Note: Descriptions are shown in the official language in which they were submitted.
1
SYSTEM AND METHOD FOR CONTINUOUS CASTING
[00011
FIELD OF THE INVENTION
[0002] The present invention relates generally to continuous casting of
metals
and, more particularly, to a twin belt casting system and method for
continuous casting
of metals.
BACKGROUND OF THE INVENTION
[0003] Continuous casting of light metal alloys such as, for example,
aluminum
alloys, has typically been performed in continuous casters, such as twin roll
casters and
twin belt casters. Twin roll casters generally include a pair of opposed,
rotating rolls
against which molten metal is fed. The centerlines of the rolls are in a
vertical or
generally vertical plane that passes though a region of minimum clearance
between the
rolls, referred to as the "nip", such that the cast strip forms in a generally
horizontal
path, although other twin roll casting apparatuses exist that produce strips
in an angled
or vertical direction.
[0004] As shown in FIG. 1, twin belt casters, on the other hand, such as
twin belt
casting apparatus 10, generally include a pair of endless belts 12, 14 carried
by a pair of
upper pulleys 16, 18 and a corresponding pair of lower pulleys 20, 22.
(Pulleys 16 and
20 are also referred to herein as nip pulleys or nip rolls. Pulleys 18 and 22
are also
referred to herein as downstream pulleys or downstream rolls.) The arrangement
of the
nip rolls 16, 18 and 20, 22 one above the other defines a mold zone, A,
bounded by the
CA 3057381 2020-01-21
CA 03057381 2019-09-19
WO 2018/191098 PCT/US2018/026197
2
belts 12, 14. The gap between the belts 12, 14 determines the thickness of the
cast strip
24. Molten metal 26 fed directly via a feeding apparatus 28 having a nozzle 30
into the
nip is confined between the moving belts 12, 14 and is solidified as it is
carried along.
Heat from the solidifying metal is withdrawn into the portions of the belts
12, 14 which
are adjacent to the metal being cast by various means known in the art.
[0005] While existing twin roll casting systems and twin belt casting
systems are
generally suitable for what can be regarded as ordinary performance,
improvements in
terms of minimum strip thickness and metallurgical quality, including surface
quality,
are desired without sacrificing productivity. For example, with twin roll
casting, where
metal is cast against the opposed nip rolls, the length of the mold is limited
to a short
distance prior to the tangent point of the opposed rolls, the diameters of
which are
limited by practical considerations such as the space that must be made
available for the
feeding apparatus. These upper limits on the diameter and circumference of the
rolls
limits casting speed, roll life and metallurgical quality.
[0006] With twin belt casting, as discussed above, molten metal is
typically fed
onto the belt at or just after the tangent point where the belts transition
from the curved
path defined by the nip rolls or pulleys to the planar path of the mold
region. Although
the belts allow for an extended mold length as compared to twin roll casting,
initial
solidification occurs in the zone immediately following the nip, where the
belts are the
most unstable. In particular, with reference to FIG. 2, a phenomenon known as
belt
"take-off" can occur in this zone 34 (referred to as belt take-off zone) as
the belt 14
transitions from a curved path of travel around the nip roll 20 to a planar
path of travel
in the mold zone where the belts 12, 14 are supported by backup rolls 32. As
used
herein, "belt take-off" refers to the natural tendency of a tensioned belt to
come away
from its radiused or planar guide surface when subjected to a bending moment
or other
force. As will be readily appreciated, metallurgical quality may be negatively
impacted
in regions of belt instability, such as in this zone immediately following the
nip,
particularly when casting alloys having broad freezing ranges.
CA 03057381 2019-09-19
WO 2018/191098 PCT/US2018/026197
3
[0007] Moreover, in twin belt casting, wherein molten metal is fed into the
substantially parallel section of the mold, casting thicknesses are also
confined to
thicker sections, typically over 15 millimeters thick. Accordingly, additional
post-
casting operations such as rolling are often required to achieve thicknesses
less than 15
millimeters, which increases overall cost. In addition, the solidification of
the internal
layers of these relatively thick cast sections is slowed considerably by the
thermal
resistance of the surface layers, which can be particularly detrimental when
casting
alloys having a broad freezing range.
[0008] In view of the above, there is a need for a system and method for
twin belt
continuous casting of metals that enables thinner metal strips to be produced
and
achieves improved metallurgical quality, including surface quality, of the
cast strip than
has heretofore been possible with existing systems and apparatuses, without
sacrificing
productivity.
