U.S. patent application number 11/264212 was filed with the patent office on 2007-05-03 for method and apparatus for electromagnetic confinement of molten metal in horizontal casting systems.
Invention is credited to David Wayne Timmons, David A. JR. Tomes, Ali Unal, Gavin Frederick Wyatt-Mair.
Application Number | 20070095499 11/264212 |
Document ID | / |
Family ID | 37909403 |
Filed Date | 2007-05-03 |
United States Patent
Application |
20070095499 |
Kind Code |
A1 |
Tomes; David A. JR. ; et
al. |
May 3, 2007 |
Method and apparatus for electromagnetic confinement of molten
metal in horizontal casting systems
Abstract
The present invention provides an apparatus for strip casting of
molten metal including a pair of casting rollers adapted to receive
molten metal along a horizontal axis, wherein a vertical distance
separating the pair of casting rollers defines a molding zone; and
an electromagnetic edge containment apparatus positioned on each
side of the molding zone having an induction coil wound about a
portion of a magnetic member to generate magnetic lines of force
upon application of a current, wherein the poles of the magnetic
member are positioned distal from to aligned to the planar sidewall
of the casting rollers and the current provides magnetic lines of
force perpendicular to said horizontal axis that contain the molten
metal in contact to the casting rollers without substantially
increasing the temperature of the molten metal.
Inventors: |
Tomes; David A. JR.;
(Sparks, NV) ; Unal; Ali; (Export, PA) ;
Wyatt-Mair; Gavin Frederick; (Lafayette, CA) ;
Timmons; David Wayne; (Reno, NV) |
Correspondence
Address: |
INTELLECTUAL PROPERTY
ALCOA TECHNICAL CENTER, BUILDING C
100 TECHNICAL DRIVE
ALCOA CENTER
PA
15069-0001
US
|
Family ID: |
37909403 |
Appl. No.: |
11/264212 |
Filed: |
November 1, 2005 |
Current U.S.
Class: |
164/428 ;
164/503 |
Current CPC
Class: |
B22D 11/0605 20130101;
B22D 11/0622 20130101; B22D 11/0662 20130101 |
Class at
Publication: |
164/428 ;
164/503 |
International
Class: |
B22D 11/06 20060101
B22D011/06; B22D 27/02 20060101 B22D027/02 |
Claims
1. An apparatus for strip casting of molten metal comprising: (a) a
pair of casting rollers adapted to receive molten metal along a
horizontal axis, wherein a vertical distance separating the pair of
casting rollers defines a molding zone; (b) an electromagnetic edge
containment apparatus positioned on each side of the molding zone,
comprising an induction coil wound about a portion of a magnetic
member to generate magnetic lines of force upon application of a
current, wherein said magnetic member comprises a first and second
pole positioned distal from and aligned with a sidewall of said
pair of casting rollers and the current provides magnetic lines of
force perpendicular to said horizontal axis that contain the molten
metal in contact with the casting rollers with substantially no
increase in temperature to the molten metal; and (c) a means for
supplying the molten metal to the molding zone along said
horizontal axis from a tundish, while ensuring said molten metal
remains substantially non-oxidized, wherein the tundish is
separated from the molding zone by a distance to substantially
eliminate wave generation within the tundish by the magnetic lines
of force.
2. The apparatus of claim 1 wherein said current comprises an
alternating current having a frequency ranging from 40 Hz to 10,000
Hz.
3. The apparatus of claim 1 wherein said current comprises less
than 2,000 amp/turns.
4. The apparatus of claim 1 which includes shield means positioned
about the magnetic member.
5. The apparatus of claim 1, wherein the magnetic member has a
generally C-shaped configuration, including a core portion and
parallel poles integral with and extending therefrom.
6. The apparatus of claim 5, wherein the induction coil is wound
about the core of the magnetic member, in which the induction coil
is coiled from 1 to 100 times around the magnetic member.
7. The apparatus of claim 1, wherein the vertical distance
separating the pair of casting rollers provides a metal head height
that allows for containment of the molten metal between the casting
rollers by the magnetic lines of force at said current without a
substantial increase in temperature of the molten metal resulting
from the magnetic lines of force.
8. The apparatus of claim 1, wherein the vertical distance
separating the pair of casting rollers is less than 1.0''.
9. The apparatus of claim 1, wherein the magnetic member is
positioned to the molding zone to position the magnetic lines of
force to produce a convex sidewall, a concave sidewall, or a
substantially flat sidewall to the molten metal within the molding
zone.
10. The apparatus of claim 1, wherein the magnetic member is formed
of a ferromagnetic material from a stack of bonded or mechanically
linked laminates or the magnetic member is formed from a solid core
of ferromagnetic material.
11. The apparatus of claim 1, wherein said pair of casting rollers
comprises a ferromagnetic material, non-ferromagnetic material, or
a non-ferromagnetic material that is at least coated with a
ferromagnetic material on at least casting surfaces and said
sidewalls of said pair of casting rollers.
12. The apparatus of claim 1, wherein said sidewall of said pair of
casting rollers is substantially planar.
13. An apparatus for strip casting of molten metal comprising: (a)
a pair of opposing endless metal belts, each of the pair of
opposing endless metal belts passing over a roller and having a
periphery substantially aligned to a sidewall of the roller, said
each of said opposing endless metal belts having a surface for
accepting molten metal, wherein a vertical dimension separation the
pair of opposing endless metal belts defines a molding zone; (b) an
electromagnetic edge containment apparatus positioned on each side
of the molding zone comprising an induction coil wound about a
portion of a magnetic member to generate magnetic lines of force
upon application of a current, wherein the current provides
magnetic lines of force that contain the molten metal within a
width and in contact to at least a portion of said pair of opposing
endless metal belts with substantially no increase in temperature
to the molten metal; and (c) a means for supplying said molten
metal to the molding zone along a horizontal axis from a tundish,
the tundish separated from said molding zone by a distance to
substantially eliminate wave generation within the tundish by the
magnetic lines of force.
14. The apparatus of claim 13, wherein the magnetic member
comprises an upper pole and a lower pole, the induction coil wound
about a portion of the magnetic member to generate magnetic lines
of force passing from one of the upper and lower poles to the
other, with the magnetic member being positioned such that the
upper and lower poles direct magnetic lines of force establish
containment forces at the edges of the pair of opposing endless
metal belts to contain the molten metal therebetween.
15. The apparatus of claim 13, wherein the vertical distance
separating the pair of opposing endless metal belts provides a
metal head height that allows for containment of the molten metal
between the pair of opposing endless metal belts by the magnetic
lines of force at said current without a substantially increase in
temperature of the molten metal resulting from the magnetic lines
of force.
16. The apparatus of claim 13, wherein the minimum vertical
distance separating the pair of opposing endless metal belts, at
the nip of the caster, ranges from about 0.025'' to 0.25''.
17. The apparatus of claim 13, wherein the magnetic member is
positioned to the molding zone to position the magnetic lines of
force to produce a convex sidewall, concave sidewall or
substantially flat sidewall to the molten metal within the molding
zone.
18. A cast metal strip comprising: a first shell; a second shell;
and a central portion between said first shell and said second
shell, said central portion comprising grains having an equiaxed
structure, wherein said cast metal strip has sidewall edges being
substantially uniform.
19. The cast metal strip of claim 18 wherein said first shell is an
upper shell and said second shell is a lower shell.
20. The cast metal strip of claim 18, wherein said cast metal strip
may be rolled without machining said sidewall edges.
21. The cast metal strip of claim 18 comprising aluminum and other
light metals such as magnesium and zinc.
22. The cast metal strip of claim 18, wherein said equiaxed
structure is substantially globular.
