Note: Descriptions are shown in the official language in which they were submitted.
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METHOD AND APPARATUS FOR ELECTROMAGNETIC CONFINEMENT OF
MOLTEN METAL IN HORIZONTAL CASTING SYSTEMS
Field of the Invention
[0001] The present invention relates to the continuous casting of inetal
strip,
and more particularly, to the electromagnetic confinement of molten metal in a
contlntioUs casting system.
Back~roitnd of the Invention
[0002] Continuous casting of metals is performed in twin - roll casters and
belt
casters or coinbinations 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.
[00031 Up to the present, the mechanical edge dams have been employed to
provide eontaimnent of the molten metal in the casting zone. Such devices have
included the caterpillar type edge danls that move with the strip (as in the
Hazelett
casters) or fixed edge danis that are pressed against the surface of the
rolls. The latter
is used in the twin-roll steel strip casting industly. Such fixed mechanical
edge dalns
have a short service life as they get eroded by contact with the cold sidewall
of the
rolls. In addition, such mechanical edge dains provide sites for the
forniation of
skulls that have a tendency to be sheared off and thus enter the cast strip to
render the
inicrostructure nletallurgically undesirable. Caterpillar edge dains, while
well proven
for the thicker slab castings (10 - 25 nun tliick.), become ixnpractical for
tliin strip
casters or twin druin casters of the steel industry where the cross section to
be
contained changes sharply along the casting zone.
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f0004] Electromagnetic edge dams have been elnployed in the prior art in the
strip casting of inetals in vertical twin drum (roller) casting systeins.
Electroinagnetic
edge dalns of a magtietic 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.
10005] The magnetic system electromagnetic edge dams use a magnetic
meinber which coinprises a yoke or core connecting two pole faces disposed on
eitlier
side of the gap on which the molten metal is to be confined. The magnetic
inember 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 contaiiunent 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 ininhnize leakage of flux in a
direction away
from the gap. Such magnetic confinement systems have the advantage that the
confineinent cuxxent need not be as high as compared to those systems using
solely an
iiiduction coil. If a stronger inagnetic field is required, it can be achieved
with the
same current level by reducing the area of the pole faces to concentrate tlle
field.
However, such systelns are not without disadvantages. For example, such
systems
typically have poor operating efficiency resulting from core losses and losses
due to
inagnetic hysterisis when an alternating magnetic field is applied to the
magnetic
material. Additionally, high teniperatLires are typically generated that need
to be
dissipated by cooling in order to prevent dainage to the nlagnetic system.
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[0007] Induction confinement systenls typically einploy 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 tiine-
va.lying
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 menlber is built arotuld the inductor to focus the
current to the
inductor surface facing the molten tnetal. Induction coil systems are
generally
siinpler in design than magnetic systems. However, induction systems are
disadvantageously limited in terins of the lnaximum metallostatic head that
can be
contained by the systezn. The lnaximum metallostatic head that can be
supported in
induction coil systems is Iimited, because induction coil systems reqtiire
very high
inductor currents to provide adequate contaimnent forces, wherein such high
currents
are accompanied by increased heat generation, which in tLirn hinders or slows
the
solidification process during casting.
[00081 Referring to Figure 1, in vertical twin roll casters, the molten znetal
head against which containtnent must be provided tends to be very high. For
typical
operating condition, the metal head heiglxt Hl is about 65 1o the radius of
the casting
rolls. Therefore, electromagnetic edge dam apparatus used in vertical twin
roll
casters znust provide a magnetic field strong enough to contain a metal pool
having a
head height Hl that is 65% the raditts of the casting rolls. Such
electroinagnetic edge
dams have not been successfillly conimercialized for two reasons. First, the
high
current required to contain the inolten inetal 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 inetal
head
formed atop vertical roller caster systems create indziction heating on the
metal pool's
sidewall, which interferes with the solidification process.
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[0009] U.S. Patent No. 4,936,374 describes a vertical casting systenl and
electromagnetic confznement apparatus having the disadvantages described
above.
Further, U.S. Patent No. 4,936,374 describes casting rollers having a rim
por.tion, in
which the containnlent magnetic field is conducted through the rim portion of
the
casting roll. In addition to induction heating and wave generation, the riin
portions of
the casting rolls disclosed in U.S. Patent No. 4,936,374 produce a ridge in
the cast
product and therefore fail to provide a casting strip having uniforin
sidewalls (edges).
The ridge formed in the casting strip produced using the apparatus and method
disclosed in U.S. Patent No. 4,936,374 n-iust 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 higll-speed continuous
casting of metals and alloys, which achieves uniforinity in the cast strip
surface,
provides good molten metal containinent in the casting zone, and results in
strip
edges which can be rolled without needing to be machined by triiruning.
SuminM 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 (sidevvalls).
The
present invention fiirFller provides a inethod and apparatus for producing a
cast inetal
strip, wllich provides a means for adjusting the profile of the cast metal
strip's
sidewall.