SUMMARY OF THE INVENTION
[0009] It is an object of the present invention to provide a twin belt
continuous
casting apparatus.
[0010] It is another object of the present invention to provide a twin belt
continuous casting apparatus that improves heat transfer rates throughout the
thickness
of the cast strip as compared to existing apparatuses.
[0011] It is another object of the present invention to provide a twin belt
continuous casting apparatus that produces thinner metal strips than has
heretofore
been possible.
[0012] It is another object of the present invention to provide a twin belt
continuous casting apparatus that improves metallurgical quality, including
surface
quality, of the cast strip.
CA 03057381 2019-09-19
WO 2018/191098 PCT/US2018/026197
4
[0013] It is another object of the present invention to provide a twin belt
continuous casting apparatus that facilitates the use of thicker belts than
has heretofore
been possible.
[0014] It is another object of the present invention to provide a method
for twin
belt continuous casting that minimizes belt take-off.
[0015] It is another object of the present invention to provide a method
for twin
belt continuous casting that enables the production of strips less than about
7
millimeters in thickness.
[0016] It is another object of the present invention to achieve the above
objectives
without sacrificing productivity.
[0017] These and other objects are achieved by the present invention.
[0018] According to one embodiment of the present invention, a continuous
casting apparatus for casting a metal strip is provided. The continuous
casting
apparatus includes a first belt carried by a first upstream pulley and a first
downstream
pulley, a second belt carried by a second upstream pulley and a second
downstream
pulley, and a mold region into which molten metal is supplied, the mold region
being
defined by a first mold support section arranged behind the first belt
intermediate the
first upstream pulley and the first downstream pulley and a second mold
support
section arranged behind the second belt intermediate the second upstream
pulley and
the second downstream pulley. The first mold support section supports the
first belt
and defines a shape of the first belt in the mold region and the second mold
support
section supports the second belt and defines a shape of the second belt in the
mold
region. At least one of the first mold support section and the second mold
support
section includes a transition portion and a generally planar portion
downstream from
CA 03057381 2019-09-19
WO 2018/191098 PCT/US2018/026197
the transition portion. The transition portion has a variable radius
configured to receive
molten metal from a metal feeding device.
[0019] According to another embodiment of the present invention, a method
for
continuous casting a metal strip is provided. The method includes arranging a
first belt
on a first upstream pulley and a first downstream pulley, arranging a second
belt on a
second upstream pulley and a second downstream pulley, forming a mold region
by
arranging a first mold support section behind the first belt intermediate the
first
upstream pulley and the first downstream pulley and arranging a second mold
support
section behind the second belt intermediate the second upstream pulley and the
second
downstream pulley, at least one of the first mold support section and the
second mold
support section having a curved transition portion downstream from the first
upstream
pulley and the second upstream pulley, and a generally planar portion
downstream
from the curved transition portion, and feeding molten metal onto the curved
transition
portion.
[0020] According to yet another embodiment of the present invention, a
continuous casting apparatus for casting a metal strip is provided. The
continuous
casting apparatus includes a first belt carried by a first upstream pulley and
a first
downstream pulley, a second belt carried by a second upstream pulley and a
second
downstream pulley, and a mold region defined by a first mold support section
arranged
behind the first belt intermediate the first upstream pulley and the first
downstream
pulley and second mold support section arranged behind the second belt
intermediate
the second upstream pulley and the second downstream pulley. The mold region
includes a first zone, a second zone downstream from the first zone, and a
third zone
downstream from the second zone.
CA 03057381 2019-09-19
WO 2018/191098 PCT/US2018/026197
6
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] The present invention will be better understood from reading the
following description of non-limiting embodiments, with reference to the
attached
drawings, wherein below:
[0022] FIG. 1 is a simplified schematic illustration of a prior art twin
belt caster.
[0023] FIG. 2 is a detailed, schematic illustration of a portion of a prior
art twin
belt caster, illustrating the phenomenon of belt take-off in a mold zone of
the caster.
[0024] FIG. 3 is a simplified schematic illustration of a twin belt casting
apparatus
according to an embodiment of the present invention.