23. A casting apparatus comprising: (a) a pair of casting rollers
adapted to receive molten metal along a horizontal axis, wherein a
vertical distance separating the pair of casting rollers defines a
molding zone; (b) a tip delivery structure positioned to supply the
molten metal to the molding zone along said horizontal axis from a
tundish while ensuring said molten metal remains substantially
non-oxidized; and (c) an edge containment apparatus positioned on
each side of the molding zone, said edge containment apparatus
comprising: a mechanical edge dam positioned overlying at least an
end portion of said tip delivery structure and partially extending
towards said molding zone, and an electromagnetic edge dam
comprises a first and second magnetic pole positioned distal from
and aligned to a sidewall of said pair of casting rollers and
overlying a portion of said mechanical edge dam partially extending
towards said molding zone, wherein said electromagnetic edge dam
provides magnetic lines of force perpendicular to said horizontal
axis that contain the molten metal in contact to the casting
rollers.
24. The casting apparatus of claim 23 wherein said tip delivery
structure has a length that substantially eliminates wave
generation within the tundish by the magnetic lines of force.
25. The casting apparatus of claim 24 wherein said electromagnetic
edge dam comprises an induction coil wound about a magnetic member
to generate magnetic lines of force upon application of a
current.
26. The casting apparatus of claim 25 wherein said current provides
magnetic lines of force that contain the molten metal in contact to
the casting rollers with substantially no increase in temperature
to the molten metal.
27. A method of forming a cast metal strip comprising providing
molten metal to a molding zone along a horizontal axis; containing
said molten metal within said molding zone with a magnetic
containment means; and casting said molten metal into a cast metal
strip, wherein sidewall geometry of said cast metal strip is
configured by adjusting said magnetic containment means.
28. The method of claim 27 wherein said sidewall geometry is flat
or is concave or convex relative to a centerline portion of said
cast metal strip.
29. The method of claim 28 wherein said magnetic containment means
comprises an induction coil wound about a magnetic member to
generate magnetic lines of force upon application of a current,
said magnetic member having a first and second magnetic pole
positioned distal from to adjacent to said molding zone.
30. The method of claim 29 wherein said adjusting said magnetic
containment means comprises increasing or decreasing said current
through said induction coil.
31. The method of claim 29 wherein said adjusting said magnetic
containment means comprises moving said first and second magnetic
poles adjacent to or distal from said molding zone.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to the continuous casting of
metal strip, and more particularly, to the electromagnetic
confinement of molten metal in a continuous casting system.
BACKGROUND OF THE INVENTION
[0002] Continuous casting of metals is performed in twin-roll
casters and belt casters or combinations thereof. Methods are
available for casting both in the horizontal and in the vertical
direction. In particular, the steel industry has recently developed
high speed twin roll strip casters which operate in the vertically
down direction.
[0003] Up to the present, the mechanical edge dams have been
employed to provide containment of the molten metal in the casting
zone. Such devices have included the caterpillar type edge dams
that move with the strip (as in the Hazelett casters) or fixed edge
dams that are pressed against the surface of the rolls. The latter
is used in the twin-roll steel strip casting industry. Such fixed
mechanical edge dams have a short service life as they get eroded
by contact with the cold sidewall of the rolls. In addition, such
mechanical edge dams provide sites for the formation of skulls that
have a tendency to be sheared off and thus enter the cast strip to
render the microstructure metallurgically undesirable. Caterpillar
edge dams, while well proven for the thicker slab castings (10-25
mm thick), become impractical for thin strip casters or twin drum
casters of the steel industry where the cross section to be
contained changes sharply along the casting zone.
[0004] Electromagnetic edge dams have been employed in the prior
art in the strip casting of metals in vertical twin drum (roller)
casting systems. Electromagnetic edge dams of a magnetic system
type use a combination of a magnet assembly and an AC coil to
generate confinement forces. Electromagnetic edge dams of an
induction system type rely solely on an AC coil to generate the
containment forces.
[0005] The magnetic system electromagnetic edge dams use a magnetic
member which comprises a yoke or core connecting two pole faces
disposed on either side of the gap on which the molten metal is to
be confined. The magnetic member is made of a ferromagnetic
material and is surrounded over a given length of the yoke by a
coil carrying an AC current. The magnetic flux generated by the
flow of the current into the coil is transmitted to the poles of
the magnet through the yoke and establishes containment forces at
the metal surface in the gap.
[0006] Typically, in magnetic systems, part of the magnetic member
is covered with an electrically conductive shield to minimize
leakage of flux in a direction away from the gap. Such magnetic
confinement systems have the advantage that the confinement current
need not be as high as compared to those systems using solely an
induction coil. If a stronger magnetic field is required, it can be
achieved with the same current level by reducing the area of the
pole faces to concentrate the field. However, such systems are not
without disadvantages. For example, such systems typically have
poor operating efficiency resulting from core losses and losses due
to magnetic hysterisis when an alternating magnetic field is
applied to the magnetic material. Additionally, high temperatures
are typically generated that need to be dissipated by cooling in
order to prevent damage to the magnetic system.
[0007] Induction confinement systems typically employ a shaped
inductor positioned close to the gap in which the molten metal is
to be contained. The AC current flowing in the inductor generates
induced currents as well as a time-varying magnetic field on the
surface of the molten metal to be contained. The interaction
between the current and the magnetic field provide containment
forces. To improve efficiency, a magnetic member is built around
the inductor to focus the current to the inductor surface facing
the molten metal. Induction coil systems are generally simpler in
design than magnetic systems. However, induction systems are
disadvantageously limited in terms of the maximum metallostatic
head that can be contained by the system. The maximum metallostatic
head that can be supported in induction coil systems is limited,
because induction coil systems require very high inductor currents
to provide adequate containment forces, wherein such high currents
are accompanied by increased heat generation, which in turn hinders
or slows the solidification process during casting.
[0008] Referring to FIG. 1, in vertical twin roll casters, the
molten metal head against which containment must be provided tends
to be very high. For typical operating condition, the metal head
height H.sub.1 is about 65% the radius of the casting rolls.
Therefore, electromagnetic edge dam apparatus used in vertical twin
roll casters must provide a magnetic field strong enough to contain
a metal pool having a head height H.sub.1 that is 65% the radius of
the casting rolls. Such electromagnetic edge dams have not been
successfully commercialized for two reasons. First, the high
current required to contain the molten metal pool creates standing
waves on the top surface of the metal pool that are too large in
magnitude for the casting process. Second, the large
electromagnetic forces needed to contain the molten metal head
formed atop vertical roller caster systems create induction heating
on the metal pool's sidewall, which interferes with the
solidification process.
[0009] U.S. Pat. No. 4,936,374 describes a vertical casting system
and electromagnetic confinement apparatus having the disadvantages
described above. Further, U.S. Pat. No. 4,936,374 describes casting
rollers having a rim portion, in which the containment magnetic
field is conducted through the rim portion of the casting roll. In
addition to induction heating and wave generation, the rim portions
of the casting rolls disclosed in U.S. Pat. No. 4,936,374 produce a
ridge in the cast product and therefore fail to provide a casting
strip having uniform sidewalls (edges). The ridge formed in the
casting strip produced using the apparatus and method disclosed in
U.S. Pat. No. 4,936,374 must be machined prior to rolling of the
casting strip. Additional machining disadvantageously adds to the
cost of the production.
[0010] Accordingly, a need remains for a method of high-speed
continuous casting of metals and alloys, which achieves uniformity
in the cast strip surface, provides good molten metal containment
in the casting zone, and results in strip edges which can be rolled
without needing to be machined by trimming.