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[00121 In one en-ibodirnent 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 wliich the sidewall of the cast inetal strip edges may be substantially
flat, or
concave or convex in relation to the cast metal strip's centerline. The cast
znetal
strip's substantially uniforin edges allows for the cast nietal strip to be
rolled without
further machining. Broadly, one einbodiment of an apparatus of the present
invention comprises:
(a) a pair of casting rollers adapted to receive niolten metal along a
horizontal axis, wherein a vertical distance separating the pair of casting
rollers
defines a molding zone;
(b) an electromagnetic edge contaiinnent apparatus positioned on each side
of the molding zone, coinprising an induction coil wound about a portion of a
magnetic inember to generate niagnetic lines of force upon application of a
current, wherein said magnetic member coinprises 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 witll
substantially no
increase in temperature to the molten metal; and
(c) a means for supplying the molten inetal to the molding zone along said
horizontal axis from a tundish while ensuring said molten metal remains
substantially non-oxidized, wllerein the tundish is separated fronl the
molding zone
by a distance to substantially eliminate wave generation within the tundish by
the
magnetic lines of force.
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[00131 In another einbodiment 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 colnbination of a mechanical edge
dain and
an electromagnetic edge dam. Broadly, the inventive casting apparattis
colnprises:
. (a) a pair of casting rollers adapted to receive molten metal along a
horizontal axis, wherein a vertical distance separating the pair of castizig
rollers
defines a molding zone;
(b) a tip delivery structure positioned to supply the inolten metal to the
znolding zone along said horizontal axis from a tundish w11ile ensuring said
molten
lnetal remains substantially non-oxidized; and
(c) an edge containment apparatLis positioned on each side of the molding
zone, said edge contaimnent apparatus comprising:
a mechanical edge dain positioned overlying at least an end portion
of said tip delivery structure and partially extending towards said molding
zone,
and
an electroinagnetic edge dam coinprises 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 dain partially extending
towards
said molding zone, wherein said electromagnetic edge dain provides magnetic
lines of force perpendicular to said horizontal axis that contain the molten
inetal in
contact to the casting rollers.
[0014] In each einbodinlent, the vertical distance separating the horizontally
disposed pair of casting rollers provides a nzetal head height that allows for
contailunent of the molten metal by magnetic lines of force that are provided
by an
electromagnetic contaimnent device witllout a substantial increase in the
teniperature
of the molten metal. For the purposes of this disclosure, the terin
"positioned distal
frorn and aligned to a sidewall of said pair of casting rollers" is intended
to denote
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that the poles of the electromagnetic edge dain do not extend towards the
casting
apparatuses centerline beyond a plane defined by the sidewall of the casting
rollers,
but are positioned within close enouph 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 daxn inay 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
embodiinent, 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 incoiporate a lip portion. In one ernbodiznent, 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.
100151 In another embodilnent of the present invention, a belt casting system
is
provided that employs electromagnetic edge contaimnent and produces a metal
strip
hdving substantially uniforin edges, wherein the substantially uniforin edges
allows
for the cast metal strip to be rolled without further machining. Broadly, the
inventive
belt casting systein for strip casting of molten metal coinprising:
(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
lnetal belts
having a surface for accepting molten nietal, wherein a vertical diinension
separating the pair of opposing endless metal belts defines a molding zone;
(b) an electromagnetic edge contailunent apparatus positioned on each side
of the molduig zone comprising an induction coil wound about a portion of a
magnetic member to generate magnetic lines of force upon application of a
cuirent, wllerein the current provides magnetic lines of force that contain
the
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inoltera 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 telnperature 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
inagnetic lines of force.
[0016] In another aspect of the present invention, a casting strip is provided
that may be forined by the above casting apparatus. Broadly, the cast strip
comprises:
(a) a first shell;
(b) a second shell; and
(c) a central portion between said first shell and said second shell,
said central portion coinprising grains having an equiaxed structure,
wherein said cast metal strip has sidewall edges being substantially
uniform.
[0017] In anotl2er aspect of the present invention, a lnethod 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:
providing molten metal to ainolding zone along a horizontal axis;
containing said molten metal within said molding -zone witll a
magnetic containment means; and
casting said molten metal into a cast metal strip, wlierein sidewall
geometry of said cast metal strip is configured by adjusting said
nlagnetic contairunent means.
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[00181 The magnetic field may be adjusted to provide a xnetal casting strip
sidewall geoanetiy that is flat or is concave or convex relative to the
centerline of the
cast metal strip. In one embodiment, the inagnetic containlnent means may
include
an induction coil wound about a magnetic member to generate magnetic lines of
force r.ipon application of a current. The magnetic nlember having a first and
second
magnetic pole positioned distal from to adjacent to the molding zone.
[0019] The niagnetic lines of force produced by the inagnetic containineiit
ineans inay be adjusted by increasing or decreasing the current through the
induction
coil or by changing the positioning of the magnetic contairunent ineans
relative to the
inolding zone. Positioning the first and second magnetic poles of the
lnagnetic
contailunent means adjacent to the molding zone may produce a cast inetal
strip
having a concave sidewall and positioning the first and second magnetic poles
of the
magnetic containnnent zneans distal from the molding zone inay produce a cast
metal
strip having a convex sidewall.
Brief Description of the Drawings
100201 Figtire 1(side cross sectional view) is a scheinatic of a poi-tion of a
vertical roller caster casting appratus depicting a molten metal head and a
pair of rolls
operated according to the prior art.
[0021] Figure 2a (side cross sectional view) is a schematic of one einbodiment
of a llorizontal casting apparatus having electroinagnetic edge dains in
accordance
witli the present invention.
