Note: Descriptions are shown in the official language in which they were submitted.
WO91/18696 PCT/US90/03243
,~ .
2~20 6
SIDEWALL CONTAINMENT OF LI~UID METAL WITH HORIZONTAL ALTERNATING
MAGNETIC FIELDS
CONTRACTUAL ORIGIN OF THE INVENTION
The United States Government has rights in this
invention under Contract No. W-31-109-ENG-38 between the
U.S. Department of Energy and the University of Chicago,
operator of Argonne National Laboratory.
BACKGROUND OF THE INVENTION
This invention relates generally to the casting of
metal sheets and is particularly directed to the ~
vertical casting of metal sheets between counter
rotating rollers.
Steel making occupies a central economic role and
represents a significant fraction of the energy con-
sumption of many industrialized nations. The bulk of
steel making operations involves the production of steel
plate and sheet. Present steel mill practice typically
produces thin steel sheets by pouring liquid steel into
a mold, whereupon the liquid steel solidifies upon
contact with the cold mold surface. The solidified
SU8srlTuTE SHEEr
WO 91/18696 2 ~ ~ ~ 2 o ~ PCI/US90/03243
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r ' . - 2 -
steel leaves the mold either as an ingot or as a
continuous slab after it is cooled typically by water
circulating within the mold wall during a solidification
process. In either case, the solid steel is relatively
thick, e.g., 6 inches or greater, and must be
subsequently processed to reduce the thickness to the
desired value and to improve metallurgical properties.
The mold-formed steel is usually characterized by a
surface roughened by defects, such as cold folds,
lo liq~lation, hot tears and the like which result primarily
from contact between the mold and the solidifying
metallic shell. In addition, the steel ingot or sheet
thus cast also frequently exhibits considerable alloy
segregation in its surface zone due to the initial
cooling of the metal surface from the direct application
of a coolant. Subsequent fabrication steps, such as
rolling, extruding, forging and the like, usually
require the scalping of the ingot or sheet prior to
working to remove both the surface defects as well as
20 the alloy deficient zone adjacent to its surface. These
additional steps, of course, increase the comple~ity and
e~pense of steel production.
Steel sheet thickness reduction is accomplished by a
rolling mill which is very capital intensive and
consumes large amounts of energy. The rolling process
SUeSTlTUTE SHEET
WO91/1~96 2 0 8 4 2 0 6 PCT/US~/03~3
, - 3 -
therefore contributes substant~ally to the cost of the
steel sheet. In a typical installat~on, a 10 inch
thick steel slab must be manipulated by at least ten
rolling machines to reduce lts thickness. The rolling
mill may extend as much as one-half mile and cost as
much as SS00 milllon.
Compared to current practice, a large reduct~on
in steel sheet total cost and in the energy required
for its production could be achieved $f the sheets
could be cast ~n near net shape, i.e. in shape and size
closely appro%imating the final desired product. This
would reduce the rolling mill operation and would
result in a large savings in energy. There are several
technologies currently under development which attempt
to achieve these advantages by forming the steel sheets
in the casti~ ~rocess.
One approach under consideration by the steel
industry to reduce processing involves roller casting
of sheets of steel. This method was originally con-
ceived by ~. Bessemer over 100 years ago as describedin British patent nos. 11,317 (1847) and 49,053 ~1857)
and a paper to the Iron and Steel Institute, U.R.
(October 1891). Th~s roller casting method produces
steel sheets by pouring molten steel between counter
rotatinq twin-rollers. The rollers are separated by a
.~
20~4206
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gap. Rotation of the rollers forces the molten metal through the
gap between the rollers. Mechanical seals are necessary to
contain the molten metal at the edges of the rollers. The
rollers are made from a metal with high thermal conductivity, such
as copper or copper alloys and water-cooled in order to solidify
the skin of the molten metal before it leaves the gap between the
rollers. The metal leaves the rollers in the form of a strip or
sheet. This sheet can be further cooled by water or other
suitable means via jets. This method has the drawback that the
mechanical seals used to contain the molten metal at the roller
edges are in physical contact with both the rotating rollers and
molten metal and therefore subject to water, leaking, clogging,
freezing and large thermal gradients. Furthermore, contact
between the mechanical seals and the solidifying metal can cause
irregularities along the edges of sheets cast in this manner
thereby offsetting the advantages of the roller method.
Accordingly, of the present invention seeks to provide an
improved method, apparatus and arrangement for casting thin metal
sheets.
