the coil
rather than at the bottom.
As noted above, side wall portions 67, 68 of
shield 65 have an interior surface which conforms to
and closely follows the exterior surface of side
walls 58, 59 on second magnetic member 50 (Fig. 5).
Cooling fluid is circulated through the hollow
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PATENT APPLICATION
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- 22 -
interior 69, 70 of shield 65 (Fig. 11) to cool the
shield and to assist in cooling second magnetic
member 50.
Although side wall portions 67, 68 on shield 65
are shown with vertical exterior surfaces (Figs. 4,
11), these exterior surfaces may curve inwardly from
top to bottom just as do the interior surfaces of
side wall portions 67, 68. In such a case, the
shape of shield 65 would resemble the shape of
second magnetic member 50 (Figs. 4, 9). However, no
matter the embodiment employed for shield 65, it is
important that the inner surfaces of sidewall
portions 67, 68 conform to and closely follow the
outer surfaces of sidewalls 58, 59 of second
magnetic member 50 and that the inside surfaces of
side walls 58, 59 on second magnetic member 50
conform to and closely follow the outside surfaces
of outside walls 51, 52 on half coils 41, 42.
Referring now to Figs. 14 and 15, these figures
show, with arrows, the magnetic field generated by
coil 40 at upper and lower elevations indicated by
section lines 14--14 and 15--15 respectively in Fig.
12. The magnetic field enters and leaves magnetic
members such as 48 and 50 at right angles to a
surface of the magnetic material. The magnetic
field generally is parallel or tangent to a surface
composed of non-magnetic, conductive material~ such
as front wall 45 of coil 40 and rolls 31, 32.
Refractory member 80 is essentially transparent to
the magnetic field. The molten metal confined in
gap 35 is shown at 111 in Figs. 14 and 15, and the
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PATENT APPLICATION
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outer boundary of molten metal 111 at open side 36
of gap 35 is shown at 112 in Figs. 14 and 15.
As noted above, each roll 31, 32 has a
peripheral side edge 37, 38 defining an edge of open
side 36 of gap 35. Ad~acent each side edge 37, 38
is a side edge portion, e.g. side edge portion 39
adjacent peripheral side edge 38 (Figs. 14-15).
Similarly, each front wall 45-46 on a half coil 41,
42, has a respective outside edge 105, 106~ each
horizontally spaced from the other, and there is an
outside edge portion 107, 108 adjacent each outside
edge 105, 106 respectively.
As shown in Fig. 12, the horizontal distance
between outside edges 105, 106 on half coil front
walls 45, 46 is greater than the horizontal distance
between the two peripheral side edges 37, 38
defining open side 36 of gap 35, at the same
vertical location along gap 35. Referring to Figs.
14-15, each outside edge portion 107, 108 on a
respective coil front wall 45, 46 is spaced in an
axial direction away from a respective side edge
portion, e.g. 39 on roll 32, to define a narrow
space 109 therebetween.
As shown in Figs. 14 and 15, outside edge
portion 107 on front wall 45 of half coil 41 and
side edge portion 39 on roll 32 cooperate to provide
increased magnetic flux density in the magnetic
field in space 109, compared to the flux density of
the magnetic field extending across open side 36 of
gap 35. The reason for this will be discussed
below. Increased magnetic flux density increases
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PATENT APPLICATION
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- 24 -
the magnetic pressure in space 109, compared to the
magnetic pressure at open side 36 of gap 35, thereby
preventing molten metal from flowing laterally
outwardly through space 109.
The depth of penetration of a magnetic field
into a non-magnetic conductor, such as molten metal
111 or front wall 45 of half coil 41 or roll 32, is
inversely proportional as the square root of the
product of (a) the magnetic permeability and (b) the
conductivity of the conductive material. Copper or
copper alloy, of which half coil front wall 45 and
roll 32 are composed, are much less penetrable by a
magnetic field than is molten steel. As a result,
the magnetic field and magnetic flux density are
more concentrated in space 109, between peripheral
edge portion 39 on roll 32 and outside edge portion
107 on half coil fronc wall 45, than between front
wall 45 and outside boundary 112 on molten metal
111, when the molten metal is steel.