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[0022] Figure 2b (side cross sectional view) depicts one embodiinent of a twin
belt caster equipped with an electroinagnetic edge dain apparatus in
accordance with
the preSellt i11ve11tlon inveiltioll.
[0023] FigL2re 3 (side cross sectional view) depicts the lnolding zone of the
inventive horizontal casting device. -
[0024] Figure 4, depicts a table sulmnarizing the magnetic field density that
is
required to contain a molten pool of alulninum at different head heights.
[0025] Figure 5 depicts a plot of the magnetic field strength produced by aii
electromagnetic contaiiunent device in accordance with the present invention
at
varying currents and distances wherein the distance is measured from the
sidewall of
the caster roll.
[0026] Figure 6 (side cross sectional views) depicts a sectional view taken
along the lines 2-2 in Figure 2a, and illustrate the positioning of the
electromagnetic
edge dams in relationship to the sidewall of the roller casters.
[0027] Figures 7a-7d provide a sectional view of the electroinagnetic edge
dain apparatus of the present invention illustrating the path of the inagnetic
lines of
force in relation to the roller casters of the horizontal roller caster
casting apparatus.
[0028] Figures 8 a-c (side view) illustrate different pole face angles and
orientations in accordance with the present invention.
[0029] Figure 9 illustrates an exeniplaly einbodiment of the present invention
wherein a magnetic member has a split core design.
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[00301 Figure 1 O illListrates all eXelnplary enlbodllnent of tlle present
Inventlon
wherein the magnetic member has a laminate design.
[0031] Figure 11 illustrates an exemplary elnbodiinent of the present
invention
wherein a mechanical edge dam'is used in conjunction with an electroinagnetic
edge
dam.
[0032] Figure 12 depicts a table summarizing the push of the electromagnetic
edge dazn.
[0033] Figures 13 a-c are pictorial representations of sidewall of a casting
strip.
[0034] FigLUes 14 a-b are photographic representations of the edges of the
strip
made with a high magnetic force in the electromagnetic dam.
[0035] Figure 15 is a pictorial representation of a casting strip having a
flat
edge profile (straight edge).
[0036] Figure 16 is a pictorial representation of -a castuzg strip following
an
87% reduction (acceptable degree of edge cracking).
Detailed Description of Preferred Embodiments
[0037] 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 systein with a magnetic field that is produced by a lower AC
cLUrent than
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was previously possible. By providing sufficient electroniagnetic containinent
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.
[00381 As discussed above, in prior vertical casting metllods with larger
molten metal head height, larger magnetic forces are required in order to
contain the
greater pressure prodticed by the molten metal, wherein larger xnagnetic
forces
typically require higher currents that generate heat. For example, to contain
molten
aluminum against a 300 nun heigllt, as representative of typical vertical
casting
inethods, a rninimuln magnetic field intensity of 0.24 T would be needed. In
the
present invention, the inetal head height is kept low, as achieved by a
horizontally
disposed casting system, so that the required containinent can be met with
relatively
low magnetic field density. For exaxnple, a 50 inin 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 aluininuin 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 accoinpanying drawings, like and/or
corresponding eleinents are referred to by like reference numbers.
[0039] RefeiTing to Figure 2a, in,.one embodiinent of the present invention, a
horizontal roller casting apparatus 10 is provided having an electromagnetic
edge
daxn 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 wllich the casting is
drawn. The
horizontal roller casting apparatus 10 is practiced using a pair of counter-
rotating
cooled rolls Rl and R2 rotating in the directions of the arrows Al and A2,
respectively.
By the terin horizontal, it is meant to denote that the cast strip is produced
along a
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horizontal plane, in which the horizontal plane is parallel to section line 2-
2, or at an
angle of plus or minus about 30 from the horizontal plane.
[0040] Referring to Figure 2b, in one embodiment of the present invention, a
horizontal belt casting apparatus 10' is provided having an electromagnetic
edge dani
15 positioned to provide magnetic lines of force to confine molten metal M
within
the niolding zone 20 of the apparatus 10, wherein the lnagnetic 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
B1 and B2 rotating in the directions of the arrows AI and A2, respectively. It
is noted
that although tlie following figures are directed towards the horizontal
roller caster 10
depicted in Figure 2a, the following description is equally applicable to the
horizontal
belt caster 10' disclosed in Figure 2b with the exception that instead of the
molten
xnetal contacting'the roller casters Rl, R, the molten metal is contacting the
counter-
rotating belts B1, B2. It is further noted, that further differences between
the
horizontal roller casting apparatus 10 and the belt casting apparatus 10' in
accordance
witl-i the present invention are noted when relevant throughout the following
portions
of the specification.
[0041] Referring to Figure 3,lnolten metal M is transported to the molding
zone 20 by a feed tip T, which may be made frozn a suitable ceramic material.
The
feed tip T distributes molten inetal M in the direction of arrow B directly
onto the
casting rolls Rl and R2 rotating in the direction of the arrows Al and A2,
respectively.'