Further the present invention seeks to produce thin metal
sheets which require little or no subsequent rolling after the
sheet is cast.
The invention further seeks to provide for continuous roller
casting of metal sheets.
Further still, this invention seeks to provide containment of
a pool of molten metal between twin-roller casters, without
sidewalls that make physical contact with the rollers and to
prevent a pool of molten metal from flowing out the ends of
~ ~ 2~8420~
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counter rotating rollers by means of a shaped horizontal
alternating magnetic field.
Still further, this invention seeks to provide an
electromagnetic stopper or seal that is capable of preventing or
regulating the flow of a molten metal in a horizontal direction.
SUMMARY OF THE INVENTION
The present invention provides for confinement of molten
metal with a horizontal alternating magnetic field. In
particular, this invention employs a magnet that can produce a
horizontal alternating magnetic field to confine a molten metal at
the edges of parallel horizontal rollers as a solid metal sheet is
cast by counter-rotation of the rollers.
The invention provides an apparatus for confining molten metal
comprising containment means having an open side, a magnet capable
of generating a mainly horizontal alternating magnetic field, the
magnet located adjacent to the open side of the containment means
whereby the field generated by the magnet is capable of inducing
eddy currents in a thin layer at the surface of the molten metal
which interact with the magnetic field producing a force that can
contain the molten metal within the containment means. The magnet
includes ma~netic poles located adjacent to the open side of the
confinement means, a core connecting the poles, a coil encircling
the core, the coil capable of being responsive to a current
source, whereby an alternating magnetic field can be generated
between the poles and parallel to the open side of the containment
means so that a molten metal can be confined within the
confinement means.
Another aspect of the invention comprehends an apparatus for
~.~.
' - 6 - 2~2~
confining molten metal comprising containment means having an open
side comprising a pair of rollers, each of the rollers including a
middle portion and a rim portion, wherein the rollers are parallel
and adjacent to each other in a horizontal plane and further
wherein the rollers are separated by a gap, whereby counter
rotation of the rollers can force the flow of a molten metal
between the gap between the rollers. A magnet is located adjacent
to the open side of the containment means comprised of magnetic
poles located adjacent to the open side of the containment means
and extending axially into the ends of the rollers, a core
connects the magnetic poles and a coil encircling the core, the
coil capable of being responsive to a current source. An
alternating magnetic field can be generated between the poles and
parallel to the open side of the containment means so that a
molten metal can be confined within the containment means. The
middle portions of the rollers have a resistivity lower than the
rim portions so that transmission of a magnetic field by the
magnet through the middle portion is less than through the rim
portion.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure la is a cross sectional front view of the present
invention.
Figure lb is a sectional view of a segment of the roller in
Figure la.
Figure 2 is a view along section line 2 - 2' of Figure la.
Figure 3 is a view along section line 3 - 3' of Figure la.
Figure 4 is a cross sectional view of the core as depicted
~, along section line 4 - 4' of Figure 2.
WO9l/1~96 PCT/US90/03~3
_7_ 2 ~ ~2 0~
Figure 5 is a perspective view of the magnet and
coil of one embodiment of this invention.
Figure 6 is a perspective view of another embodiment
of the magnet and coil of this invention.
Figure 7 is a cross section of the yoke as depicted
in Figure 6.
Figure 8 is a perspective view of another embodiment
of the magnet core of this invention.
Figure 9 is a front sectional vertical front view of
lo another embodiment of this invention.
Figure 10 is a vertical sectional front view of
still another embodiment of the magnet of this invention
and a sideview of the rollers.
Figure 11 is a horizontal sectional view of another
embodiment of this invention.
Figure 12a is a front view of a portion of another
embodiment of the roller rim of this invention.
Figure 12b is a top view of the embodiment of the
roller rim of this invention as depicted in Figure 12a.
Figure 13a is a view of a portion of a roller
showing another embodiment of the roller rim of this
invention.
Figure 13b is a sectional view along line 13b-13b'
of Figure 10.
Figure 14 is a side view of another embodiment of
this invention.
SU~S I I I UTE SHEET
WO9l/1~96 PCT/US90/03~3
- 8 - _
Figure lSa is a side view of still another embodi-
~Q ment of this invention.
Figure 15b is a horizontal view along line 15b-
lSb' of Figure 15a.