The magnetic pressure developed by the magnetic
field is proportional to the square of the magnetic
flux density which in turn is determined by the
cross-sectional area of the magnetic flux. Because
the magnetic field is squeezed in space 109, the
cross-sectional area of the magnetic flux in space
109 is smaller than the cross-sectional area of the
flux in the space between coil 40 and molten metal
111. As a result, the magnetic flux density is
increased in space 109, compared to the magnetic
flux density between coil 40 and molten metal 111,
thereby increasing the magnetic pressure in space
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PATENT APPLICATION
09007/9619
109 compared to the magnetic pressure between coil
40 and molten metal 111.
The depth of penetration of the magnetic field
is also inversely proportional to the angular
frequency of the alternating electric current. At a
frequency of 3,000 ~ertz, the relative penetrations
of the magnetic field into molten steel and copper
is abou~ 10.9 and 1.2 mm, respectively. A typical
operating frequency for coil 40 is about 3,000
Hertz. If the frequency is too much lower than
that, secondary re-circulating flows can be
developed in the molten metal, and that would be
undesirable. The higher the frequency, the greater
the amount of heat that is generated in the coil,
and that in turn requires increased cooling. The
frequency employed cannot be greater than the
available cooling capacity.
The magnetic pressure directly opposite front
edge 49 on first magnetic member 48 is less than the
magnetic pressure elsewhere along open side 36 of
gap 35, because of the directionality of the
magnetic field opposite front edge 49 (Fig. 14). As
a result, molten metal boundary 112 projects further
outwardly toward coil 40 at a location directly
opposite first magnetic member 48.
The smaller the width of first magnetic member
48, the less spreading the magnetic field will
undergo directly in front of first magnetic member
48, producing a smaller decrease in magnetic
pressure there. If first magnetic member 4& is
relatively wide, molten metal 111 may touch
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PATENT APPLICATION
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- 26 -
refractory member 80 in front of first magnetic
member 48, possibly producing solidification of the
molten metal there. If first magnetic member 48 is
relatively narrow, the magnetic field will be
sufficiently concentrated in front of first magnetic
member 48 to prevent the molten metal from touching
refractory 80 at that location. First refractory
member 48 can be as narrow as 0.020 inches (0.508
mm) and as wide as the separation between rolls 31,
32 at the narrowest portion of gap 35 (e.g. 0.1-0.25
inches) (2.54~6.35 mm).
In Fig. 15, which shows the magnetic field at
essentially the narrowest portion of gap 35, the
magnetic pressure directly in front of first
magnetic member 48 will be sufficiently high to
prevent the molten steel from contacting refractory
member 80 at that location. The increased magnetic
pressure at the elevation depicted in Fig. 15 is due
to the smaller magnetic path length at that
elevation and the closer proximity to the front edge
49 of first magnetic member 48 of space 109 in which
the magnetic field is squeezed to increase the flux
density thereof.
First magnetic member 48 need not be uniform in
width along its vertical dimension. However, if the
width of first magnetic member 48 is varied, the
minimum width should be at the bottom thereof.
Referring now to Figs. 16-23, indicated
generally at 130 (Figs. 16-17 and 23) is an
apparatus constructed in accordance with another
embodiment of the present invention.
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PATENT APPLICATION
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- 27 -
Apparatus 130 comprises a current-conducting
coil 140 having a multiplicity of vertically
disposed coil turns 141 wrapped around a vertically
disposed magnetic member 150. Each coil turn 141
comprises a vertically disposed front portion 142
facing open side 36 of gap 35. Alternating current
is conducted through coil 140, and this directly
generates a horizontal magnetic field which, because
of the proxi~ity of coil 140 to open side 36, causes
the magnetic field to extend from front portions 142
of coil turns 141, through open side 36 of gap 35,
to the molten metal in the gap, and with sufficient
strength to exert a confining pressure against the
molten metal in the gap.
Except for the coil turn 141 located furthest
to the left as viewed in Fig. 23, each coil turn 141
includes a top portion 143 connected to that coil
turn's front portion 142, a bottom portion 144
connected to the bottom of that coil turn's front
portion 142 and a back portion 145 connecting the
bottom portion 144 of a coil turn 141 to the top
portion 143 of an adjacent coil turn 141 (Fig 17).