Gaps Gl and G2 between the feed tip T and the respective rolls Rl and R2 are
maintained as sinall as possible to prevent molten metal from leaking out and
to
mininlize the exposure of the molten metal to the atmosphere. A suitable
dimension
of the gaps Gi and G2 is about 0.01 inch (0.25 nun). A plane L through the
centerline
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of the rolls R, and R2 passes throiigli a region of minimu.m clearance between
the
rolls RI and R2 referred to as the roll nip N.
[0042] The molten metal M delivered from the feeding tip T directly contacts
the cooled rolls Rl and R2 at regions 18 and 19, respectively. Upon contact
with the
rolls RI and R2, the metal M begins to cool and solidify. The cooling metal
produces
an upper.shell 16 of solidified metal adjacent the roll RI and a lower shell
17 of
solidified metal adjacent to the roll R2. 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 inetal M. The large
dendrites 21
are broken and dragged into a center portion 12 of the slower moving flow of
the
nlolten metal M and are carried in the direction of arrows C1 and C2.
[0043] The dragging action of the flow can cause the large dendrites 21 to be
broken fi.irEher into smaller dendrites 22 (not shown to scale). In the
central poi-tion
12 upstreain of the nip N, the metal M is senli-solid including a solid
co7nponent
including solidified small dendrites 22 and a molten metal component. The
metal M
in the region 23 has a inushy consistency due in part to the dispersion of the
snlall
dendrites 22 therein. At the location of the nip N, sonie of the molten metal
is
squeezed backwards in a direction opposite to the arrows Ct and C2. The
forward
rotation of the rolls Rl and R2 at the nip N advances substantially only the
solid
portion of the metal (t11e upper and lower shells 16 and 17 and the small
dendrites 22
in the central portion 12) while forcing molten lnetal in the central portion
12
upstream from the nip N such that the metal is conlpletely solid as it leaves
the point
of tiie nip N.
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[0044] Downstream of the nip N, the central portion 13 is a solid central
layer
13 containing the small dendrites 22 sandwiclzed between the upper shell 16
and the
lower shell 17. In the central layer 13, the small deindrites 22 may be about
20 to
about 50 microns in size and have a generally equaixed (globular) shape, as
opposed
to having a colurnnar shape. The three layers of the upper and lower shells 16
and 17
and the solidified central layer 13.constitute a solid cast strip.
(0045] The rolls Ri and R2 serve as heat siiiks for the heat of the molten
metal
M. In the present invention, heat is transferred froin the molten metal M to
the rolls
RI and R2 in a uniform manner to ensure uniformity in the surface of the cast
strip.
Surfaces D1 and D2 of the respective rolls Ri and R2 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) whicli contact the
nlolten
lnetal M. The surface irregularities may serve to increase the heat transfer
froin the
surfaces D1 and D2. The rolls Rj and R2 may be coated with a material to
enliance
separation of the cast strip from the rolls Rl and R2 such as chromium or
nickel. In a
preferred eznbodiment, the rolls RI and R2, including surfaces D1 and D2,
coinprise a
ferroznagnetic material. In the einbodiments of the present invention, in
which the
rolls Rl and R2 do not conzprise a ferroinagnetic material, the casting
surfaces DI, D2
of the roller as well as the roller's sidewall may be coated with a
ferroznagnetic
materials.
[0046] The control, maintenance and selection of the appropriate speed of the
rolls RI and R2 may impact the operability of the present invention. The roll
speed
detennines the speed that the niolten 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 celitral portion 12 and break into the small dendrites
22.
Accordingly, the present invention is suited for operation at high speeds such
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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 Rl and R2 may be less than the speed of tlie rolls RI
and R2 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 Dt
and D2
ensi.ire unifonn heat transfer from the molten metal M.
[0047J 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
wen the
stip is being plastically deforined by the rolls during roll castiong. A
significant
benefit of the present invention is that solid strip is not produced until the
metal
reaches the nip N. The thickness is deterlnined by the dimension of the nip N
between the rolls Ri and R2. The roll separating force may be sufficiently
great to
squeeze molten inetal upstream and away from the nip N. Excessive molten metal
passing througli 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
niisaligned. Insufficient inolfien metal reaching the nip N causes the strip
to form
premati.irely as occurs in conventional roll casting processes. A prelnaturely
formed
strip 20 may be deformed by the rolls R1 and R2 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 alunlinum alloy in
order to
remove the heat froin the tllick 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.
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[0048] 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 niagnitude less than
in
conventional processes. Aluminuin alloy strip may be produced at thicknesses
of
about 0.1 incll or less at casting speeds of 25 to about 400 feet per minute.
Thicker
gauge aluminum alloy strip nlay also be produced using the method of the
present
invention, for exainple at a tliickness of about 1/4 incll.
[0049] The aluininum alloy strip 20 continuously cast according to the present
invention includes a first layer of an alumininn alloy and a second layer of
the
aluminum alloy (corresponding to the shells 16 and 17) witll an intermediate
layer
(tlle solidified central layer 13) therebetween. The grains in the aluminuzn
alloy strip
of the present invention are substantially undeforined because the force
applied by
the rolls is low (300 pounds per inch of width or less). The strip is not
solid until it
reaclzes the nip N; hence it is not hot rolled in the manner of conventional
twin roll
casting and does not receive typical therino-mechanical treatnlent. In the
absence of
conventional hot rolling in the caster, the grains in the strip 20, are
substantially
undeforined and retain their initial structure achieved upon solidification,
i.e. an
equiaxed structlire, such as globular.