DETAILED DESC~IPTION OF THE INVENTION
The present invention overcomes the problems of
roller casting with a novel design which features
electromagnetic containment of the liquid metal at the
roller edges in place of mechanical seals thereby
overcoming the problems associated with mechanical
seals. The present invention provides a shaped hori-
zontal alternating magnetic field to confine a pool of
molten metal between the cylindrical surfaces of a pair
of rollers as the molten metal is cast into a thin
vertical sheet by counter rotation of the rollers which
force the molten metal between them. The horizontal
alternating magnetic field of the present invention can
also be used to prevent or regulate the flow of molten
metal from weirs or orifices of other geometries. The
pressure, p, exerted by the molten pool of metal consists
essentially of ferrostatic pressure Ph and pressure Pr
induced by the rollers via the solidifying metal to be
cast
p s Ph + Pr . (1)
The magnetic pressure, Pm, exerted by the hor-zontal
alternating magnetic field, B, must balance the pressure
WO91/1~96 ~ PCT/US90/03243
- _ 9 ~
from the top of the metal pool to the region where the
shell of the metal has solidified sufficiently thick to
withstand the pressure. The magnetic pressure is given
by
p = B2/ ~ (2)
where the constant ~ O is the permeability of free space.
The ferrostatic pressure Ph exerted by the molten
pool of metal increases linearly with increasing down-
ward distance h from the surface of the pool
Ph = 9~ h (3)
where ~ is the density of the metal and g is the acceler-
ation of gravity. The magnetic field required to con-
tain the ferrostatic pressure can be found by equating
the magnetic and ferrostatic pressure,
B = (2~ o g ~ h)l/2 = khl/2 (4
For casting steel k is approximately 4~0 if h is measured
in cm and B in gauss.
The roller induced pressure Pr depends on the pro-
perties of the metal being cast, the roller diameter
and speed and the thickness of the metal strip or sheet
being cast. In case of steel sheets, it is estimated
that Pr can be many times larger than the hydrostatic
pressure Ph-
The frequency of the alternating magnetic fieldchosen is as low as practicable consistent with the
WO91/18696 PCT/US90/03~3
~Q distance between the rollers and the distance between
the end of the rollers, typically between 39 Hz and
16,000 Hz.
Figure la depicts a cross sectional view of the
roller casting arrangement of the present invention. A
pair of rollers lOa and lOb (referred to collectively as
rollers 10) are parallel and adjacent to each other and
lie in a horizontal plane so that a molten metal 12 can
be contained between them above the point where the
lo rollers are closest together. Rollers 10 are separated
by a gap, d (shown in Figure 2). Counter rotation of
rollers lOa and lOb (in the direction shown by the
arrows lla and llb), operating with gravity, forces the
molten metal 12 to flow through the gap d between the
rollers 10 and out the bottom.
Magnetic poles 16a and 16b located on both sides of
the gap d between rollers lOa and lOb generate an
alternating magnetic field which exerts an
electromagnetic inward force that prevents the molten
liquid 12 from flowing out the sides at the edges of the
rollers lOa and lOb. Throughout this application
references will be made to confinement at one end of a
pair of rollers. It should be understood that
confinement of molten metal between a pair of counter
rotating rollers as provided by the present invention
will be used at both ends of the pair of rollers.
SU~STIT~ITE SHE~T
WO91/1~96 ~ 0 ~ 4 ~ ~ ~ PCT/US90/03243
-- 11 --
Rollers 10 include a cooling means to cool and
thereby solidify the molten metal by conduction as it
passes between rollers 10. Referring to Figure lb, the
cooling means may comprise a plurality of circulating
water-cooled channels 13 located inside the surface
wall of the roller. Referring again to Figure la,
after emerging from rollers 10, the metal has solidified
into a sheet 18 hav~ng a thickness equal to the gap, d,
between the rollers 10. Jets 22 located below the
10 rollers further cool the cast metal sheet by spraying a
coolant (such as water or air) on it. The cast metal
sheet is guided, supported and carried away from the
rollers by mechanical guides 23.
Referring to Figure 2, there is depicted a hori-
zontal sectional view of the invention along section
line 2-2' of Figure la. Figure 2 depicts the arrange-
ment of magnetic poles with respect to the rollers.
Rollers lOa and lOb are separated by a gap, d, through
which the metal being cast 18 can pass. Magnet 24 is
20 comprised of a yoke 26 and poles 16a and 16b. Coils
28a and 28b wind around the magnet. Coils 28a and 28b
carry an electric current supplied by an alternating
current source thereby magnetizing the magnet 24 and
inducing a magnetic field between poles 16a and 16b.
