Note: Descriptions are shown in the official language in which they were submitted.
CA 02596473 2007-07-31
WO 2006/089419 PCT/CA2006/000267
1
METHOD OF AND MOLTEN METAL FEEDER FOR CONTINUOUS CASTING
TECHNICAL FIELD
This invention relates to the continuous casting of molten metals, preferably
aluminum and aluminum alloys. More particularly, the invention relates to a
method of
introducing the molten metal into the casting cavity of a continuous caster
and the
design of a metal feeder used for this purpose.
BACKGROUND ART
Continuous casting of metals has been carried out for many years, e.g. by
using
a twin belt caster, twin roll caster or rotating block caster. Continuous
casters of this
kind usually have a horizontal, or slightly downwardly-sloping, casting cavity
formed
between two confronting and continuously rotating casting surfaces. The molten
metal
is introduced into one end of the casting cavity and it is cooled and
solidified as it is
drawn through the casting cavity by the rotating casting surfaces. A cast
ingot, slab or
strip of solidified metal emerges from the casting cavity at the opposite end.
Molten metal is introduced into the casting cavity by some form of molten
metal
feeder that introduces a stream of molten metal between the casting surfaces.
The
feeder may be in the form of an open topped trough, in which molten metal is
directed
by means of an open spout or channel into the casting cavity (referred to as
"pool
feeding"), or more preferably by means of a nozzle which encloses and confines
the
molten metal until it emerges from a tip at the extreme end of the nozzle.
As-cast ingots produced by both DC (direct chill) and continuous strip casters
produce metal slabs or strips having surface defects of various kinds. In DC
casting
such surface defects are often renloved by means of "scalping" (i.e. removing
a thin
surface layer from the cast article). However, in continuous strip casting,
scalping may
not be practical or economical and it is desirable to provide an article at
the outset
having a minimum of surface defects.
Surface defects may be produced by a variety of mechanisms, including reaction
with the refractory materials of the metal delivery system and localized
cooling non-
CA 02596473 2007-07-31
WO 2006/089419 PCT/CA2006/000267
2
uniformities, and many improvements have been made to reduce the size and
number of
such defects.
Another common mechanism involves entrainment of surface oxides that form
"cold shuts". Such defects arise from the inevitable surface oxides that form
on the
meniscus surface of the molten metal where it exits the metal feeder to
contact the
moving casting surface. As the meniscus is dragged along by the moving casting
surface, the oxide film becomes strained and breaks, causing relatively large
and visible
surface defects of an irregular nature. This not only affects the appearance
of the cast
article, but also can introduce structural weaknesses that cause rollability
problems.
The defects are particularly critical in surface critical applications such as
foil stock, can
stock and automotive sheet, and can limit the speed of casting.
There are various prior references disclosing feeder design and methods of
metal introduction into a continuous casting cavity, e.g. U.S. Patent
5,636,681 which issued
on June 10, 1997, and U.S. Patent 6,725,904 which issued on April 27, 2004.
These
patents disclose feeder designs that are intended to produce non-turbulent
metal flow
into the casting cavity.
European Patent No. EP 0 962 271 B 1, which was granted on December 17,
2003 to Hazelett Strip-Casting Corporation (inventor Valerie G. Kagan)
discloses a belt
casting apparatus with a metal delivery device that "pours" the metal onto a
belt. The
tip of the delivery device is spaced a distance away from the surface of the
belt and it
terminates at an end surface disposed at right angles to the metal-contacting
inner
surface of the delivery device.
U.S. Patent 4,648,438 which issued on March 10, 1987 to Hazelett Strip-Casting
Corporation (inventor Robert W. Hazelett, et al.) discloses a belt caster and
metal
delivery device in which the end of the tip is "squared" and is arranged at
right angles to
the casting surface.
The following are examples of strip casters having tips in which the interior
of
the tip is tapered in the direction of the tip:
U.S. Patent 3,774,670 which issused on November 27, 1973 to Prolizenz AG
(inventor Ivan Gyongyos); U.S. Patent 5,660,757 which issued on August 26,
1997 to
CA 02596473 2007-07-31
WO 2006/089419 PCT/CA2006/000267
3
Hunter Engineering Co., Inc. (inventor Denis M. Smith); and U.S. Patent
6,173,755
which issued on January 16, 2001 to Aluminum Company of America (inventor Nai-
Yi
Li, et al.).