[0025] FIG. 4 is an enlarged, detail view of a mold support section of the
twin
belt casting apparatus of FIG. 3, according to an embodiment of the present
invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0026] Referring to FIG. 3, a twin belt casting apparatus 100 according to
an
embodiment of the present invention is illustrated. As shown therein, the
casting
apparatus 100 includes a first endless belt 112 carried by a first upstream
pulley or roll
116 and a first downstream pulley or roll 118, and a second endless belt 114
carried by a
second upstream pulley or roll 120 and a second downstream pulley or roll 122.
Each
roll is mounted for rotation about its longitudinal axis and serves to rotate,
guide
and/ or tension the belts 112, 114. Either or both of the upper rolls 116, 118
and the
lower rolls 120, 122 may be driven by a suitable motor (not shown). The belts
112, 114
are endless and are preferably formed of a metal which has low reactivity or
is non-
reactive with the metal being cast. As illustrated in FIG. 3, the upstream
rolls 116, 120
are positioned one above the other, some distance apart to allow room for a
metal
CA 03057381 2019-09-19
WO 2018/191098 PCT/US2018/026197
7
feeding apparatus 128 to be positioned in the space, and define a plane P1
extending
through the respective tangents of the rolls 116, 120.
[0027] Molten metal 126 to be cast is supplied through the feeding
apparatus 128
having a nozzle 130 located so as to deliver a horizontal stream of molten
metal at a
point 129 downstream from the plane P1 into the mold region of the apparatus
100, as
discussed in detail hereinafter. In an embodiment, an edge containment means
that
eliminates the need for travelling edge dam blocks may be employed to contain
the
molten metal at the mold entry and/or throughout the mold region. For example,
stationary edge dams located between the first and second belts 112, 114 may
be
employed to effectuate side containment of the molten metal adjacent to first,
second
and/or third zones of a mold region of the apparatus, as discussed
hereinafter.
[0028] As further shown in FIG. 3, the casting apparatus also includes a
pair of
opposed mold support sections 132, 134 located along the path of the moving
belts 112,
114, which support the belts 112, 114, respectively, and define at least a
portion of the
path of travel of the moving belts 112, 114. The mold support sections 132,
134 define
therebetween a mold region 136 downstream from Pi. Importantly, the mold
region 136
is formed by separate mold support sections 132, 134 located distal from and
approximately mid-way between the upstream rolls 116, 120 and the downstream
rolls
118, 122, rather than in close proximity to the nip rolls 116, 120. As
discussed
hereinafter, one or both of the mold support sections 132, 134 may include
curved
sections of large radii that support the belts 112, 114 upon which the molten
metal 126 is
fed. This configuration allows a belt, even when lightly tensioned about the
mold
support sections 132, 134, to inherently exert an effective hold-down force
that conforms
the belt shape to the shape of the curved mold support sections 132, 134.
While the
embodiments herein show the supporting structure that supports the moving
belts and
defines the shape of the moving belts in the mold region 136 as solid "mold
support
sections" other supporting devices such as an array of backup rolls or platens
may also
be utilized to define the support the moving belts 112, 114 and define the
shape of the
CA 03057381 2019-09-19
WO 2018/191098 PCT/US2018/026197
8
moving belts 112, 114 in the mold region 136 the without departing from the
broader
aspects of the present invention.
[0029] With reference to FIG. 4, one or both of the mold support sections
132, 134
may include a first, small radius portion 138 defining a first zone (Zone I)
of the belt
pass, a second, large radius transition portion 140 adjoining the small radius
portion 138
and defining a second zone (Zone II) of the belt pass, and a third,
substantially planar
portion 142 adjoining the large radius portion 140 and defining a third zone
(Zone III)
of the belt pass. In an embodiment, the small radius portion 138 and the large
radius
portion 140 may have a radius from about 0.4 meters to about 1.5 meters, where
the
large radius portion 140 has a radius that is different from, and larger than
a radius of
the small radius portion 138. In an embodiment, the small radius portion 138
may have
a constant or variable radius of curvature from about 0.3 meters to about 1
meter, and
the large radius portion 140 may have a constant or variable radius of
curvature from
about 0.5 meters to about 25 meters. In an embodiment, the large radius
portion 140
may have a radius of curvature that increases (as slope decreases)
progressively from
the small radius portion 138 to the planar portion 142 (i.e., a variable or
changing radius
of curvature). In an embodiment, the large radius portion 140 defining Zone II
of the
belt pass may have a radius of curvature that changes continuously from the
upstream
end to the downstream end.