[0050] Continuous casting of aluminiun alloys according to the present
invention is achieved by initially selecting the desired dimension of the nip
N
correspond'ulg to the desired gauge of the strip S. The speed of the rolls Ri
and R2
may be increased to a desired production rate, or to a speed that is less than
the speed
at wllich the roll separating force increases to a level that indicates that
plastic
deforlna.tion of the casting strip is occurring between the rolls RI and R2.
Casting at
the rates contemplated by the present invention (i.e. about 25 to about 400
feet per
minute) solidifies the aluminuxn alloy strip about 1000 times faster than
aluminum
17
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alloy cast as an ingot cast and improves the properties of the strip over
alulninum
alloys cast as an ingot.
[0051] The molten nletal M being delivered from the feed tip T is confined
within the molding zone 20 by at least an electromagnetic edge daln 15 that is
positioned to diiect 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 apparatLis. In a preferred enibodiment,
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 dain 15,
wherein the mechanical edge dani 55 is positioned proximate to the feed tip T
and the
electromagnetic edge dain 15 is positioned overlying the terminating end of
the
mechanical edge dain 55 and provides confinement forces along the entire
length of
the molding zone 20, as depicted in Figures 6 and 11.
[0052] 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 dazns.
In prior casting apparatus employing electroinagnetic edge dains, high
magnetic force
fields where required to contain the nzolten metal, which resulted in
induction heating
witliin the inolten metal that disadvantageously effected the solidification
process. In
the present invention, by reducing the magnitude of the required
electromagnetic
force, the current and/or ~iequency 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.
[0053] Without wishing to be boLuld, but in the interest,of fitrtlier
describing
the present invention, applicants' believe that the reduction in the
electromagnetic
18
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force that is requlred to contain the metal Wltllln the molding zone is
related to the
decreased head height 1-12 of the molten metal froni the feed tip T, as
depicted in
Figure 3, as opposed to the greater lieight H1 of the molten metal pool
disposed atop
the roller caster in prior vertical casting apparatuses, as depicted in Figure
1. As
discussed above, the height H1 (or depth) of the molten pool atop the
vertically
disposed casting rollers is approxiYnately 65% the heigllt of the casting
roller Rl, R2
and can range from 8 inches to 20 inches, as depicted in Figure 1. Referring
to
Figure 3, in the present invention, the height H2 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 soine
exainples 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"inolten inetal head".
[00541 The relationship between tlie height of the inolten metal head H2 and
the magnetic field density required for containing inolten aluminuin at
different head
level's is best described tllrough 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 - PgH2
where p is is the magnetic pressure in Pa, p is the density of the metal, g is
the acceleration of gravity and H2 is the height of the molten metal head. The
pressure produced by the molten metal head in turn deterinines the strength of
the
magnetic field that inust be produced by electrornagnetic edge contaiiunent
device
15 to contain the molten metal head witllin the molding zone 20. In the
present
'invention, the height of the nlolten metal head H2 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 vaiying heigllt HZ,
from
the feed tip T of the present horizontal roller casting apparatus 10, was
deterinined
19.
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using the above equation and is listed in the Table depicted in Figure 4. To
sulnmarize the pressure ranged from about 125 Pa for a metal head height H2 of
approximately 0.5 inches (12.7 nnn) to abotit 2,492 Pa for a metal head height
H2
of approximately 10 inclles (254 mni).
[0055] The pressure required to contain the molten metal head H2 within the
molding zone 20 is then used in the following equation to determine the
required
niagnetic field density (B):
p = B212 ga
where p is the magnetic presstire in Pa (Pascals), B is the magnetic field
density in T (Tesla) and o is the permeability of air (=4n xl0'7 H/ni).
Referring
to Figure 4, from the above equation, it is calculated that for a relatively
high
molten metal head heigllt H, for feed tip T delivery of approximately 254 mnr
(10
inch), the magnetic field density needed is 0.079 T (790 Gauss) and a molten
metal
head height H2 of approximately 12.7 min (0.5 inch), themagnetic field density
needed is approximately 0.0177 T. As illustrated in Figure 4, reducing the
molten
metal head height H2 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
einbodiment,
the electromagnetic edge daiil operates at approximately 2000 ampere turns
(i.e. a
coil of 10 ttirns drawing 200 A).
[0056] In another aspect of the present invention, the physical positioning of
the electromagnetic edge dain, the molten metal head heigllt and the strength
of the
nlagnetic 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 Ri, R2
sidewall. The
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strength of tlie magnetic field at different distances froin the face (edge)
of the roller
casters may be calculated by the following equation:
BL = ( o nI/1)/{(2D/H)sinh(L/1) + (w/1)cosh(L/l)}
where:
BL = magnetic field intensity at a distance L(in) in the gap from the roll
face.
nI = coil turns and current.
w = roll gap 1= 4( , 8w/2) in which r = relative pernieability of steel
caster roll (taken as 600), S= skin depth for steel (material of the caster
roll), and
w is the roll gap.
D = distance between electromagnet pole and the roll face.
H= height of magnet pole.