The coil turn furthest to the left, as viewed in
Fig. 23, does not include a back portion. Instead,
bottom portion 144 on that coil turn communicates
with other structure to be subsequently described.
Coil 140 is composed of hollow copper tubing
through which a cooling fluid is circulated. The
cooling fluid enters coil 140 through an inlet
conduit 192 connected to the top portion 143 of the
coil turn 141 located furthest to the right as
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PATENT APPLICATION
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- 28 -
viewed in Fig. 23. The cooling fluid exits from
coil 140 through an outlet conduit 1~3 connected to
the bottom portion 144 of the coil turn 141 located
furthest to the left in Fig. 23. A pair of bus bars
194, 195 are electrically connected respectively to
inlet conduit 192 and outlet conduit 193 to conduct
alternating electric current through coil 140.
Apparatus 130 comprises structure for
preventing the magnetic field from dissipating in a
direction away from open side 36 of gap 35. This
structure restricts the magnetic field generated by
coil 140 substantially to the gap's open side 36.
Referring to Figs. 16-18 and 23, conductively
attached to each front portion 142 of a respective
coil turn 141, and facing open side 36 of gap 35, is
a vertically disposed metal strip 148 constituting a
non-magnetic conductor, composed of copper, for
e~ample.
As shown in Fig. 16, each metal strip 148 has a
width which narrows downwardly along the vertical
dimension of the strip in conformity with a
narrowing in the width of open side 36 of gap 35, so
that, when current flows through coil 140 and strips
148, the current density in the strip increases with
decreasing strip width. As noted above, the static
pressure developed by the molten metal in gap 35
increases with increased depth. However, because
incrèased current density produces increased
magnetic pressure, the configuration of the
conductor defined by strips 148 brings about an
increase in magnetic pressure in conformity with the
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PATENT APPLICATION
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- 29 -
increased static pressure developed by the molten
metal in gap 35.
The non-magnetic conductor defined by strips
148 and located be~ween coil 140 and the open side
of the gap, is sufficiently proximate to open side
36 to confine the magnetic field generated by coil
140 substantially to the open side of the gap. As
shown in Figs. 16 and 17, the conductor defined by
strip 148 occupies substantially the entire area, at
the front of the coil, between upper and lower
portions 143, 144 of each coil turn 141.
Magnetic member 150 is composed of magnetic
material, it is associated with coil 140, and it
cooperates with the coil to produce a low reluctance
return path for the directly generated magnetic
field produced by coil 140 and which extends through
open side 36 of gap 35. As shown in Figs. 18 and
23, magnetic member 150 has a front surface 151
facing open side 36 of gap 35. Each front portion
20 142 of each coil turn 141 is located in front of
front surface 151 of magnetic member 150. Each
front portion 142 of a coil turn 141 has a pair of
sides 146, 147 each covered by a strip of magnetic
material 160, 161 respectively tFig. 18). Each
25 strip of magnetic material 160, 161 extends between
(a) front surface 151 of magnetic member 150 and (b)
metal strip 148 attached to front portion 142, to
concentrate the electric current flowing through
coil turn front portion 142 on metal strip 148.
There is a thin insulating film between front
surface 151 of magnetic member 150 and front portion
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PATENT APPLICATION
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142 of coil turn 141. Similarly, there is a thin
film of electrical insulating material between each
side 146, 147 of front portion 142 and the
corresponding magnetic strip 160, 161 covering sides
146, 147 respectively. Strips 148 are in
substantially abutting, side-by-side relation
separated only by a thin film of electrical
insulating material. The electrical insulating
material described in the preceding paragraph is the
same as that used in apparatus 30 illustrated in
Figs. 1-15 to separate coil 40 from magnetic members
48 and 50.
Magnetic member 150 and magnetic strips 160,
161 may be composed of the same magnetic material as
are the magnetic members 48 and 50 in apparatus 30.