The major portions of magnetic poles 16a and 16b are
WO91/1~96 PCT/US90/03243
- 12 -
.,
located inside the outer edges 30a and 30b of the
rollers. The magnetic pol~s 16a and 16b are stationary
and radially separated from the rollers lOa and lOb by
a space clearance large enough to allow free rotat~on
of the rollers 10. The poles 16 extend axially into
the ends of the rollers 10 a short d~stance.
The cylindr~cal surfaces of rollers 10 have a middle
middle portion 32 which comes in contact with the molten
metal. The middle portions 32 are constructed of a
materlal which has high thermal conductivity so that a
cooling means, used in conjunction with the rollers,
can remove heat from the molten metal thereby facilit-
ating the casting process. In the present embodiment,
the cooling means used in conjunction with the rollers
comprises water cooled channels 13 in the interior of
rollers 10 as shown in Figure lb. In this embodiment,
the middle portions 32 of rollers 10 are made of copper
alloy.
The rollers 10 also have outer rims 34a and 34b
which form extensions of middle portions 32 of rollers
10. Rims 34 are located in the area between the mag-
netic poles 16. Poles 16 generate a magnetic field
that penetrates through the rims 34 of rollers 10 in
this embodiment. Therefore, for this embodiment rims
34 must be made of a material suitable for the transmis-
WO91/18696 2 ~ PCT/US90/03243
- 13 -
s~on of a magnetic field. In this embodiment of the
present invention, the rims are made of stainless
steel.
The resistivity of stainless steel (approximately
75 micro-ohm-cm at room temperature) matches reasonably
the resistivity of molten steel ~approximately 140
micro-ohm-cm); therefore, the horizontal magnetic flux
can penetrate both metals. ~ue to eddy currents in the
molten metal, the field decays exponentially as the
axial distance, z, from the edge of the pool increases.
Therefore, a magnet force Fl at the pool edge is
larger than the oppositely directed force F2 further
into the pool, as shown in Figure 3, resulting in a net
containing force F
F = 1~ oJzl 3 ~B dz. (5)
As a result, the molten metal can be contained between
the rollers.
Referring again to Figure 2, the edges 30 of
rollers 10 are curved and tapered on their interior
portions to accomodate the magnetic poles 16. Likewise
poles 16 generally conform in shape to the exterior
portion of the rollers 10. Shield 33 encloses yoke
26 and portions of poles 16 except ~or the pole ends.
Yoke 26 may be made of a laminated core. Shield 33
encloses the core 26 without forming an electrically
WO91/18696 PCT/US90/03~3
- 14 -
ili:9
shorted turn as ~llustrated by Figure 4. The shield
33 may be formed by two U-channels 33a and 33b made from
copper sheets and insulated from each other by at least
one gap 35. Shield 33 should be made of a material
with low resistivity to prevent transmission of a mag-
netic field by means of eddy current shielding and
thereby serve to reduce flux leakage, enhance shaping
the magnetic field and improve circuit efficiency.
Shield 33 may also serve as a heat shield for the mag-
net and may be water cooled for th~s purpose. A materialwith low resistivity and high thermal conductivity,
such as copper or copper alloy, is ideal for use as
shield 33.
Referring to Figure 3, there is depicted a hor$-
zontal cross section of the present invention as viewed
along section line 3-3' of Figure la. Figure 3 depicts
a section between the rollers at a point dispiaced
vertically from the horizontal axes of the rollers 10.
Figure 3 shows containment of the molten metal 12 by
the rollers 10 and the interaction of magnetic field,
B, and eddy currents i. Figure 3 depicts rollers 10
having middle portion 32 and rims 34. Also shown in
Figure 3 is the magnet 24 having a yoke 26, poles 16
coil 28 and shield 33.
Figure 3 also depicts molten metal 12 retained
between the ends of rollers 10 by the magnet field, B
WO9l/18696 2 ~ PCT/US90/03243
-- 15 --
(shown as the dashed lines), between poles 16. The
magnetic field, 8, causes eddy currents, ~, in the
molten metal, indicated by arrow heads out oE the page
and arrow ta$1s into the page, and a resultant electro-
magnetic force, F, directed toward the interior of the
pool to contain the molten metal. The containment
forces, F, are due to the interaction of the horizontal
field, B, with the eddy currents, i, in the molten
metal, induced in the molten metal by the magnetic
f~eld, B.