SUMMARY OF THE INVENTION
[0011] The present invention overcomes the above-described
obstacles and disadvantages by providing an electromagnetic
confinement apparatus incorporated into a horizontal casting
apparatus, wherein the positioning of the electromagnetic
confinement apparatus and a magnetic field that is produced by an
alternating current provides a cast metal strip having
substantially uniform edges (sidewalls). The present invention
further provides a method and apparatus for producing a cast metal
strip, which provides a means for adjusting the profile of the cast
metal strip's sidewall.
[0012] In one embodiment of the present invention, the current
applied through the electromagnetic confinement apparatus, as well
as, the positioning of the electromagnetic confinement apparatus to
the molding zone of the horizontal casting apparatus is selected to
provide a cast metal strip having substantially uniform edges, in
which the sidewall of the cast metal strip edges may be
substantially flat, or concave or convex in relation to the cast
metal strip's centerline. The cast metal strip's substantially
uniform edges allows for the cast metal strip to be rolled without
further machining. Broadly, one embodiment of an apparatus of the
present invention comprises: [0013] (a) a pair of casting rollers
adapted to receive molten metal along a horizontal axis, wherein a
vertical distance separating the pair of casting rollers defines a
molding zone; [0014] (b) an electromagnetic edge containment
apparatus positioned on each side of the molding zone, comprising
an induction coil wound about a portion of a magnetic member to
generate magnetic lines of force upon application of a current,
wherein said magnetic member comprises a first and second pole
positioned distal from and aligned to a sidewall of said pair of
casting rollers and the current provides magnetic lines of force
perpendicular to said horizontal axis that contain the molten metal
in contact to the casting rollers with substantially no increase in
temperature to the molten metal; and [0015] (c) a means for
supplying the molten metal to the molding zone along said
horizontal axis from a tundish while ensuring said molten metal
remains substantially non-oxidized, wherein the tundish is
separated from the molding zone by a distance to substantially
eliminate wave generation within the tundish by the magnetic lines
of force.
[0016] In another embodiment of the apparatus of the present
invention, a horizontal roller casting apparatus is provided in
which containment of the metal through the apparatus is provided by
the combination of a mechanical edge dam and an electromagnetic
edge dam. Broadly, the inventive casting apparatus comprises:
[0017] (a) a pair of casting rollers adapted to receive molten
metal along a horizontal axis, wherein a vertical distance
separating the pair of casting rollers defines a molding zone;
[0018] (b) a tip delivery structure positioned to supply the molten
metal to the molding zone along said horizontal axis from a tundish
while ensuring said molten metal remains substantially
non-oxidized; and [0019] (c) an edge containment apparatus
positioned on each side of the molding zone, said edge containment
apparatus comprising: [0020] a mechanical edge dam positioned
overlying at least an end portion of said tip delivery structure
and partially extending towards said molding zone, and [0021] an
electromagnetic edge dam comprises a first and second magnetic pole
positioned distal from and aligned to a sidewall of said pair of
casting rollers and overlying a portion of said mechanical edge dam
partially extending towards said molding zone, wherein said
electromagnetic edge dam provides magnetic lines of force
perpendicular to said horizontal axis that contain the molten metal
in contact to the casting rollers.
[0022] In each embodiment, the vertical distance separating the
horizontally disposed pair of casting rollers provides a metal head
height that allows for containment of the molten metal by magnetic
lines of force that are provided by an electromagnetic containment
device without a substantial increase in the temperature of the
molten metal. For the purposes of this disclosure, the term
"positioned distal from and aligned to a sidewall of said pair of
casting rollers" is intended to denote that the poles of the
electromagnetic edge dam do not extend towards the casting
apparatuses centerline beyond a plane defined by the sidewall of
the casting rollers, but are positioned within close enough
proximity to the castings roller's sidewall to provide a sufficient
magnetic field to contain molten metal within the molding zone. It
is noted that the poles of the electromagnetic edge dam may be
adjusted from adjacent to the casting rollers sidewall to any
distance from the sidewall, so long as sufficient containment
forces are provided by the poles to the molding zone. In one
embodiment, the sidewall of the casting roller may be substantially
planar. The term "substantially planar" with respect to the casting
roller's sidewall denotes that the casting roller does not
incorporate a lip portion. In one embodiment, the electromagnetic
lines of force are produced by an alternating current having a
frequency ranging from 40 Hz to 10,000 Hz through the
electromagnetic edge containment device.
[0023] In another embodiment of the present invention, a belt
casting system is provided that employs electromagnetic edge
containment and produces a metal strip having substantially uniform
edges, wherein the substantially uniform edges allows for the cast
metal strip to be rolled without further machining. Broadly, the
inventive belt casting system for strip casting of molten metal
comprising: [0024] (a) a pair of opposing endless metal belts, each
of the pair of opposing endless metal belts passing over a roller
and having a periphery substantially aligned to a periphery of the
roller, said each of said opposing endless metal belts having a
surface for accepting molten metal, wherein a vertical dimension
separating the pair of opposing endless metal belts defines a
molding zone; [0025] (b) an electromagnetic edge containment
apparatus positioned on each side of the molding zone comprising an
induction coil wound about a portion of a magnetic member to
generate magnetic lines of force upon application of a current,
wherein the current provides magnetic lines of force that contain
the molten metal within a width and in contact to at least a
portion of said pair of opposing endless metal belts with
substantially no increase in temperature to the molten metal; and
[0026] (c) a means for supplying said molten metal to the molding
zone along a horizontal axis from a tundish, the tundish separated
from said molding zone by a distance to substantially eliminate
wave generation within the tundish by the magnetic lines of
force.
[0027] In another aspect of the present invention, a casting strip
is provided that may be formed by the above casting apparatus.
Broadly, the cast strip comprises: [0028] (a) a first shell; [0029]
(b) a second shell; and [0030] (c) a central portion between said
first shell and said second shell, said central portion comprising
grains having an equiaxed structure, wherein said cast metal strip
has sidewall edges being substantially uniform.
[0031] In another aspect of the present invention, a method is
provided for casting a metal strip in which a magnetic field is
utilized to control the geometry of the metal strip's sidewall.
Broadly, the inventive method comprises: [0032] providing molten
metal to a molding zone along a horizontal axis; [0033] containing
said molten metal within said molding zone with a magnetic
containment means; and [0034] casting said molten metal into a cast
metal strip, wherein sidewall geometry of said cast metal strip is
configured by adjusting said magnetic containment means.
[0035] The magnetic field may be adjusted to provide a metal
casting strip sidewall geometry that is flat or is concave or
convex relative to the centerline of the cast metal strip. In one
embodiment, the magnetic containment means may include an induction
coil wound about a magnetic member to generate magnetic lines of
force upon application of a current. The magnetic member having a
first and second magnetic pole positioned distal from to adjacent
to the molding zone.
[0036] The magnetic lines of force produced by the magnetic
containment means may be adjusted by increasing or decreasing the
current through the induction coil or by changing the positioning
of the magnetic containment means relative to the molding zone.
Positioning the first and second magnetic poles of the magnetic
containment means adjacent to-the molding zone may produce a cast
metal strip having a concave sidewall and positioning the first and
second magnetic poles of the magnetic containment means distal from
the molding zone may produce a cast metal strip having a convex
sidewall.
BRIEF DESCRIPTION OF THE DRAWINGS
[0037] FIG. 1 (side cross sectional view) is a schematic of a
portion of a vertical roller caster casting apparatus depicting a
molten metal head and a pair of rolls operated according to the
prior art.
[0038] FIG. 2a (side cross sectional view) is a schematic of one
embodiment of a horizontal casting apparatus having electromagnetic
edge dams in accordance with the present invention.