[0057] Referring to Figure 5, using the above equation, the magnetic field
strength was calculated and plotted as a ftinction of the frequency of current
(Hz)
couducted tlirot,igh the electromagnetic edge daln 15, in which the distance
at which
the magnetic field strength was calculated ranged from 10 imn to 80 inm inward
from
the sidewall of steel casting rolls (reference line 1= 10 nun, reference line
2= 20
nun, reference line 3 = 30 in, reference line 4 = 40 intn, reference line 5=
60 mm,
and reference line 6= 60 znin). In each of the calculations, the height (H) of
the
magnetic pole was set at 8 nun, the distance (D) between the electromagnetic
pole
and the roll face was set at 4 irun, and the roll gap (w) was set at 4 inin.
Additionally,
reference lines where plotted to indicate the mininluni the field strength
required to
contain a metal head having a height H2 equal to 250 inin (reference line 7),
150 inm
(reference line 8), 100 nnn (reference line 9), and 50 nun (reference line
11). The
plot depicted in Figure 5 illustrates that the 0.079 T field density required
for the 250
nun metal head 8 could be created by this electromagnet in distances as far as
20 mm
into the roll gap.
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[0058] The edge of the casting strip can therefore be contained inwards from
the casting roll Rl, R2 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 nun, can be
contained
in distances 40 trun or greater by the operation of this edge darn at 2000 amp
turns.
The range of coiitaixunent can be extended fi.irther, 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 t11e edge
dain.
[0059] It is fiirther noted that the plot depicted in Figtire 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.
[00601 In addition to the height of the molteil metal head and the magnetic
field density, the positioning of the electromagnetic edge dain witll respect
to casting
rollers may also be adjusted to provide electromagnetic force lines to confine
the
nlolten metal M within the inolding zone 20. Referring to Figure 6, the
electromagnetic edge dain 15 may be positioned wherein the poles of the
magnetic
member are aligned to the sidewalls 13 of the casting rollers Rl, R2. In one
embodiment, the electroinagnetic edge dani may be positioned wherein the poles
of
the magnetic meznber are distal from tlie sidewalls of eacll casting roller
RI, R2. In
the einbodiments of the present invention in which a horizontal belt casting
apparattis
is employed as depicted in Figure 2a, the electromagnetic edge dazn 15 inay be
positioned wherein each pole of the magnetic melnber is distal from to aligned
to the
adjacent sidewall of the casting belts Ba, B2. For the purposes of this
disclosure the
term "distal froin to aligned to the adjacent sidewall of the casting belts"
is intended
22
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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 enouph proximity to the sidewall of the
castings
belts to provide a sufficient magnetic field to contain molten metal within
the
molding zone.
[0061] The inventive electromagnetic edge dain will also perform in casters
with rolls made froni a non-magnetic (non-ferromagnetic) material, such as
copper.
However, when the rollers comprise a non-magnetic inaterial, the penetration
of the
magnetic field into the roll gap may be limited and thus contairunent will
typically
occur on a plaiie 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 contaiznnent, as depicted in Figure 8d.
[0062] It is noted that prior casting apparatuses typically shape the magnetic
poles of the electromagnetic devices and the casting rolls to focus the
nlagnetic field
towards the molding zone. In one exaniple, prior casting rollers employ lips
extending froni the sidewall of each roller and may have fiirther included
magnetic
poles having a geometry corresponding to the extending lips of prior casting
rollers.
Contraiy 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 danl. In one embodiinent of the present invention, the
sidewalls 113 of the casting rollers Rl, R2 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 dani 15
is in
close proximity to the casting rollers Rl, R2. The electromagnetic edge dam 15
may
23
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also be positioned distal from the casting roller's sidewall 113. Regardless
of the
positioning of.the electroinagnetic edge dam 15, the electromagnetic edge dam
15 is
positioned to provide sufficient electrolnagnetic force to contain the molten
metal M
within the molding zone 20.
[0063] The positioning of the edge dams 15 may be dependent on the current
or frequency utilized in the edge dam. For exaniple, lower currents may
provide
lower magnitude electrolnagnetic force line and therefore be more likely to
require
that the edge dam 15 be positioned in closer proxiunity to the molding zone
20. The
higher the current conducted through the electromagnetic edge dain the greater
the
inagnitude of the electromagnetic force lines and hence the father away the
electromagnetic edge dams may be positioned from the molding zone.
[0064] Referring to Figures 7a-7c, in one einbodiment, the positioning of the
electroinagnetic edge dain 15 and the inagnitude of the electromagnetic force
lines
are selected to fornl a substaiitially flat sidewall (Figure 7a), a convex
sidewall
(Figure 7b), or concave sidewall (Figure 7c) in the inolten inetal M witllin
the
molding zone 20. In one exaxnple, a current of 2200 Ainp/turns produces a
casting
strip having a concave sidewall; a current of 1200 .Axnp/turns produces a
casting strip
having a substantially flat or straigllt sidewall; aiid a current of on the
order of 1200
.Amp/turns produces casting strip having a substantially convex sidewall. It
is noted
that the above exainples 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 suffzcient contaimneilt forces to
the
molding zone 20 and does not result in excessive induction heating. In some of
the
prefelred-einbodiments of the present invention, in which the casting strip's
sidewall
is concave or convex, the curvahire of the sidewall may be defined by a radius
that is
approximately half the lnolten head height.