In the present invention, a number of different
magnet and coil geometries can be employed to adapt to
particular requirements of the casting process. Figure
5 is an perspective view of the magnet 24 and coil 28
as depicted in Figures 1-4. The magnet has laminated
yoke 26 and po es 16a and 16b. The poles 16 are arced
in shape and may conform to the shape of the interior
portions of rollers 10. The coil compr.ses a coil
pair 28a and 28b which encircle laminated core portions
40a and 40b of magnet 24. Coils 28 are connected to an
alternating current supply 36 which prov-des an alter-
na-ting current, Is, which energizes the magnet 24. The
pair of coils may be connected in ser~es to the current
source or in parallel depending upon design considerations.
For simplicity, the eddy current shield around the
magnet ~s not shown.
WO91/18696 PCT/US90/03~3
~ - 16 -
~,
Another embodiment of the magnet and coil is
depicted in Figure 6. In this embodiment, the magnet
42 has a square shaped core 44 connecting poles 46a and
46b. Poles 46a and 46b in this embodiment have shaped
pole faces 48a and 48b but squared off backs 50 to con-
form to the square shape of the core 44. As illustrated
by the cutaway view of pole 46b, an insulated copper
shield 51 encloses the core to reduce leakaqe flux. A
gap 52 in the shield 51 prevents the shield from being a
shorted turn around the magnet core. Coil 60 encircles
core 44 and shield 51. In this embodiment, the coil 60
is a single layer coil instead of a coil pair as in the
previous embodiment. Coil 60 is connected to a alter-
nating current supply 36 which provides an alternating
current, Is~ whtch energizes the magnet 42. The leakage
flux could be reduced further by also enclosing the
coil 60 with a copper shield 53a and 53b as depicted in
Figure 7. This additional shield 53a and 53b would
reduce the cross sectional area available in the air
space for the leakage flux around the coil windings and
thereby reduce such leakage flux. In still another
embodiment, the inner shield 51 could be deleted and
the core and coil assembly enclosed by only an outer
shield 53. --
Figure 8 depicts another varia_ion of the magnet
used in the present invention. In this embodiment,
WO91/18696 2 ~ 8 ~ 2 ~ ~ PCT/US90/03~3
- 17 -
magnet 54 has a generally truncated trapezoidal shaped
core with rectangular flat arms 55 connecting the
trapezoidal yoke 56 to the poles 57a and 57b. Similar
to the magnet design in Figure 5, this magnet may have
the advantage of being simpler to construct.
A further modification to the magnet is depicted in
Figure 9. In Figure 9, a molten liquid 12 is being cast
into a sheet 18 between rollers lO. As in the previous
embodiments, magnet poles 59a and 59b confine the molten
metal at the edges of the rollers lO. In this
embodiment, the magnetic poles 59 are adjustable in
position. The poles 59a and 59b can be slanted and
moved to be closer to or further away from the roller
rims. This feature enables adjustment of the magnetic
field. As depicted in Figure 9, the upper parts of the
poles 59 have been moved further away from the roller
rims as compared to the bottom part of the poles. As
shown by the dashed lines representing the magnetic
field B in Figure 9, with the top ends of poles 59
further apart, the magnetic field can be made relatively
stronger near the lower end and weaker at the higher end
as compared to the pole configuration shown in Figure
la. This adjustability can be utilized for casting
metal sheets of different thickness where different
forces of confinement may be necessary.
SUB~ ~ EET
WO9l/18696 PCT/US90/03243
- 18 -
Figure 10 shows still another variation of the
G~
magnet in the present invention. This variation offers
the most fle~ibility of any of the designs shown so
far. (Figure 10 depicts just one magnet pole; it should
be understood that 'an identical pole would be positioned
opposite this pole in the other roller.)
In Figure 10, each magnet pole is divided into
three discreet separate magnetic elements 61a, 61b, and
61c. Each of these elements is an independent magnet
comprising cores 62, excitation coils 63, and eddy-current-
shields 33, which enclose their respective coils and cores,
except for an air gap which prevents the shields f rom
becoming a shorted turn such as dep-cted in Figures 4 or
7. Magnetic element 61a contains the upper portion of
the sidewall of the molten metal pool 12, element 61b
contains the center of the pool sidewail and element
61c contains the lower portion of the pool sidewall.