DISCLOSURE OF THE INVENTION
An object of the present invention is to improve the continuous casting of
molten metal, especially molten aluminum and its alloys, particularly with a
view to
reducing surface defects of the cast article and more particularly to reduce
the incidence
of oxide incorporation into the cast surface.
According to one aspect of the present invention, there is provided a feeder
for
delivery of molten metal into a mold formed between confronting casting
surfaces of a
continuous casting machine. The feeder comprises a nozzle having a projecting
tip
including at least one wall provided with a molten-metal-contacting inner
surface, an
outer surface, and an end surface at an outer extremity of the tip extending
between the
inner and outer surfaces. The inner surface and the end surface interconnect
at a line
and form an included angle of less than 88 , and the wall of the nozzle has a
thickness
adjacent to the line in the range of 0.5 to 3 mm.
According to another embodiment, there is provided a continuous casting
machine comprising a pair of endless casting surfaces confronting each other
across a
casting cavity, means for moving the casting surfaces in the same direction at
the same
speed, and a feeder for introducing molten metal into the casting cavity at
one end of the
cavity. The feeder comprises a nozzle having a projecting tip including at
least one wall
provided with a molten-metal-contacting inner surface, an outer surface, and
an end
surface at an outer extremity of the tip extending between the inner and outer
surfaces.
The inner surface and the end surface interconnect at a line and form an
included angle
of less than 88 degrees. The wall has a thickness adjacent to the line in the
range of 0.5
to 3 mm, and the line is positioned during casting at a spacing from an
adjacent casting
surface within the range of 0.5 to 3 mm.
According to yet another embodiment of the invention, there is provided a
process of continuous strip casting a molten metal to form a cast metal strip
article. The
process comprises feeding a molten metal which develops an oxide layer when in
CA 02596473 2007-07-31
WO 2006/089419 PCT/CA2006/000267
4
contact with air from a nozzle having a projecting nozzle tip onto at least
one moving
casting surface such that the metal forms a meniscus having a surface coating
of metal
oxide between an extremity of the tip and the casting surface. The metal is
fed from a
tip having a wall provided with an inner surfac.e and an end surface that
interconnect at
a line and form an included angle of less than 88 degrees. The wall has a
thickness
adjacent to the line in the range of 0.5 to 3 mm. The tip is positioned during
casting
such that the spacing of the line from an adjacent casting surface is within
the range of
0.5 to 3 mm.
The present invention is concerned with obtaining a continuously cast strip
article of good surface quality. As the inventors obtained improvements in
surface
quality by making general improvements to the casting technique and apparatus,
they
noticed the presence of periodic surface striations extending across the cast
article at
right angles to the direction of casting. The inventors found that these
striations were
due, at least in part, to oscillations of the meniscus formed between the
casting tip and
the casting surface. When casting reactive metals, such as aluminum, the
meniscus is
coated with a layer of metal oxide and the oscillation of the meniscus can
cause this to
break. The underlying metal thus exposed rapidly grows a new layer of oxide
upon
reaction with air, but the break forms a visible defect in the surface of the
cast product
as the oxide layer is drawn onto the casting surface. It is tlieorized that
meniscus
oscillations are inherent in continuous casters, at least in belt casters used
for casting
aluminum and its alloys, as well as other reactive metals, and that they
cannot be
entirely eliminated. The inventors therefore took another approach, i.e. of
increasing
the uniformity of the oscillations to produce a cast article having small,
regularly spaced
striations that do not manifest themselves as surface defects because of their
regular and
fine appearance. In particular, it was found that an oscillation of the
meniscus of at
least 50 Hertz, e.g. in the range of 50 to 200 Herz, was required to impart an
acceptable
appearance to the cast product.
The meniscus tends to oscillate at right angles to its surface, i.e. it tends
to
become more rounded and then less rounded in the region extending from the
nozzle to
the casting surface with each such cycle representing one oscillation.