[0039] FIG. 2b (side cross sectional view) depicts one embodiment
of a twin belt caster equipped with an electromagnetic edge dam
apparatus in accordance with the present invention.
[0040] FIG. 3 (side cross sectional view) depicts the molding zone
of the inventive horizontal casting device.
[0041] FIG. 4 depicts a table summarizing the magnetic field
density that is required to contain a molten pool of aluminum at
different head heights.
[0042] FIG. 5 depicts a plot of the magnetic field strength
produced by an electromagnetic containment device in accordance
with the present invention at varying currents and distances
wherein the distance is measured from the sidewall of the caster
roll.
[0043] FIG. 6 (side cross sectional views) depicts a sectional view
taken along the lines 2-2 in FIG. 2a, and illustrate the
positioning of the electromagnetic edge dams in relationship to the
sidewall of the roller casters.
[0044] FIGS. 7a-7d provide a sectional view of the electromagnetic
edge dam apparatus of the present invention illustrating the path
of the magnetic lines of force in relation to the roller casters of
the horizontal roller caster casting apparatus.
[0045] FIGS. 8a-c (side view) illustrate different pole face angles
and orientations in accordance with the present invention.
[0046] FIG. 9 illustrates an exemplary embodiment of the present
invention wherein a magnetic member has a split core design.
[0047] FIG. 10 illustrates an exemplary embodiment of the present
invention wherein the magnetic member has a laminate design.
[0048] FIG. 11 illustrates an exemplary embodiment of the present
invention wherein a mechanical edge dam is used in conjunction with
an electromagnetic edge dam.
[0049] FIG. 12 depicts a table summarizing the push of the
electromagnetic edge dam.
[0050] FIGS. 13a-c are pictorial representations of sidewall of a
casting strip.
[0051] FIGS. 14a-b are photographic representations of the edges of
the strip made with a high magnetic force in the electromagnetic
dam.
[0052] FIG. 15 is a pictorial representation of a casting strip
having a flat edge profile (straight edge).
[0053] FIG. 16 is a pictorial representation of a casting strip
following an 87% reduction (acceptable degree of edge
cracking).
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0054] The present invention provides an electromagnetic edge dam
that confines molten metal to the molding zone of a horizontally
disposed roller casting or belt casting system with a magnetic
field that is produced by a lower AC current than was previously
possible. By providing sufficient electromagnetic containment means
at lower AC currents, the present invention utilizes
electromagnetic confinement without creating a substantial increase
in the temperature of the molten metal or producing wave generation
effects.
[0055] As discussed above, in prior vertical casting methods with
larger molten metal head height, larger magnetic forces are
required in order to contain the greater pressure produced by the
molten metal, wherein larger magnetic forces typically require
higher currents that generate heat. For example, to contain molten
aluminum against a 300 mm height, as representative of typical
vertical casting methods, a minimum magnetic field intensity of
0.24 T would be needed. In the present invention, the metal head
height is kept low, as achieved by a horizontally disposed casting
system, so that the required containment can be met with relatively
low magnetic field density. For example, a 50 mm head height in a
horizontal casting apparatus consistent with the present invention
requires a magnetic field density of only 0.055 T to contain molten
aluminum in the horizontal position while casting. The present
invention is now discussed in more detail referring to the drawings
that accompany the present application. In the accompanying
drawings, like and/or corresponding elements are referred to by
like reference numbers.
[0056] Referring to FIG. 2a, in one embodiment of the present
invention, a horizontal roller casting apparatus 10 is provided
having an electromagnetic edge dam 15 positioned to provide
magnetic lines of force to confine molten metal M within the
molding zone 20 of the apparatus 10, wherein the magnetic lines of
force extend along a plane perpendicular to the plane on which the
casting is drawn. The horizontal roller casting apparatus 10 is
practiced using a pair of counter-rotating cooled rolls R.sub.1 and
R.sub.2 rotating in the directions of the arrows A.sub.1 and
A.sub.2, respectively. By the term horizontal, it is meant to
denote that the cast strip is produced along a horizontal plane, in
which the horizontal plane is parallel to section line 2-2, or at
an angle of plus or minus about 30.degree. from the horizontal
plane.
[0057] Referring to FIG. 2b, in one embodiment of the present
invention, a horizontal belt casting apparatus 10' is provided
having an electromagnetic edge dam 15 positioned to provide ma
genetic lines of force to confine molten metal M within the molding
zone 20 of the apparatus 10, wherein the magnetic lines of force
extend along a plane perpendicular to the plane 2-2 on which the
casting is drawn. The horizontal belt casting apparatus 10' is
practiced using a pair of counter-rotating belts B.sub.1 and
B.sub.2 rotating in the directions of the arrows A.sub.1 and
A.sub.2, respectively. It is noted that although the following
figures are directed towards the horizontal roller caster 10
depicted in FIG. 2a, the following description is equally
applicable to the horizontal belt caster 10' disclosed in FIG. 2b
with the exception that instead of the molten metal contacting the
roller casters R.sub.1, R.sub.2 the molten metal is contacting the
counter-rotating belts B.sub.1, B.sub.2. It is further noted, that
further differences between the horizontal roller casting apparatus
10 and the belt casting apparatus 10' in accordance with the
present invention are noted when relevant throughout the following
portions of the specification.
[0058] Referring to FIG. 3, molten metal M is transported to the
molding zone 20 by a feed tip T, which may be made from a suitable
ceramic material. The feed tip T distributes molten metal M in the
direction of arrow B directly onto the casting rolls R.sub.1 and
R.sub.2 rotating in the direction of the arrows A.sub.1 and
A.sub.2, respectively. Gaps G.sub.1 and G.sub.2 between the feed
tip T and the respective rolls R.sub.1 and R.sub.2 are maintained
as small as possible to prevent molten metal from leaking out and
to minimize the exposure of the molten metal to the atmosphere. A
suitable dimension of the gaps G.sub.1 and G.sub.2 is about 0.01
inch (0.25 mm). A plane L through the centerline of the rolls
R.sub.1 and R.sub.2 passes through a region of minimum clearance
between the rolls R.sub.1 and R.sub.2 referred to as the roll nip
N.
[0059] The molten metal M delivered from the feeding tip T directly
contacts the cooled rolls R.sub.1 and R.sub.2 at regions 18 and 19,
respectively. Upon contact with the rolls R.sub.1 and R.sub.2, the
metal M begins to cool and solidify. The cooling metal produces an
upper shell 16 of solidified metal adjacent the roll R.sub.1 and a
lower shell 17 of solidified metal adjacent to the roll R.sub.2.
The thickness of the shells 16 and 17 increases as the metal M
advances towards the nip N. Large dendrites 21 of solidified metal
(not shown to scale) are produced at the interfaces between each of
the upper and lower shells 16 and 17 and the molten metal M. The
large dendrites 21 are broken and dragged into a center portion 12
of the slower moving flow of the molten metal M and are carried in
the direction of arrows C.sub.1 and C.sub.2.
[0060] The dragging action of the flow can cause the large
dendrites 21 to be broken further into smaller dendrites 22 (not
shown to scale). In the central portion 12 upstream of the nip N,
the metal M is semi-solid including a solid component including
solidified small dendrites 22 and a molten metal component. The
metal M in the region 23 has a mushy consistency due in part to the
dispersion of the small dendrites 22 therein. At the location of
the nip N, some of the molten metal is squeezed backwards in a
direction opposite to the arrows C.sub.1 and C.sub.2. The forward
rotation of the rolls R.sub.1 and R.sub.2 at the nip N advances
substantially only the solid portion of the metal (the upper and
lower shells 16 and 17 and the small dendrites 22 in the central
portion 12) while forcing molten metal in the central portion 12
upstream from the nip N such that the metal is completely solid as
it leaves the point of the nip N.