In this embod~ment, each individual discreet mag-
netic element is individually controlled and provided
with individual currents, Isa, Isb, and IsC~ These
three magnetic elements may be energized from a single
alternating current power source 64 or from three
ind~vidual power sources. W~th a single power source,
two variable reactors would be connected in series with
the coils of two of the three magnetic elements in
order that the magnetic f~elds of the three magnetic
WO91~18696 ~ PCT/US90/03243
-- 19 --
elements can be adjusted independently; the time
constant (L/R) of the reactors is designed to be the
same as the time constant of the magnets in order that
the flux generated by the three independent magnets is
in phase. With three independent power sources, care
must be taken that the three sources have the correct
phase relation. Because each element can be
individually adjusted there is provided a high degree of
adjustability for the total magnetic field as well.
lo This adjustability can be utilized to optimize operation
under varying conditions, such as with different sheet
thicknesses, different molten metals or alloys,
different temperature conditions, start-up and shut-down.
Feedback loops can utilize sensors 65 to monitor the
position of the upper, middle and lower portions of the
electromagnetically contained sidewall. Any deviation
from a present position will produce an error signal
which, after suitable amplification, will change the
power supplied to the respective magnetic elements in
order to restore the preset containment position of the
respective sidewall portion. These sensors may take the
form of discreet beams (rays) that are transmitted
parallel to the sidewall from one side and detected by a
receiver on the other side (the beam being interrupted
when the sidewall moves closer to the magnet).
Alternately, the sensors may take the form of
SUEtSTITU ~ E ~- ET
WO 91/18696 ~ PCT/US90/03243
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'~
discreet beams that are transmitted normal to the side-
wall and their reflection from the surface of the side-
wall being detected by a receiver and used to determine
the posltion of the sidewall. The sensors ~ay take the
form of variable capacitors where the monitored sidewall
portion is one electrode of the capacitor and the other
is a suitable electrode mounted a fixed distance and in
parallel to the sidewall. In a still further alternative,
the sensor may take the form of an impedance measurement
of the magnet excitation which changes with the flux
linkage between the magnet and the liquid metal of the
respective sidewall portion.
A st~ll further embodiment of the magnet desisn is
depicted on Figure ll. Figure ll depicts a horizontal
sectional view of one end of one roller pair. In this
embodiment the pole assemblies 66a and 66b are hoop-
shaped and contained inside and attached to the rollers
lOa and lOb behind rims 34a and 34b, respectively.
Accordingly, poles 66 will rotate with rims 34 and
rollers lO. Port~on 68 of shield 69 is located between
core sections 72a and 72b and close to the area where
the casting takes place. Poles 66a and 66b are circular
and made of a ferromagnetic material. The coil 60
magnetizes yoke 70 and magnet a-rms 72a and 72b as in
the previous embodiments. Eddy current shields 69 and
79 confine the magnetic flux to the yoke 70, magnet
WO91/18696 ~ ~ ~ 4 ~ ~ ~ PCT/US90/03243
- 21 -
arms 72 and poles 66 (reducing leakage flux) as described
earlier. Shields 69 and 79 may also incorporate heat
shielding or cooling means to protect the coil or the
magnet. Poles 66a and 66b though separated from magnet
arms 72a and 72b and rotating with rollers lOa and lOb,
are magnetized by the~r close proximity to arms 72a and
72b via relatively small gaps 74a and 74b. This embodi-
ment has the advantage that the poles can be located as
close together as physically possible, i.e. inside the
rims. This design simplifies the shape of the magnet
yoke and permits the use of different magnet yokes and
coils when the assembly of rollers 10 and poles 66 is
used to cast different thicknesses of metal sheets.
Casting sheets, i.e. 0.4~ thick would utilize a more
powerful magnet assembly than casting 0.04~ thick metal
sheets.
As described previously and shown in ~ sures 2, 3,
and 11, the magnetic f~eld penetrates through the outer
rim portion of the rollers to confine the molten metal.
The present invention can also be practiced without a
special rim portion provided a suitable material is
used for the rollers, such as a ceramic, which enables
penetration by a magnetic field without generating eddy
- currents in the roller. However, in the preferred
embodiment, use of a r~m portion on the rollers provides
WO91/18696 PCT/US90/03~3
- 22 -
for shaping the maqnetic fie]d by establishing a well
defined transition from the area of a high magnetic flux
near the edge of the roller to an area of low magnetic
flux further away from the roller's edge. Shaping the
magnetic field in this manner provides the advantages of
better control of the magnetic field that contains the
sidewall of the molten pool of metal.
The present invention provides for shaping the
magnetic field by using a material with a low
resistivity, such as copper or copper alloy, for the
main portion of the roller and a material with a higher
resistivity for the rim portion. The copper or copper
alloy used for the main portion will effectively prevent
penetration of the magnetic field (except for a small
negligible skin layer on the surface) and will, at the
same time, cool the molten metal efficiently causing it
to solidify.