The inventors found that the frequency of the meniscus oscillations can be
affected by various parameters, e.g. the application of external forces, such
as
CA 02596473 2007-07-31
WO 2006/089419 PCT/CA2006/000267
pneumatic pressure in the small gap between the nozzle and the casting belt,
and the
application of a varying magnetic field in the area of the meniscus. However,
the
inventors found that the most effective way to increase the frequency of
oscillation is by
improving the design and placement of the nozzle used for injecting the molten
metal
5 onto the casting surface.
With regard to the design of the casting nozzle, the conventional nozzle
employs
a pair of projecting walls that define a molten metal channel between
confronting inner
metal-contacting surfaces. The channel has an exit at the tip of the nozzle
where the
projecting walls terminate at a flat end surface that extends at riglit angles
to the inner
metal-contacting surfaces. The walls also have outer surfaces that in use
extend along
the casting surfaces with a small gap. Using this type of nozzle, it was
noticed that not
only was the frequency of oscillation of the meniscus slow and erratic, but
the
oscillations bring the metal into contact with the end surfaces of the nozzle
walls,
causing oxide whiskers to form and build up, thereby causing further sticking
and
interference with the oscillations. The inventors found that this effect could
be
minimized or eliminated by "cutting-back" the end surface by a few degrees or
more.
This means that the end surface is caused to slope in a rearward direction
from the line
of contact with the inner metal-contacting surface to form an included angle
(the angle
within the material of the nozzle wall) of less than 88 degrees, more
preferably 85
degrees or less, even more preferably 80 degrees or less, and most preferably
75 degrees
or less. The minimum angle is preferably 15 degrees, because a smaller angle
may be
currently impractical for constructing the nozzle (although the desirable
effect would
still be apparent if the constructional limitations could be overcome). A more
desirable
lower limit for the cut-back angle is 30 degrees. An even more preferred lower
limit is
45 degrees. Most preferably, the end surface of the nozzle wall is flat along
its full
length, i.e. from the intersections with the inner and outer walls, and from
side to side
across the nozzle. This has the advantage of making the striations more
regular,
particularly across the width of the casting cavity.
The line of contact mentioned above (the apex of the included angle) forms a
so-
called "take-off point" for the meniscus, i.e. the point at which the molten
metal loses
contact with the nozzle and is briefly supported by surface tension before
contacting the
casting surface. It is necessary to provide such a line (i.e. an abrupt change
of direction
CA 02596473 2007-07-31
WO 2006/089419 PCT/CA2006/000267
6
where generally planar surfaces meet at an acute angle). Witliout limitation
as to the
theory of operation of the invention, it appears that the line and the acute
angle at this
position fixes the point of metal departure from the tip so that it does not
wander onto
the end face of the nozzle wall. Oscillations of the metal instead appear to
be localized
at the take-off line where they break the oxide layer on a regular and
frequent basis as
the metal leaves the tip, thereby causing regular and fine striations in the
final product.
Again without limitation as to theory, it seems that the amplitude of the
meniscus oscillations increases with casting speed (i.e. the speed of the
casting belt). A
greater amplitude of the oscillations increases the risk of meniscus "wander"
onto the
end face of the nozzle, so the "cut-back" angle may desirably be decreased as
the
casting speed increases (i.e. the angle of the end surface with the inner
surface should
desirably be made smaller, e.g. within the range of 15 to 80 degrees). For
fast casting,
the angle should preferably be made no larger than 75 degrees, and 70 degrees
or even
65 degrees is a more preferred upper limit.
Ideally, the meniscus should be caused to oscillate at a frequency of 50 herz
to
about 200 Herz at least for aluminum and aluminum alloys. The range of
frequencies
depends on physical properties of the metal, e.g. density, viscosity and
surface tension,
but only for significant changes in these properties. The variation among
aluminum
alloys is quite minor, but a change of base metal (e.g. aluminum to copper)
may
produce significant changes that affect the oscillations more noticeably.
In the present invention, it is found that the spacing between the "take-off
point"
and the casting surface should be quite specific. If it becomes too large, it
is difficult or
impossible to maintain a stable meniscus as it may "wander" onto the end wall
of the tip
and the metal may run back under the tip of the nozzle. The flow
characteristics change
to become more like pouring a liquid rather than casting at speed. However,
the spacing
should be large enough to allow a meniscus to form between the take-off point
and the
casting surface. The minimum distance is controlled by restrictions placed on
the tip by
methods of construction and the need for the tip to be spaced slightly from
the casting
surface. The preferred spacing (take-off line to casting surface) is about 1
mm 0.5
mm. However, the invention is effective with normal nozzle wall thicknesses
(usually
about lmm or 1/32 inch) and a spacing up to about 3 mm.