[0061] Downstream of the nip N, the central portion 13 is a solid
central layer 13 containing the small dendrites 22 sandwiched
between the upper shell 16 and the lower shell 17. In the central
layer 13, the small dendrites 22 may be about 20 to about 50
microns in size and have a generally equaixed (globular) shape, as
opposed to having a columnar shape. The three layers of the upper
and lower shells 16 and 17 and the solidified central layer 13
constitute a solid cast strip.
[0062] The rolls R.sub.1 and R.sub.2 serve as heat sinks for the
heat of the molten metal M. In the present invention, heat is
transferred from the molten metal M to the rolls R.sub.1 and
R.sub.2 in a uniform manner to ensure uniformity in the surface of
the cast strip. Surfaces D.sub.1 and D.sub.2 of the respective
rolls R.sub.1 and R.sub.2 may be made from a material of good
thermal conductivity such as steel or copper or other metallic
materials and are textured and include surface irregularities (not
shown) which contact the molten metal M. The surface irregularities
may serve to increase the heat transfer from the surfaces D.sub.1
and D.sub.2. The rolls R.sub.1 and R.sub.2 may be coated with a
material to enhance separation of the cast strip from the rolls
R.sub.1 and R.sub.2 such as chromium or nickel. In a preferred
embodiment, the rolls R.sub.1 and R.sub.2, including surfaces
D.sub.1 and D.sub.2, comprise a ferromagnetic material. In the
embodiments of the present invention, in which the rolls R.sub.1
and R.sub.2 do not comprise a ferromagnetic material, the casting
surfaces D.sub.1, D.sub.2 of the roller as well as the roller's
sidewall may be coated with a ferromagnetic materials.
[0063] The control, maintenance and selection of the appropriate
speed of the rolls R.sub.1 and R.sub.2 may impact the operability
of the present invention. The roll speed determines the speed that
the molten metal M advances towards the nip N. If the speed is too
slow, the large dendrites 21 will not experience sufficient forces
to become entrained in the central portion 12 and break into the
small dendrites 22. Accordingly, the present invention is suited
for operation at high speeds such as about 25 to about 400 feet per
minute or about 100 to about 400 feet per minute or about 150 to
about 300 feet per minute. The linear speed that molten aluminum is
delivered to the rolls R.sub.1 and R.sub.2 may be less than the
speed of the rolls R.sub.1 and R.sub.2 or about one quarter of the
roll speed. High-speed continuous casting according to the present
invention may be achievable in part because the textured surfaces
D.sub.1 and D.sub.2 ensure uniform heat transfer from the molten
metal M.
[0064] The roll separating force may be a parameter in practicing
the present invention. The roll separating force is the force
present between the rolls due to the presence of the strip within
the roll gap. The roll force is particularly high when the strip is
being plastically deformed by the rolls during roll casting. A
significant benefit of the present invention is that solid strip is
not produced until the metal reaches the nip N. The thickness is
determined by the dimension of the nip N between the rolls R.sub.1
and R.sub.2. The roll separating force may be sufficiently great to
squeeze molten metal upstream and away from the nip N. Excessive
molten metal passing through the nip N may cause the layers of the
upper and lower shells 16 and 17 and the solid central portion 13
to fall away from each other and become misaligned. Insufficient
molten metal reaching the nip N causes the strip to form
prematurely as occurs in conventional roll casting processes. A
prematurely formed strip 20 may be deformed by the rolls R.sub.1
and R.sub.2 and experience centerline segregation. Suitable roll
separating forces are about 25 to about 300 pounds per inch of
width cast or about 100 pounds per inch of width cast. In general,
slower casting speeds may be needed when casting thicker gauge
aluminum alloy in order to remove the heat from the thick alloy.
Unlike conventional roll casting, such slower casting speeds do not
result in excessive roll separating forces in the present invention
because fully solid aluminum strip is not produced upstream of the
nip.
[0065] In prior applications, roll separating force has been a
limiting factor in producing low gauge aluminum alloy strip product
but the present invention is not so limited because the roll
separating forces are orders of magnitude less than in conventional
processes. Aluminum alloy strip may be produced at thicknesses of
about 0.1 inch or less at casting speeds of 25 to about 400 feet
per minute. Thicker gauge aluminum alloy strip may also be produced
using the method of the present invention, for example at a
thickness of about 1/4 inch.
[0066] The aluminum alloy strip 20 continuously cast according to
the present invention includes a first layer of an aluminum alloy
and a second layer of the aluminum alloy (corresponding to the
shells 16 and 17) with an intermediate layer (the solidified
central layer 13) therebetween. The grains in the aluminum alloy
strip of the present invention are substantially undeformed because
the force applied by the rolls is low (300 pounds per inch of width
or less). The strip is not solid until it reaches the nip N; hence
it is not hot rolled in the manner of conventional twin roll
casting and does not receive typical thermo-mechanical treatment.
In the absence of conventional hot rolling in the caster, the
grains in the strip 20 are substantially undeformed and retain
their initial structure achieved upon solidification, i.e. an
equiaxed structure, such as globular.
[0067] Continuous casting of aluminum alloys according to the
present invention is achieved by initially selecting the desired
dimension of the nip N corresponding to the desired gauge of the
strip S. The speed of the rolls R.sub.1 and R.sub.2 may be
increased to a desired production rate, or to a speed that is less
than the speed at which the roll separating force increases to a
level that indicates that plastic deformation of the casting strip
is occurring between the rolls R.sub.1 and R.sub.2. Casting at the
rates contemplated by the present invention (i.e. about 25 to about
400 feet per minute) solidifies the aluminum alloy strip about 1000
times faster than aluminum alloy cast as an ingot cast and improves
the properties of the strip over aluminum alloys cast as an
ingot.
[0068] The molten metal M being delivered from the feed tip T is
confined within the molding zone 20 by at least an electromagnetic
edge dam 15 that is positioned to direct magnetic lines of force
perpendicular to the plane 2-2 on which the casting is being drawn.
In one embodiment, an electromagnetic edge dam 15 is positioned on
each side of the casting apparatus. In a preferred embodiment, the
molten metal M is confined within the molding zone 20 during
casting by a mechanical edge dam 55 in combination with an
electromagnetic edge dam 15, wherein the mechanical edge dam 55 is
positioned proximate to the feed tip T and the electromagnetic edge
dam 15 is positioned overlying the terminating end of the
mechanical edge dam 55 and provides confinement forces along the
entire length of the molding zone 20, as depicted in FIGS. 6 and
11.
[0069] The current and/or frequency utilized by the electromagnetic
edge dam 15 to maintain the molten metal M within the molding zone
20 is substantially less than typically required in prior casting
apparatuses using electromagnetic edge dams. In prior casting
apparatus employing electromagnetic edge dams, high magnetic force
fields where required to contain the molten metal, which resulted
in induction heating within the molten metal that disadvantageously
effected the solidification process. In the present invention, by
reducing the magnitude of the required electromagnetic force, the
current and/or frequency conducted through the electromagnetic edge
dam is also reduced, which in turn advantageously reduces the
incidence of induction heating on the sidewall of the molten metal
in the molding zone.