In the rim portion of the roller, it is essential to
allow penetration of the magnetic field to confine the
sidewall of the molten metal between the two roller
surfaces. The present invention includes several
different embodiments of the rim portion designed to
allow penetration of the magnetic field. In one
embodiment, this is accomplished by connecting a rim
made of a material with a much higher resistivity, such
as stainless steel, to the edges of the copper rollers.
SUE~STITUTE SHEET
WO91/18696 - 23 - $ ~ ;~i ~ PCT/US90/03~3
, ,
Figures 2, 3 and 11 depict stainless steel rims 34 of
this type. The stainless steel rims may be connected to
the copper rollers by brazing, bolting or other suitable
methods. In addition to allowing penetration of the
magnetic field, the stainless steel rims provide a
smooth surface for the casting surface in case the
molten metal encroaches on the rim.
Another embodiment of the rim portion is depicted in
Figures 12a and 12b. The roller 80 is made of a low
o resistivity material such as copper. At the edges
around the circumference of the rollers are a plurality
of slots 82 all the way through the roller. The slots
82 extend a short distance, s, in the axial direction of
the roller. The slots 82 permit the magnetic flux in
the edge portion, or rims of the rollers, defined by the
slots. Although the slots can be left empty, it is
preferred that the slots be filled with a material of
relatively high resistivity such as ceramic or stainless
steel, which is insulated from the sides of the slots,
or filled with a material of high magnetic
permeability. Alternatively, the slot can be filled
with laminations of high permeability metal which are
insulated from each other and from the sides of the
slots. Leaving the slots empty would require that the
magnetic field is shaped such that the molten metal is
kept away from the slots at all times. Filling the
slots provides a smooth surface in case the molten metal
encroaches on part of the rim during the casting
process. Slot dimensions can be determined based upon
the application. An advantage of the slotted copper
SUB~i 111 ~JTg; SHEET
WO9l/1~96 PCT/US90/03~3
24 -
rim design is that it features a low reluctance path for
the magnetic flux, i.e. the slots, filled with highly
permeable material or with air, thereby enabling a high
frequency alternating magnetic field. For example,
whereas the roller design with stainless steel rims can
operate at relatively low frequencies, e.g. up to 500
Hz, the roller design with slotted rims can operate with
a much wider frequency range, e.g. up to at least 16 kHz.
Other embodiments of the rim portion are shown in
Figures 13a and 13b. Figure 13b is a horizontal cross
section along line 13b-13b~ of figure 10. The
water-cooled rollers 10 are made of high thermal
conductivity material such as copper. At the edges and
around the circumference of the rollers are one or more
hoop-shaped extensions 91 of rollers 10. Arranged
between these hoop-shaped extensions 91 are similar
hoop-shaped members 92 made of copper. These hoops, 91
and 92, are insulated from each other and mounted to the
rollers 10 with bolts 93. The bolts 93 are insulated
from the hoops to prevent electrical contact between the
individual hoops and between the hoops and the roller.
The hoop-shaped extensions 91 serve the same purpose as
the slots 82 in the previous embodiment, i.e. to
transmit the magnetic field to the confinement region.
Extensions 91 can be made of similar materials as slots
82. Extensions 91 can be
SUB~ ~ JTE S~IEET
WO91/1~96 2 Q ~ 4 2 ~ 6 ~ ; PCT/US90/03~3
- 25
made of an ~nsulating material, such as ceram~c, having
a high resist~vity and relatively low permeability and,
therefore, no eddy currents. Extensions 91 can be made
of a non-magnetic, high res~stiYely metal, such as
stainless steel, wh~ch also has relat~vely low perme-
ab~1ity, but has h~gher thermal conductivity than
ceramic. Alternately, extensions 91 can be made of a
magnetic material, such as silicon steel, which has
high magnetic permeabiltty and reasonable thermal
conductivity. With a high permeability material the
hoop-shaped extensions themselves become magnetized.
Thin insulated laminations of a ferromagnetic material
could be used. W~th hoop-shaped extens~ons of stainless
steel or ferromagnetic material, each hoop should be
insulated from adjacent copper hoops. The alternating
flux emanating from the magnet pole penetrates the
roller through the hoops 91 and through the skin depth
of the copper hoops 92. A portion of this flux ~nduces
eddy currents in the molten metal 12 between the rollers.