CA 02596473 2007-07-31
WO 2006/089419 PCT/CA2006/000267
7
The metal can be fed into a closed nozzle or to an open-topped nozzle from a
conventional head box or tundish. The present invention may be used with both
types
of nozzle, but a closed nozzle is preferred.
When the nozzle is of the closed type, the two walls forming the nozzle may be
flat and parallel throughout their length, or they may be "flared" or
"divergent" at the
end, i.e. with the walls adjacent to the metal delivery end bending outwardly
at an angle
of usually no greater than about 8 degrees. This allows the walls to converge
towards
the casting surfaces by a small angle at the extreme end of the tip.
The present invention may be used with both horizontal and vertical continuous
casting machines, e.g. twin-belt casters, revolving block casters and even
twin-roll
casters (twin-roll casters are preferably operated at high speed when the
invention is
employed).
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a side view, partly in cross-section, of a twin-belt caster (without
metal
feed apparatus) of a type with which the present invention may be employed;
Fig. 2 is a cross-section of a metal feeder and adjacent parts of a twin-belt
caster
of a type with which the present invention may be employed;
Fig. 3 is a cross-section of a part of a prior art nozzle and an adjacent
casting
belt and molten metal flow, showing the development of a metal meniscus;
Fig. 4 is a view similar to Fig. 3, but showing a part of a nozzle in
accordance
with the present invention;
Fig. 5 is a top plan view of a test device used in the Example described
below;
Fig. 6 is a cross-sectional view of the test device of Fig. 5;
Figs. 7A, 7B, 7C, and 7D show cross-sections of tips used in the Example 1
described below;
CA 02596473 2007-07-31
WO 2006/089419 PCT/CA2006/000267
8
Fig. 8 shows a macrophotograph of the surface of an aluminum alloy strip cast
with a prior art nozzle having an angle between inner surface and end surface
of 93
degrees;
Fig. 9 shows a macrophotograph of the surface of an aluminum alloy strip cast
with a nozzle in accordance with the present invention having an angle between
inner
surface and end surface of 88 degrees;
Fig. 10 shows a macrophotograph of the surface of an aluminum alloy strip cast
with a nozzle in accordance with the present invention having an angle between
inner
surface and end surface of 78 degrees;
Fig. 11 shows a macrophotograph of the surface of an aluminum alloy strip cast
with a nozzle in accordance with the present invention having an angle between
inner
surface and end surface of 48 degrees; and
Fig. 12 shows a macrophotograph of the surface of an aluminum alloy strip cast
with a nozzle in accordance with the present invention having an angle between
inner
surface and end surface of 33 degrees.
BEST MODES FOR CARRYING OUT THE INVENTION
As noted above, continuous casting of metals to form strip articles (often
referred to as sheets, plates, slabs, ingots, billets, layers, etc.) has been
carried out for
many years and in many different types of continuous casting machines. For
example, a
twin-belt caster is disclosed in detail in U.S. Patent 4,061,177 which issued
on
December 6, 1977 to Alcan Research and Development Limited (inventor Olivo
Giuseppe Sivilotti), and casting machines of this kind (as well as others) are
suitable for
carrying out the present invention. The disclosure of this patent is
incorporated herein
by reference, and a brief and simplified description is provided below.
The illustrated twin belt caster 10 has upper and lower endless rotating metal
belts 12 and 14 arranged so that closely spaced moving confronting casting
surfaces 16, 18 of the belts are disposed essentially parallel to each other
through a
region where they define a casting cavity (casting mold) 20 from a cavity
entrance 21 to
a cavity exit 22. The belts are guided as they rotate through suitable oval or
otherwise
CA 02596473 2007-07-31
WO 2006/089419 PCT/CA2006/000267
9
looped return paths between the entrance and the exit of the casting cavity.