[0030] Importantly, the presence of a large radius portion or section 140
(i.e.,
Zone II) near the transition to the planar portion or section 142 of the mold
136
eliminates or substantially reduces the possibility of belt take-off at the
tangent of the
comparatively small, fixed-radius roll 120 (or its equivalent) where the belt
transitions
from a curved to planar path, and at least separates the mold entry point 129
where
molten metal is first supplied away from any area of the apparatus 100 where
belt take-
off is possible. Furthermore, the geometry of the curved portions of the mold
support
sections 132, 134 functions to support the belt 114 (or 112) in what has
heretofore been
the unsupported belt take-off region 34. As a result, the very stable nature
of this mold
entry region (including mold entry point 129) where the molten metal is fed
allows
CA 03057381 2019-09-19
WO 2018/191098 PCT/US2018/026197
9
casting at thicknesses that are as much as an order of magnitude thinner than
is
typically possible on existing twin belt casters. For example, the
configuration of the
twin belt casting apparatus 100 of the present invention allows for the
casting of thin
cast sections under approximately 7 millimeters thick and, more preferably
under
approximately 5 millimeters thick, which has heretofore not successfully
achieved on
existing twin belt casting apparatuses.
[0031] Moreover, the small radius portion 138 (Zone I) preceding the large
radius
portion 140 (Zone II) accommodates the metal feeding apparatus 128 and
associated
supporting structures.
[0032] Zone III, defined by the planar portion 142 of the mold support
sections
132, 134, for its part, performs the functions of mold forces control, cooling
control, and
belt-stabilization from thermo-mechanica I forces.
[0033] In an embodiment, the radius of the respective zones of the mold
support
sections 132, 134 may be based on a mathematical function such as a parabola,
hyperbola or other higher order functions. In an embodiment, concatenating
several
sections may include bringing different forms together in a tangential manner,
utilizing
variable radiuses, continuous radiuses, and intermittent straight sections. In
an
embodiment, the shape and contour of the mold support sections 132, 134 may be
designed to match the natural contour of the belt in the belt take-off zone 34
during
operation (which may be dependent upon the level of heat input, speed/
dynamics,
tension level, belt thickness, belt material, alloy/ solidification nuances,
etc). In certain
embodiments, the mold 136 may be constructed so that its physical shape may be
varied while casting metal or in-between casting campaigns. In an embodiment,
the
upper mold support section 132 may have a shape, contour or configuration that
is
different than the lower mold support section 134.
[0034] It is further contemplated that the radius of the converging belts
112, 114
may be increased or decreased (by increasing or decreasing the radius of the
radiused
CA 03057381 2019-09-19
WO 2018/191098 PCT/US2018/026197
portion 138 of the mold support sections 132, 134) to accommodate moving the
solidification zone further into the apparatus 100 or bring it closer to the
metal feeding
tip 130. In an embodiment, the generally parallel, planar portion of the mold
136,
defined by the opposed planar portions 142 of the mold support sections 132,
134, could
be tapered slightly and adjusted as needed to provide even cooling from both
belts as
the strip 124 shrinks without inducing hot-work to the cooling metal. In an
embodiment, the upper or lower mold support section 132, 134 may be spring
loaded or
otherwise biased towards the other of the upper of lower mold support section
(e.g.,
mechanical, fluid, electric, etc.). The exit end of the mold could also be
adjusted to
shorten or lengthen the effective cooling region of the casting apparatus 100
without
having to alter casting speed.
[0035] In connection with the above, in operation, molten metal 126 is fed
onto
the belts 112, 114 in a zone where the tensioned belts, supported on a
comparatively
large radius by means other than by nip rolls, are converging. For example, in
an
embodiment, the molten metal 126 is fed onto the large radius portion of the
belt path
defined by large radius portion 140 (Zone II) of the mold support sections
132, 134. The
combination of belt tension and the curvature of the belt provided by the
supporting
profile of the mold support sections 132, 134 provides a very stable belt
condition in the
zone where initial solidification occurs. Thinner strips may therefore be cast
at higher
solidification rates, achieving metallurgical improvements compared to
existing twin
belt casting machines, especially for broad freezing range alloys. In
addition, the ability
to cast thinner strips reduces or eliminates the requirement for subsequent
rolling to
finished gauge, which reduces both capital and operating costs.