The interaction between the flux and the eddy currents
in the molten metal contains the sidewall of the molten
metal pool between the rollers as described before.
The thickness of the hoop-shaped extensions 91, the
- number of hoop-shaped extensions, the hoop-shaped
extension material, and the magnet are designed to
contain the sidewalls of the molten pool between the
WO91/18696 PCT/US90/03243
- 26 -
rollers. W~th the hoop-shaped extension made from
highly permeable magnetic material, the electromagnetic
conta~nment circult is most efficient. ~n this case
the reluctance of the magnetic circuit is mainly deter-
mined by the reluctance of the molten metal 12 and by
the small air gap, 94, between hoops 91 and magnet
pole 61c; all other designs have much larger air gaps and
resultant larger leakage flux.
Another embodiment of this invention is shown in
Figure 14. This embodiment of the invention may be
used where conditions are such that the edge of the
cast metal sheet is not fully solidified by the time it
exits from between the rollers. This condition may
occur for a number of reasons dictated by the casting
process, such as the need for high magnetic fields of
relatively high frequency resulting in large eddy
current heating of the edges of the metal being cast,
insufficient cooling effect of the rollers near the
edges, thick cast sheet dimensions, or a combination of
these or other factors. Figure 14 depicts the rollers
10 and molten metal 12 a5 in previous embodiments.
Figure 14 also shows poles 95a and 95b which extend
below the center line of rollers 10. Th~s has the
effect of also extending the magnetic field below the
center l~ne of the rollers thereby extending the e~ec-
tromagnetic containment of the edges.
WO9l/18696 PCT/US90/03~3
aos42~
- 27 -
Wheel induced forces on the liquid edge of the metal
sheet vanish when the sheet leaves the rollers. Only
gravitational forces act on the still molten edges which
may be cooled by gas flow or by water spray. The
magnetic forces between poles 95 decrease as the sheet
moves further from the rollers; this is compatible with
the solidifying edge as the sheet moves down. However,
if the edge of the sheet is not quite solid near the end
of the magnetic field between poles 95, further
confinement of the still molten edges of the sheet can
be provided by supplemental magnet with poles 96a and
96b which extend the magnetic field well below rollers
10 until the metal sheet is sufficiently hard enough to
be supported by mechanical guides 23.
Another embodiment of this invention is depicted in
Figures 15a and 15b. This embodiment presents a
combination of a magnetic and mechanical means to
contain a molten metal at the edges of a roller casting
system. As mentioned above, the problem of using
mechanical seals to contain a molten metal at the edges
of counter-rotating casting rollers was that the mi~ture
of the molten and solidifying metal in combination with
the rotation of the rollers would clog up around the
mechanical seals. As described above, the present
invention shows how a magnetic field can be used to
contain the sidewalls of the molten metal. The present
SUBSTITUTE S~
WO91/18696 PCT/US90/03~3
- 28 -
embodiment uses both a mechanical seal and a magnet~'c
field to advantage. As in previous embodiments, rolLers
10 and poles 16 contaln a molten metal 12. The present
embodiment also includes a mechanical dam 100 positioned
between poles 16a and 16b. Mechanical dam 100 is
shaped so that it will contain the molten metal in that
area wbere there is lit~le likelihood of clogging or
deforming the cast sheet, i.e. away from the solidifying
effects of the rollers. As depicted in Figures 15a and
lSb, mechanical dam 100 is spaced away from rollers 10.
It is in the areas close to rollers 10 that the metal
is solidifying and where the likelihood o~ clogging is
greatest. Magnetic confinement with the poles 16 is
used to confine the molten and solidifying metal in the
gaps between the mechanical dam 100 and rollers 10.
Mechanical dam 100 may be made of a ferromagnetic
material 101 so that it provides a low reluctance path
for the flux between the poles 16. The side of the dam
facing the molten metal pool may be made of a layer of
high temperature ceramic 102 covering a water cooled heat
shield 103 in front of the high permeability material
which may be made from steel laminations or from high
temperature ferrite. This embodiment has the advantage
of requiring less energy than the previous embodiments
because the magnetic field along the molten metal
extends only over the gaps between the rollers 16 and
WO91/18696 2 ~ ~ 4 2 ~ ~ PCT/US90/03243
~_ - 29 -
mechanical dam 100. Also, because the volume of the
molten metal contained by the magnetic field is sma;ler,
there is less heating of the molten metal due to eddy
currents. Various mechanical dam shapes can be designed
for shaping flux density suitable for different castinq
requirements.