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[00651 In another embodiment, the electromagnetic edge dam 15 may be
configured to provide molten metal within the inolding zone having a convex
sidewall relative to the centerline of the molten metal M within the molding
zone 20.
Preferably, the sidewall of the nlolteli metal within tlle molding zone is
substantially
aligned to the planar surface of the roller casters, as depicted in Figures 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
inolten metal is confined interior to the edge of the roller casters, as
depicted in.
Figures 8b and 8d.
[0066] The electromagnetic edge dain's 15 structure is illustrated in detail
in
Figure 8a, representing a sectional view of the edge danl apparatus 15
illustrated in
Figure 2a. In the preferred embodinlent of the invention, the electromagnetic
edge
dam 15 is a magnet type of confinement system and includes a generally C-
shaped
magnetic meinber. The magnetic member 30 thus includes a core 32 having an
upper
arln 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
coinprising a coil wound about the core 32 of the magnetic meinber 30 to
establish an
induction coil. Thus, the winding is composed of a plurality of conductors
wotuld
about the core 32 of the magnetic member 30. The core wind'ulgs 3 8 about the
core
32 ean be, i.nade of solid metal such as copper wire.
[0067] Still referring to Figure 8a, the upper arzn 34 terininates in a pole
face
42 where as the lower arm 36 tertninates 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
CA 02625569 2008-04-09
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30 with its induction coil 38 pass froin one of the pole faces 42 to the other
pole face
44, as illustrated by the magnetic lines of force 48.
[0068] Figures 9a-9c illustirate 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. Figure 9a illustrates a cross section of a lnagnetic znember 30
wherein the
pole faces 42 and 44 have a negative angle relative to the ver-tical plane
substantially
perpendicular to the plane on which the casting is being drawn. Tlie negative
angle
nieans 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
inagnetic member shown in Figure 9c are stronger at the outside edge of each
pole
face than at the inside edge of each pole face. Figure 9b illustrates a cross
section of
a magnetic meinber 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 saine at
the inside
edge of each pole face and the otitside edge of each pole face. As a result,
the
magnetic field created by the magnetic member shown in Figure 9b is relatively
uniforin across each pole face. Figure 9c ilh.istrates a cross section of a
inagnetic
member 30 having pole faces 42 and 44 that are parallel in pc-u-t and not
parallel in
part. The inside region of the pole faces 42 and 44 have a negative angle
relative to
the horizontal.
[0069] In one embodinient of the present invention, the magnetic member 30 is
forined from a ferromagnetic material such as silicon steel, and can be formed
from a
solid piece of such ferroinagnetic material. Alternatively, the nlagiietic
meinber 30
can be formed frozn znultiple ferromagnetic materials, such as the split core
design
depicted in Figure 10. In another embodiment, the magnetic member 30 can be
26
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fornled frolll a series of lanlnlated ele111ents lnachlned a11d seCLlred
together using
111echalllcal 111ealls, a11 adhesive or like ffieans to yield the desired
confl.guratloll, as
depicted in Figure 11. In many instances, the use of such laininates is
preferable,
because laminates may serve to more uniformly distribute tlle flux lines in
the
magnetic member and reduce loss due to saturation of the magnetic meinber. In
addition, for a magnetic lnember made of laminated ferromagnetic material,
tlie
electrical energy dissipated as heat is also more evenly distributed atld more
easily
removed, particularly where the adhesive einployed to hold the laminate
elements
together has good thermal conductivity.
[00701 Referring back to Figures 8a-8d, siirrounding the magnetic inember 30
is an outer shield 50, whicll is preferably made of a material, and most
preferably a
metal, having structural rigidity and extreinely high electrical and-thernlal
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
xnagnetic ineznber while the good tliermal 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 chamlels therein or brazed tLibes
thereon to
distribute cooling fluid tllrough or at the surface of the outer shield to
fixrther aid in
the reinoval of heat generated by the electromagnetic field. For exainple, an
inlet.can
be employed to pass a cooling fluid tllrough the outer shield for removal from
a
discharge port wllen 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.
[0071] The electromagnetic edge dam employed in the practice of the present
inventioxi also includes an inner shield 56 dinzensioned to fit witllin the C-
shaped
27
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configuration of the nlagnetic nlelnber 30. The iiuier shield 561ikewise
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
meinber 30. In addition, it is also possible, and some times desirable, to
include
within the inner shield conduit ineans for the passage of a cooling fluid tl-
ierethrough
where it is desired to increase the ability to dissipate heat from the magnet.
It is also
possible to do away with the inrier shield; especially so when using grain
oriented
silicon steellaininates wllere the field lines prefer to flow within the
laminates.
[0072] The path of the magnetic field of the pesent invention is indicated in
Figures 8a thortiglz 8d. In Figure 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-ferroinagnetic (such as copper).
The field
creates the containinent forces on the end faces of the rolls. Figure 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 acliieved by the application of a feiromagnetic coating 200 of
sufficient
depth to the end faces and end of the casting surface of a non-ferxomagnetic
roll
material, as depicted in Figure 8d.