[0070] Without wishing to be bound, but in the interest of further
describing the present invention, applicants' believe that the
reduction in the electromagnetic force that is required to contain
the metal within the molding zone is related to the decreased head
height H.sub.2 of the molten metal from the feed tip T, as depicted
in FIG. 3, as opposed to the greater height H.sub.1 of the molten
metal pool disposed atop the roller caster in prior vertical
casting apparatuses, as depicted in FIG. 1. As discussed above, the
height H1 (or depth) of the molten pool atop the vertically
disposed casting rollers is approximately 65% the height of the
casting roller R.sub.1, R.sub.2 and can range from 8 inches to 20
inches, as depicted in FIG. 1. Referring to FIG. 3, in the present
invention, the height H.sub.2 of the molten metal as delivered from
the tip feed T to the molding zone 20 can be on the order of about
1 inch, and in some examples may be further reduced to 0.5 inches.
Hereafter, the difference in vertical location of the metal level
in the tundish and that of the center of the strip being cast is
referred to as a "molten metal head".
[0071] The relationship between the height of the molten metal head
H.sub.2 and the magnetic field density required for containing
molten aluminum at different head levels is best described through
the following equations. First, the pressure exterted by the molten
metal head, which the magnetic field must contain within the
molding zone 20 is calculated from: p=.rho.gH.sub.2 [0072] where p
is is the magnetic pressure in Pa, .rho. is the density of the
metal, g is the acceleration of gravity and H.sub.2 is the height
of the molten metal head. The pressure produced by the molten metal
head in turn determines the strength of the magnetic field that
must be produced by electromagnetic edge containment device 15 to
contain the molten metal head within the molding zone 20. In the
present invention, the height of the molten metal head H.sub.2 that
is being horizontally delivered to the molding zone 20 by the feed
tip T may be as low as 0.5 inches. The pressure that is produced by
the molten metal head of varying height H.sub.2, from the feed tip
T of the present horizontal roller casting apparatus 10, was
determined using the above equation and is listed in the Table
depicted in FIG. 4. To summarize the pressure ranged from about 125
Pa for a metal head height H.sub.2 of approximately 0.5 inches
(12.7 mm) to about 2,492 Pa for a metal head height H.sub.2 of
approximately 10 inches (254 mm).
[0073] The pressure required to contain the molten metal head
H.sub.2 within the molding zone 20 is then used in the following
equation to determine the required magnetic field density (B):
p=B.sup.2/2.mu..sub.o [0074] where p is the magnetic pressure in Pa
(Pascals), B is the magnetic field density in T (Tesla) and
.mu..sub.o is the permeability of air (=4.pi..times.10.sup.-7 H/m).
Referring to FIG. 4, from the above equation, it is calculated that
for a relatively high molten metal head height H.sub.2 for feed tip
T delivery of approximately 254 mm (10 inch), the magnetic field
density needed is 0.079 T (790 Gauss) and a molten metal head
height H.sub.2 of approximately 12.7 mm (0.5 inch), the magnetic
field density needed is approximately 0.0177 T. As illustrated in
FIG. 4, reducing the molten metal head height H.sub.2 decreases the
magnetic field density that is needed to contain the molten metal M
within the molding zone 20. The magnetic field density required to
contain metal head heights consistent with the present invention
can be obtained with electromagnets at relatively low current
levels. In one embodiment, the electromagnetic edge dam operates at
approximately 2000 ampere turns (i.e. a coil of 10 turns drawing
200 A).
[0075] In another aspect of the present invention, the physical
positioning of the electromagnetic edge dam, the molten metal head
height and the strength of the magnetic field can be varied to
control the positioning of the edge of the molten metal within the
molding zone with respect to the roller casters R.sub.1, R.sub.2
sidewall. The strength of the magnetic field at different distances
from the face (edge) of the roller casters may be calculated by the
following equation: B.sub.L=(.mu..sub.o nI/1)/{(2D/H)sin
h(L/1)+(w/1)cos h(L/1)} [0076] where: [0077] B.sub.L=magnetic field
intensity at a distance L (m) in the gap from the roll face. [0078]
nI=coil turns and current. [0079] w=roll gap 1= (.mu..sub.r
.delta.w/2) in which .mu..sub.r=relative permeability of steel
caster roll (taken as 600), .delta.=skin depth for steel (material
of the caster roll), and w is the roll gap. [0080] D=distance
between electromagnet pole and the roll face. [0081] H=height of
magnet pole.
[0082] Referring to FIG. 5, using the above equation, the magnetic
field strength was calculated and plotted as a function of the
frequency of current (Hz) conducted through the electromagnetic
edge dam 15, in which the distance at which the magnetic field
strength was calculated ranged from 10 mm to 80 mm inward from the
sidewall of steel casting rolls (reference line 1=10 mm, reference
line 2=20 mm, reference line 3=30 m, reference line 4=40 mm,
reference line 5=60 mm, and reference line 6=60 mm). In each of the
calculations, the height (H) of the magnetic pole was set at 8 mm,
the distance (D) between the electromagnetic pole and the roll face
was set at 4 mm, and the roll gap (w) was set at 4 mm.
Additionally, reference lines where plotted to indicate the minimum
the field strength required to contain a metal head having a height
H.sub.2 equal to 250 mm (reference line 7), 150 mm (reference line
8), 100 mm (reference line 9), and 50 mm (reference line 11). The
plot depicted in FIG. 5 illustrates that the 0.079 T field density
required for the 250 mm metal head 8 could be created by this
electromagnet in distances as far as 20 mm into the roll gap.
[0083] The edge of the casting strip can therefore be contained
inwards from the casting roll R.sub.1, R.sub.2 face (sidewall), if
desired, by increasing the current in the edge dam. It is noted
that the field density decreases rapidly at longer distances from
the roll face and only small metal head heights, on the order of 50
mm, can be contained in distances 40 mm or greater by the operation
of this edge dam at 2000 amp turns. The range of containment can be
extended further, if needed, by increasing the magnetomotive force
(nI) on the edge dam. When increasing the electromagnetic force,
due consideration need to be given to the heating effect of the
edge dam.
[0084] It is further noted that the plot depicted in FIG. 5 also
illustrates that the electromagnetic edge dam as utilized in the
present invention would operate effectively at any chosen
frequency. The loss in magnetic field becomes noticeable only for
operation at frequencies greater than 10 kHz.
[0085] In addition to the height of the molten metal head and the
magnetic field density, the positioning of the electromagnetic edge
dam with respect to casting rollers may also be adjusted to provide
electromagnetic force lines to confine the molten metal M within
the molding zone 20. Referring to FIG. 6, the electromagnetic edge
dam 15 may be positioned wherein the poles of the magnetic member
are aligned to the sidewalls 13 of the casting rollers R.sub.1,
R.sub.2. In one embodiment, the electromagnetic edge dam may be
positioned wherein the poles of the magnetic member are distal from
the sidewalls of each casting roller R.sub.1, R.sub.2. In the
embodiments of the present invention in which a horizontal belt
casting apparatus is employed as depicted in FIG. 2a, the
electromagnetic edge dam 15 may be positioned wherein each pole of
the magnetic member is distal from to aligned to the adjacent
sidewall of the casting belts B.sub.1, B.sub.2. For the purposes of
this disclosure the term "distal from to aligned to the adjacent
sidewall of the casting belts" is intended to denote that the poles
of the electromagnetic edge dam do not extend towards the casting
apparatuses centerline beyond a plane defined by the sidewall of
the casting belts, but are positioned within close enough proximity
to the sidewall of the castings belts to provide a sufficient
magnetic field to contain molten metal within the molding zone.
[0086] The inventive electromagnetic edge dam will also perform in
casters with rolls made from a non-magnetic (non-ferromagnetic)
material, such as copper. However, when the rollers comprise a
non-magnetic material, the penetration of the magnetic field into
the roll gap may be limited and thus containment will typically
occur on a plane close to the end of of the rolls. It may be
possible to obtain penetration into the gap by coating with a
ferromagnetic material (such as iron, nickel or cobalt) the end
faces and casting surfaces 200 of such rolls to the required depth
of containment, as depicted in FIG. 8d.