Referring to Figs. 19-20, magnetic member 150
comprises, in addition to front surface 151, a-rear
surface 152, and a pair of arcuate downwardly
converging sidewalls 153, 154 which conform the
shape of member 150 substantially to the shape of
open side 36 of gap 35. Magnetic member 150 has
cut-out portions 155 (Fig. 19) adjacent each
sidewall 153, 154 and through which pass the bottom
portions 144 of coil turns 141. Top portions 143 of
each coil turn 141 extend over the top of magnetic
member 150 (Fig. 17). As shown in Fig. 17, front
portion 142 of each coil turn 141 is located in
front of front surface 151 of magnetic member 150,
and each back portion 145 of a coil turn is located
behind the rear surface 152 of magnetic member 150
and extends between the bottom portion 144 of that
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PATENT APPLICATION
09007/9619
coil turn and the top portion 143 of an adjacent
coil turn 141.
Each coil turn 141 has a vertical dimension
differing from the vertical dimension of an adjacent
coil turn 141 and substantially corresponding to the
vertical dimension of that part of magnetic member
150 around which the coil turn is wrapped. Each
vertically disposed metal strip 148 is substantially
vertically coextensive with the coil front portion
142 to which strip 148 is conductively attached.
Each strip 148 has a pair of side edges, and the
side edges of adjacent strips 148 define a space
therebetween which is insubstantial (Fig. 16) and
which contains a thin film of electrical insulating
material to prevent electrical shorting between
adjacent strips.
Magnetic member 150 has a width which (a)
varies in a vertical direction along member 150 and
(b) corresponds substantially to the width of open
side 36 of gap 35 in the same horizontal plane.
Surrounding magnetic member 150 is a shield 165
composed of a conducting material such as copper
(Figs. 16-17 and 23). As shown in Figs. 21-22,
shield 165 comprises a rear wall 166 and a pair of
sidewalls 167, 168. Rear wall 166 is cut out at 169
to accommodate the passage through rear wall 166 of
bottom portions 144 of coil turns 141. Rear wall
166 of shield 165 closely encloses rear surface 152
of magnetic member 150 and is separated therefrom by
a thin film of electrical insulating material. Each
sidewall 167, 168 of shield 165 has a respective
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PATENT APPLICATION
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downwardly c~nverging inner surface 171, 172 which
closely encloses a respective downwardly converging
sidewall 153, 154 of magnetic member 150 and is
separated therefrom by a thin film of electrical
insulating material.
Shield 165 serves substantially the same
function in apparatus 130 as does shield 65 in
appaxatus 30 of Figs. 1-15.
Referring now to Fig. 23, sidewalls 153, 154 of
magnetic member 150 have front ends 163, 164
respectively. Extending between these sidewalls, at
their front ends, is a refractory member 180 which
performs the same function in apparatus 130 as does
refractory member 80 in apparatus 30, namely
protecting coil 140 and strips 148 from the molten
metal in gap 35, refractory member 80 being disposed
between strips 148 and open side 36 of gap 35.
However, additionally in apparatus 130, there
is a space 181 between refractory member 180 and
strips 148. Space 181 comprises a medium through
which a cooling gas can be passed, e.g. from an air
knife 182 which is situated to direct a cooling gas
through space 181 (Fig. 17).
The magnetic field generated by apparatus 130
extends horizontally across open side 36 of gap 35
between front ends 163, 164 of sidewalls 153, 154 on
magnetic member 150. There is a space 149 between
end 163 of sidewall 153 and the adjacent peripheral
side edge 37 of roll 31; and there is a similar
space 149 between end 164 of sidewall 154 and
peripheral side edge 38 of roll 32. The magnetic
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PATENT APPLICATION
09007/9619
field is squeezed in spaces 149 thereby increasing
the magnetic flux density and magnetic pressure
there compared to those existing at open side 36 of
gap 35. This enhances the resistance to escape of
molten metal through spaces 149.
Indicated generally at 230 in Figs. 25-26 is
another embodiment of apparatus constructed in
accordance with the present invention. Apparatus
230 is positioned adjacent open side 36 of gap 35
similar to the positioning of apparatus 30, and
apparatus 230 employs the proximity effect to exert
a confining pressure against the molten metal in gap
35, in a manner similar to that described above in
connection with apparatus 30, except for such
differences as are noted below.