[0073] In designing the electromagnetic contaimnent apparatus employed in
the practice of this invention, a nuYnber of different tecbniques can be used
in
dissipating heat generated by the electromagnetic field. As shown in Figure
8c, the
windings 40 inay be fonned of an annular conductor having a central opening 41
extending tlierethrough. 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 Figure 12, the core 30 may also be equipped
witll
a cooling conduit 47 extending therethrougli; in that way, a cooling fluid can
be
28
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passed tlirough the cooling conduit 47 to aid in the dissipation of heat
generated by
the electromagnetic field.
[0074] Figure 12 illustrates one preferred embodiment of the present
invention, wherein a mechanical edge dain 55 is used in conjunction with an
electromagnetic edge dam 15 having a magnetic nzeniber 30. The magnetic
znember
30 is preceded by the mechanical edge ciam 55. The mechanical edge dam 55
shown
should ideally have a cerailiic-less surface and comprise magnetic material to
reduce
the reluctance at the mouth of the molding zone. A ceramic material inay also
be
used to inake Ynechanical edge dam 55 if process conditions preclude the use
of a
metallic material. In one embodiment of the present invention, the mechanical
edge
danz 55 is positioned to ensure that the molten metal is contained witllin 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, containtnent forces are provided by the
electroli'lagnetic edge dam 15. In this arrangement, the seivice life of the
mechanical
edge dain 55 is increased by the electromagnetic edge dani 15, since the
electromagnetic edge dam 15 is positioned in the most hostile portion of the
casting
apparatus.
[0075] The followhlg ea.amples are provided to fiirtlier illustrate the
present
invention and delnonstrate some advantages that arise therefrom. It is not
intended
that the invention be liunited to the specific exainples disclosed.
EXAMPLE 1- CONFIRMATION OF ELECTROMAGNETIC PUSH
[0076] Aluminum strip was cast in accordance with the present invention using
a caster with steel rolls. The strip was then metallograpliically tested to
confirln the
effect of the electromagnetic force on the molten metal witllin tlie molding
zone.
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Test specimens were forined using a horizontal roller caster and a combination
of
electrolliagnetic and nlechanical edge dains consistent witll the present
disclosure.
Casting strips of three different thicknesses (2.44 nun, 2.291xun, and 2.16
inln) were
then cast operating the electroinagnetic edge dam at 2180 A turns. Sanlples
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 vvas
pushed
inwards as compared to the outer surfaces of the strip, as shown in Figures
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.
[0077] 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 ixun. From this measurement, the
width
of the strip within the inolding zolie can be estimated as 406 rnnl by adding
the
contraction that occurred during solidification and cooling to room
tenlperature.
[0078] Taking into account that the width of the casting roll is approximately
432 nun, it is evident that the magnetic field pushed the molten center of the
casting
strip a distance of approximately 13 mm (13 nu11= (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 widtli of the casting strip in the molding zone
from the
width of the casting roller the total displaceinent produced by the
electromagnetic
edge dains is calculated. The amount of displaceinent produced by a single
edge dam
is calculated by the nunlber of edge dains ernployed, which in this
experiinent
consisted of two electromagnetic edge danis positioned at opposing ends of the
casting rollers.
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[0079] SinZilar electroinagnetic push effects were observed for all three
different strip tliiclaless (strip gauge), as suirunarized in the Table
depicted in Figure
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 lnagnetic push
between
the two sides (drive side and operator side) of the caster rolls, as
summarized in
Figure 13, is attributed to variations in the location of the electroinagnets
and the
mechanical edge dains.
EXAMPLE 2- CONTROL OF CAST STRIP EDGE PROFILE
[0080] The edge profile of the as-cast strip was checked for operation at
different inagnetoinotive force levels in the electromagnet. The edge profile
obtained
at 2180 A ttirn operation shown in Figure 14 were considered unsuitable for
subseqtient rolling of the strip unless the edges were trinuned prior to
rolling. In order
to provide cast strip having edge profiles suitable for rolling without
additional
machining, the inagnetomotive force of the electroinagnet was reduced to
decrease
the push on the central portion of the castiilg strip so that the edge profile
of the strip
would be flat or slightly convex.
[0081] 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 electrolnagnet. To obtain a flat
edge
profile, the magnetic field should be selected to jtist offset the pressure
produced by
the molten metal in the molding zone, which is produced by the metal head with
a
minor contribtition small roll presstire. Referring to Figure 15, the edge of
the casting
strip made under these conditions was flat and highly straigllt indicating
that it could
31
CA 02625569 2008-04-09
WO 2007/053808 PCT/US2006/060215
be rolled without trimining the edges of the casting strip or other additional
macllining.
10082] This strip was rolled in-line successfi.illy through four stands of
rolling
mills. The casting strip was rolled fronl a 2.7 null (0.107 inch) as-cast tl-
iickness to a
thickness of approximatelyl 0.36 mm (0.014 inch), which corresponded to an 87
%
reduction in thickness. Referring to Figure 16, the sheet made by this method
showed
only minor cracks at the edges, wliicll could be removed by triinining prior
to coiling.
[0083] Following proper adjusttnent of the electromagnetic edge dam, high
quality edges are obtained in the as-cast strip which perinits rolling to
higli reduction
ratios saving materials and ixnproving the efficiency of the process.
[0084] While the present invention has been particularly shown and described
witll respect to preferred enlbodiments 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 forins and
details
described and illustrated, but fall within the scope of the appended claims.
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