[0036] In addition to the above-described benefits, the casting apparatus
100 of
the present invention also enables the use of much thicker casting belts as
compared to
the casting belts utilized on existing belt casters with comparatively small,
fixed-
diameter nip pulleys or their equivalent. In particular, practical belt
thicknesses are
limited by the minimum radii that it must conform to under tension. Generally,
this
means that the diameter of the pulleys (or their equivalent) on belt casting
machines
CA 03057381 2019-09-19
WO 2018/191098 PCT/US2018/026197
11
must be approximately 400 - 600 times the thickness of a high-strength low
alloy steel
belt at ambient temperatures. Any smaller a ratio and the outer fibers of the
belt can be
stressed beyond their yield point. For a 1.2 millimeter thick belt, this
translates to a
pulley diameter of 600 millimeters (0.6 meters). Under conditions of high heat
transfer,
the outer fibers of the steel belt are further stressed, requiring even larger
pulley radii.
[0037] By utilizing mold support sections 132, 134 having a large radius
portion
140, and feeding onto such large radius portion 140 rather than the smaller
radius
pulley or nip rolls, thicker belts may be utilized than has heretofore been
possible. This
is particularly desirable because thicker belts have a higher heat capacity
and promote
higher heat transfer rates, which are helpful particularly when casting broad
freezing
range alloys. By combining thin cast sections, e.g., less than about 7
millimeters thick,
while utilizing thick belts, e.g., approximately 2 millimeters or more, heat
transfer rates
of an order of magnitude greater than are typical on existing belt casters can
be
achieved while maintaining belt stability. In an embodiment, the belts may be
in the
range of about 1-4 millimeters thick. This, in turn, allows very broad
freezing range
alloys to be cast on twin belt casters at high production rates, with superior
metallurgical and surface qualities.
[0038] In addition to the advantages described above, utilizing the mold
support
sections 132,134 to support the moving belts and to form the mold region 136
downstream from the upstream pulleys al lows the belts to expand and contract
on the
essentially frictionless supporting mold support sections. This is in stark
contrast to
existing devices where expanding and contracting of the moving belts on the
rotating
entrance/ upstream pulleys can contribute to instability. Indeed, the present
invention
essentially separates the mold region 136 from the upstream pulleys or rolls
which
drive the belts.
[0039] While the embodiments described above disclose that the mold
sections
132, 134 include first and second radiused portions that lead to a generally
planar
portion, it is contemplated that the mold sections 132, 134 may alternatively
be formed
CA 03057381 2019-09-19
WO 2018/191098 PCT/US2018/026197
12
with a single curved or radiused portion upstream from the generally planar
portion
onto which the molten metal is fed. In an embodiment, this radiused,
transition portion
may have a radius that increases progressively from an upstream end of the
mold
section to the planar portion of the mold section. In yet other embodiments
the mold
sections 132, 134 may have more than two distinct radiused or curved portions,
either
with constant or variable radius, such as three, four, five, or more radiused
portions
leading up to the generally planar portion.
[0040] In connection with the above, certain combinations of thicker belts
and
thinner cast strips allow for the use of the natural thermal capacitance of
the belt as a
conductive cooling means at levels considerably higher than that experienced
in
existing casting systems, which allows for more rapid solidification of the
cast strip. In
prior art systems, heat is actively removed from the belt in, and proximate
to, the mold
zone due to the limited proportion of thermal capacity of thinner belts (e.g.,
about less
than ¨1.2 millimeters) with respect to thicker strips (e.g., in excess of
about 15
millimeters). Conversely, a more advantageous proportion of thermal capacity
is
offered by thicker belts (up to about 4 millimeters) casting thinner strips
(between about
2-6 millimeters), as contemplated by the present invention, which enables belt
thermal
conduction to more rapidly accomplish initial solidification of the cast
strip.
Accordingly, heat removal from the belt may then be accomplished either by a
combination of belt cooling both proximate to and remote from the mold region,
or
entirely remote from the mold region.
[0041] Although this invention has been shown and described with respect to
the
detailed embodiments thereof, it will be understood by those of skill in the
art that
various changes may be made and equivalents may be substituted for elements
thereof
without departing from the scope of the invention. In addition, modifications
may be
made to adapt a particular situation or material to the teachings of the
invention
without departing from the essential scope thereof. Therefore, it is intended
that the
invention not be limited to the particular embodiments disclosed in the above
detailed
CA 03057381 2019-09-19
WO 2018/191098 PCT/US2018/026197
13
description, but that the invention will include all embodiments falling
within the scope
of this disclosure.