Apparatus 230 comprises a single turn coil 240
composed of what are substantially two half-coils
comprising a front half-coil 241 connected at its
bottom end by a shorting element 243 to a rear half-
coil 242 which functions also as a shield, as willbe subsequently described.
Alternating current flows from a bus bar (not
shown) downwardly through front half-coil 241, then
through shorting element 243 to rear half coil 242,
upwardly through the latter (which functions as a
return path for the current) and then away from coil
240 through another bus bar (not shown) connected to
half coil 242.
Front half-coil 241 has a front wall 245,
constituting the front surface portion of coil 240,
side walls 251, 252 and a rear wall 255. A magnetic
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PATENT APPLICATION
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member 250 closely encloses the front half-coil's
rear wall 255 and side walls 251, 252, similar to
the enclosure of corresponding walls on coil 40 by
magnetic member 50 (Figs. 4-5). A thin insulating
layer (not shown) separates magnetic member 250 from
half coil walls 251, 252 and 255.
The arrangement described in the preceding
paragraph concentrates the current, flowing
downwardly through half coil 241, on front surface
portion 245 thereof.
The shield defined by rear half coil 242 has a
rear wall portion 266 and side wall portions 267,
268 which closely enclose a rear wall 257 and side
walls 258, 259 on magnetic member 250. A thin
insulating layer (not shown) separates the wall
portions of the shield from the walls of the
magnetic member.
Coil 240 directly generates a magnetic field
which is disposed horizontally and substantially
uniformly across the full horizontal width of front
surface portion 245 of half coil 241 and through the
open side 36 of gap 35. Front surface portion 245
is a non-magnetic electrical conductor which faces
the gap's open side 36 and is positioned
sufficiently proximate to open side 36 to confine
the magnetic field substantially to the gap's open
side.
Magnetic member 250 comprises a low reluctance
return path for the directly generated magnetic
field which extends through the gap's open side.
Shield 242 confines that part of the directly
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PATENT APPLICATION
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- 35 -
generated magnetic field which is outside of the low
reluctance return path substantially to a space
defined on one side by front surface portion 245 of
half coil 241 and on the other side by the molten
metal in gap 35.
A refractory member 280 cooperates with the
other components of apparatus 230 in the same manner
as refractory member 80 cooperates with the
components of apparatus 30. Refractory member 280
functions like refractory member 80.
Apparatus 230 differs from apparatus 30
principally in that apparatus 230 eliminates the gap
in the horizontal magnetic field generated by
apparatus 30 and resulting from the location of
first magnetic member 48 between half coils 41 and
42 (Fig. 14). Apparatus 230 provides a magnetic
field which is disposed fully across front surface
portion 245 of coil 240 and which has a more uniform
horizontal component than the magnetic field
generated by apparatus 30. Because of this greater
uniformity, the magnetic field will tend to
penetrate further into gap 35, although apparatus
230 requires twice the current flow required by
apparatus 30.
Half coil 241 has a vertical extension 273 for
attachment, e.g. at 274, to a bus bar to supply
incoming current to half coil 241. Half coil 242
has an upper portion 275 for attachment, e.g. at
276, to a bus bar for return flow of current away
30 from half coil 242. Components 241-243, 273 and 275
are hollow. Cooling fluid is circulated through
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PATENT APPLICATION
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- 36 -
half coils 241 and 242, through shorting member 243,
through extension 273 on half coil 241 and through
upper portion 275 on half coil 242. Appropriate
guide members and passages for the cooling fluid are
provided within all of the components described in
the preceding paragraph, these being structural
expedients which are within the skill of the art.
Apparatus 230 is easier to cool than apparatus
30 because apparatus 230 does not employ a magnetic
member like first magnetic member 48 employed in
apparatus 30. First magnetic member 48, composed of
iron laminates and located in a slot between half
coils 41 and 42, renders apparatus 30 relatively
more difficult to cool.
The foregoing detailed description has been
given for clearness of understanding only, and no
unnecessary limitations should be understood
therefrom, as modifications will be obvious to those
skilled in the art.