[0087] It is noted that prior casting apparatuses typically shape
the magnetic poles of the electromagnetic devices and the casting
rolls to focus the magnetic field towards the molding zone. In one
example, prior casting rollers employ lips extending from the
sidewall of each roller and may have further included magnetic
poles having a geometry corresponding to the extending lips of
prior casting rollers. Contrary to prior casting apparatuses, the
present invention does not require specially configured casting
rollers to facilitate the focus of the magnetic field produced by
the electromagnetic edge dam. In one embodiment of the present
invention, the sidewalls 113 of the casting rollers R.sub.1,
R.sub.2 may be substantially planar. Further, the electromagnetic
edge dam 15 of the present invention may be positioned so that the
face of the electromagnetic edge containment device is aligned to
the face of the casting roller's planar sidewall 113, wherein the
electromagnetic edge dam 15 is in close proximity to the casting
rollers R.sub.1, R.sub.2. The electromagnetic edge dam 15 may also
be positioned distal from the casting roller's sidewall 113.
Regardless of the positioning of the electromagnetic edge dam 15,
the electromagnetic edge dam 15 is positioned to provide sufficient
electromagnetic force to contain the molten metal M within the
molding zone 20.
[0088] The positioning of the edge dams 15 may be dependent on the
current or frequency utilized in the edge dam. For example, lower
currents may provide lower magnitude electromagnetic force line and
therefore be more likely to require that the edge dam 15 be
positioned in closer proximity to the molding zone 20. The higher
the current conducted through the electromagnetic edge dam the
greater the magnitude of the electromagnetic force lines and hence
the father away the electromagnetic edge dams may be positioned
from the molding zone.
[0089] Referring to FIGS. 7a-7c, in one embodiment, the positioning
of the electromagnetic edge dam 15 and the magnitude of the
electromagnetic force lines are selected to form a substantially
flat sidewall (FIG. 7a), a convex sidewall (FIG. 7b), or concave
sidewall (FIG. 7c) in the molten metal M within the molding zone
20. In one example, a current of 2200 Amp/turns produces a casting
strip having a concave sidewall; a current of 1200 Amp/turns
produces a casting strip having a substantially flat or straight
sidewall; and a current of on the order of 1200 Amp/turns produces
casting strip having a substantially convex sidewall. It is noted
that the above examples are provided for illustrative purposes only
and are not intended to limit the present invention, as any current
is applicable to the present invention, so long as the current
provides sufficient containment forces to the molding zone 20 and
does not result in excessive induction heating. In some of the
preferred embodiments of the present invention, in which the
casting strip's sidewall is concave or convex, the curvature of the
sidewall may be defined by a radius that is approximately half the
molten head height.
[0090] In another embodiment, the electromagnetic edge dam 15 may
be configured to provide molten metal within the molding zone
having a convex sidewall relative to the centerline of the molten
metal M within the molding zone 20. Preferably, the sidewall of the
molten metal within the molding zone is substantially aligned to
the planar surface of the roller casters, as depicted in FIGS. 8a
and 8c. Alternatively, the electromagnetic edge dam 15 may be
configured to project magnetic lines of force beyond the sidewall
113 of the casting rollers, wherein the molten metal is confined
interior to the edge of the roller casters, as depicted in FIGS. 8b
and 8d.
[0091] The electromagnetic edge dam's 15 structure is illustrated
in detail in FIG. 8a, representing a sectional view of the edge dam
apparatus 15 illustrated in FIG. 2a. In the preferred embodiment of
the invention, the electromagnetic edge dam 15 is a magnet type of
confinement system and includes a generally C-shaped magnetic
member. The magnetic member 30 thus includes a core 32 having an
upper arm or pole 34 and a lower arm or pole 36 extending therefrom
to define a generally C-shaped cross section. The core 32, includes
an induction coil winding 38 comprising a coil wound about the core
32 of the magnetic member 30 to establish an induction coil. Thus,
the winding is composed of a plurality of conductors wound about
the core 32 of the magnetic member 30. The core windings 38 about
the core 32 can be, made of solid metal such as copper wire.
[0092] Still referring to FIG. 8a, the upper arm 34 terminates in a
pole face 42 where as the lower arm 36 terminates in a pole face
44, respectively, with the molten metal M being maintained
therebetween. The pole faces 42 and 44 thus define the surface from
which the magnetic lines of force generated by the magnetic element
30 with its induction coil 38 pass from one of the pole faces 42 to
the other pole face 44, as illustrated by the magnetic lines of
force 48.
[0093] FIGS. 9a-9c illustrate different pole face 44 angles and
orientations in accordance with the present invention. It will be
appreciated by those skilled in the art that as the inter-pole-face
gap 43 increases, the strength of the field across the gap
decreases. FIG. 9a illustrates a cross section of a magnetic member
30 wherein the pole faces 42 and 44 have a negative angle relative
to the vertical plane substantially perpendicular to the plane on
which the casting is being drawn. The negative angle means that the
inter-pole-face gap 43 is less at the outside edge of each pole
than at the inside edge of each pole face. As a result, the
containment forces created by the magnetic member shown in FIG. 9c
are stronger at the outside edge of each pole face than at the
inside edge of each pole face. FIG. 9b illustrates a cross section
of a magnetic member 30 wherein the pole faces 42 and 44 have no
angle relative to the vertical plane substantially perpendicular to
the plane on which the casting is being drawn. The zero angle means
that the inter-pole-face gap 43 is the same at the inside edge of
each pole face and the outside edge of each pole face. As a result,
the magnetic field created by the magnetic member shown in FIG. 9b
is relatively uniform across each pole face. FIG. 9c illustrates a
cross section of a magnetic member 30 having pole faces 42 and 44
that are parallel in part and not parallel in part. The inside
region of the pole faces 42 and 44 have a negative angle relative
to the horizontal.
[0094] In one embodiment of the present invention, the magnetic
member 30 is formed from a ferromagnetic material such as silicon
steel, and can be formed from a solid piece of such ferromagnetic
material. Alternatively, the magnetic member 30 can be formed from
multiple ferromagnetic materials, such as the split core design
depicted in Figure 10. In another embodiment, the magnetic member
30 can be formed from a series of laminated elements machined and
secured together using mechanical means, an adhesive or like means
to yield the desired configuration, as depicted in FIG. 1l. In many
instances, the use of such laminates is preferable, because
laminates may serve to more uniformly distribute the flux lines in
the magnetic member and reduce loss due to saturation of the
magnetic member. In addition, for a magnetic member made of
laminated ferromagnetic material, the electrical energy dissipated
as heat is also more evenly distributed and more easily removed,
particularly where the adhesive employed to hold the laminate
elements together has good thermal conductivity.
[0095] Referring back to FIGS. 8a-8d, surrounding the magnetic
member 30 is an outer shield 50, which is preferably made of a
material, and most preferably a metal, having structural rigidity
and extremely high electrical and thermal conductivities.
Preferably, the outer shield 50 is fabricated of copper, although
other metals such as silver and gold can likewise be used. The high
electrical conductivity of the outer shield 50 aids in containing
the magnetic lines of force within the magnetic member while the
good thermal conductivity aids in the dissipation of heat from the
overall apparatus. As will be appreciated by those skilled in the
art, the outer shield 50 may be provided with cooling channels
therein or brazed tubes thereon to distribute cooling fluid through
or at the surface of the outer shield to further aid in the removal
of heat generated by the electromagnetic field. For example, an
inlet can be employed to pass a cooling fluid through the outer
shield for removal from a discharge port when additional cooling
capability is required. Thus, the cooling fluid can be passed
through a conduit within the outer shield to remove heat generated
by the electromagnetic field.
[0096] The electromagnetic edge dam employed in the practice of the
present invention also includes an inner shield 56 dimensioned to
fit within the C-shaped configuration of the magnetic member 30.
The inner shield 56 likewise serves to contain the magnetic lines
of force generated by the coil 38 of the magnetic member 30,
insuring that the magnetic lines of force are maintained within the
magnetic member 30. In addition, it is also possible, and some
times desirable, to include within the inner shield conduit means
for the passage of a cooling fluid therethrough where it is desired
to increase the ability to dissipate heat from the magnet. It is
also possible to do away with the inner shield; especially so when
using grain oriented silicon steel laminates where the field lines
prefer to flow within the laminates.
[0097] The path of the magnetic field of the present invention is
indicated in FIGS. 8a thorough 8d. In FIG. 8a, magnetic field flows
from one pole of the edge dam to the other in a plane essentially
parallel to the side faces of the rolls. It is applicable to
metallic rolls which are non-ferromagnetic (such as copper). The
field creates the containment forces on the end faces of the rolls.
FIG. 8b illustrates the case when the field penetrates into the gap
and contains the molten metal inwards from the roll faces. This
will be the case for ferromagnetic rolls and strong fields. It can
also be achieved by the application of a ferromagnetic coating 200
of sufficient depth to the end faces and end of the casting surface
of a non-ferromagnetic roll material, as depicted in FIG. 8d.
[0098] In designing the electromagnetic containment apparatus
employed in the practice of this invention, a number of different
techniques can be used in dissipating heat generated by the
electromagnetic field. As shown in FIG. 8c, the windings 40 may be
formed of an annular conductor having a central opening 41
extending therethrough. Thus, cooled water can be passed through
the central opening of the windings 40 to aid in the dissipation of
heat generated by the electromagnetic field. As shown in FIG. 12,
the core 30 may also be equipped with a cooling conduit 47
extending therethrough; in that way, a cooling fluid can be
370044-00038 passed through the cooling conduit 47 to aid in the
dissipation of heat generated by the electromagnetic field.
[0099] FIG. 12 illustrates one preferred embodiment of the present
invention, wherein a mechanical edge dam 55 is used in conjunction
with an electromagnetic edge dam 15 having a magnetic member 30.
The magnetic member 30 is preceded by the mechanical edge dam 55.
The mechanical edge dam 55 shown should ideally have a ceramic-less
surface and comprise magnetic material to reduce the reluctance at
the mouth of the molding zone. A ceramic material may also be used
to make mechanical edge dam 55 if process conditions preclude the
use of a metallic material. In one embodiment of the present
invention, the mechanical edge dam 55 is positioned to ensure that
the molten metal is contained within the casting apparatus while
being delivered from the tundish H to the feed tip T. Once the
molten metal M reaches the feed tip T, containment forces are
provided by the electromagnetic edge dam 15. In this arrangement,
the service life of the mechanical edge dam 55 is increased by the
electromagnetic edge dam 15, since the electromagnetic edge dam 15
is positioned in the most hostile portion of the casting
apparatus.
[0100] The following examples are provided to further illustrate
the present invention and demonstrate some advantages that arise
therefrom. It is not intended that the invention be limited to the
specific examples disclosed.
EXAMPLE 1
Confirmation of Electromagnetic Push
[0101] Aluminum strip was cast in accordance with the present
invention using a caster with steel rolls. The strip was then
metallographically tested to confirm the effect of the
electromagnetic force on the molten metal within the molding zone.
Test specimens were formed using a horizontal roller caster and a
combination of electromagnetic and mechanical edge dams consistent
with the present disclosure. Casting strips of three different
thicknesses (2.44 mm, 2.29 mm, and 2.16 mm) were then cast
operating the electromagnetic edge dam at 2180 A turns. Samples
were then cut from the edges of the strips and were prepared for
metallographic examination. It was observed that the center part of
the casting strip was pushed inwards as compared to the outer
surfaces of the strip, as shown in FIGS. 14a and 14b. This
observation confirms the confinement effect of the electromagnetic
edge dam during casting, since the central portion of the strip is
the last to solidify.
[0102] The depth of the confinement effect into the roll gap was
estimated by first measuring the width of the casting strip at room
temperature, wherein the width of the casting strip was
approximately 400.5 mm. From this measurement, the width of the
strip within the molding zone can be estimated as 406 mm by adding
the contraction that occurred during solidification and cooling to
room temperature.
[0103] Taking into account that the width of the casting roll is
approximately 432 mm, it is evident that the magnetic field pushed
the molten center of the casting strip a distance of approximately
13 mm (13 mm=(432(width of roller caster)-406)/2) from the casting
roll face on each side of the casting roll. More specifically, by
subtracting the calculated width of the casting strip in the
molding zone from the width of the casting roller the total
displacement produced by the electromagnetic edge dams is
calculated. The amount of displacement produced by a single edge
dam is calculated by the number of edge dams employed, which in
this experiment consisted of two electromagnetic edge dams
positioned at opposing ends of the casting rollers.
[0104] Similar electromagnetic push effects were observed for all
three different strip thickness (strip gauge), as summarized in the
Table depicted in FIG. 13. The degree of magnetic push was measured
as the depth of the center portion of the strip with respect to the
edges. The magnetic push was somewhat higher for thinner gauge
strip, since the narrower roll gap would create a higher field
density at any given distance. It is believed that the difference
in the magnetic push between the two sides (drive side and operator
side) of the caster rolls, as summarized in FIG. 13, is attributed
to variations in the location of the electromagnets and the
mechanical edge dams.
EXAMPLE 2
Control of Cast Strip Edge Profile
[0105] The edge profile of the as-cast strip was checked for
operation at different magnetomotive force levels in the
electromagnet. The edge profile obtained at 2180 A turn operation
shown in FIG. 14 were considered unsuitable for subsequent rolling
of the strip unless the edges were trimmed prior to rolling. In
order to provide cast strip having edge profiles suitable for
rolling without additional machining, the magnetomotive force of
the electromagnet was reduced to decrease the push on the central
portion of the casting strip so that the edge profile of the strip
would be flat or slightly convex.
[0106] A flat edge profile was obtained in the casting strip at a
current level of 180 A (or 1620 A turns) being applied to the
electromagnet. To obtain a flat edge profile, the magnetic field
should be selected to just offset the pressure produced by the
molten metal in the molding zone, which is produced by the metal
head with a minor contribution small roll pressure. Referring to
FIG. 15, the edge of the casting strip made under these conditions
was flat and highly straight indicating that it could be rolled
without trimming the edges of the casting strip or other additional
machining.
[0107] This strip was rolled in-line successfully through four
stands of rolling mills. The casting strip was rolled from a 2.7 mm
(0.107 inch) as-cast thickness to a thickness of approximately 0.36
mm (0.014 inch), which corresponded to an 87% reduction in
thickness. Referring to FIG. 16, the sheet made by this method
showed only minor cracks at the edges, which could be removed by
trimming prior to coiling.
[0108] Following proper adjustment of the electromagnetic edge dam,
high quality edges are obtained in the as-cast strip which permits
rolling to high reduction ratios saving materials and improving the
efficiency of the process.
[0109] While the present invention has been particularly shown and
described with respect to preferred embodiments thereof, it will be
understood by those skilled in the art that the foregoing and other
changes in forms and details may be made without departing from the
spirit and scope of the present invention. It is therefore intended
that the present invention not be limited to the exact forms and
details described and illustrated, but fall within the scope of the
appended claims.
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