The upper
belt 12 passes around a cylindrical driving roll 24 and then travels along an
upper path
where it may be further supported, if desired, by rows of idler rollers or the
like (not
shown), and then around a semi-cylindrical bearing 25. The lower belt follows
an
essentially identical but mirror image path including a drive rol126 and a
semi-
cylindrical bearing 27 similar to the bearing located immediately above.
Molten metal
is introduced into the casting cavity by a feeder 30 (not shown in Fig. 1, but
illustrated
in Fig. 2) incorporating a nozzle 32 having a projecting tip 34 provided with
a molten
metal outlet 35 at the outer extremity (extreme end) 36 of the tip. Hence,
molten metal
enters the apparatus from the feeder 30 in the direction shown by arrow A
(Fig. 1), the
metal solidifies within the casting cavity 20 and a cast strip article emerges
from the
apparatus at the exit of the casting cavity in the direction of arrow B as
shown. The
reverse (inner) surfaces 17, 19 of the casting belt are generally cooled by
means of jets
of cooling water, e.g. as shown at 23.
Figure 2 shows an enlargement of the end of the caster adjacent to the
entrance 21 of the casting cavity 20. The belts 12 and 14 are shown in dash-
dot lines.
As previously noted, the apparatus is provided with a molten metal feeder 30
which
may be of the type disclosed, for example, in US patent No. 5,636,681issued on
June 10,
1997 to Alcan International Limited (the disclosure of which is incorporated
herein by
reference). The feeder 30 comprises top and bottom nozzle mounts 38 which hold
metal delivery nozzle 32 in place so that its tip 34 projects between the two
moving
belts 12 and 14 of the belt caster. The mounts 38 are bolted to the caster
structure (not
shown) and support the nozzle 32 such that an upstream opening 40 can mate
with a
similar opening in a tundish or feed-box (not shown) used to feed the caster
with molten
metal. A resilient refractory seal (also not shown) is used between upstream
faces 41 of
the nozzle and the tundish or feed-box. The nozzle 32 is fabricated from
refractory
materials, for example as described in US patent No. 5,636,681, and the tip 34
has a
slightly divergent shape as shown. Spacers 46, in the form or wire mesh or
metal strips,
are provided between the outer surfaces of the nozzle and the adjacent casting
belts to
maintain a fixed and controlled spacing between the nozzle and belts. The
nozzle
includes refractory walls 53 that have inner molten-metal-contacting surfaces
55
confronting each other across a molten-metal-conveying channel 50 leading from
the
CA 02596473 2007-07-31
WO 2006/089419 PCT/CA2006/000267
upstream opening to the metal outlet 35. The walls have end surfaces 56 that
interconnect with the inner surfaces 55 at lines 65. The walls also have outer
surfaces 54 that confront the casting belts 12 and 14.
The present invention is primarily concerned with the delivery of molten metal
5 into the casting cavity in the region of the nozzle tip 34. This is
explained in more
detail with reference to Figs. 3 and 4.
Figure 3 shows a conventional nozzle tip 34 in which only the lower wal153 of
the tip is shown adjacent the lower casting belt 14. The upper wall of the tip
and the
upper casting belt can be visualized as mirror images of the lower parts. The
illustrated
10 tip has an outer surface 54 extending generally parallel to the surface of
the adjacent
casting belt 14, a molten-metal-contacting inner surface 55, and a narrow end
surface 56
that is disposed at right angles to the inner surface 55 of the tip (as
indicated by the
small rectangle). In the delivery and casting of molten metals, a meniscus 58
(i.e. an
unsupported metal surface) bridges the gap between the nozzle tip 34 and the
adjacent
casting belt 14. For reactive metals (i.e. metals that spontaneously form an
oxide layer
when in contact with air) such as aluminum and its alloys, the meniscus is
covered by
an oxide layer 60. As the belt 14 moves through the casting cavity, the oxide
layer 60 is
drawn along by the belt by friction and is subject to stress in the region of
the meniscus.
It is found that the oxide layer on the meniscus will periodically rupture and
the exposed
metal will instantly form a new layer of oxide. The resulting oxide breakage
and re-
growth causes surface defects in the cast article. The inventors of the
present invention
have found that the effect of the inevitable oxide rupture on surface quality
can be
reduced or minimized by ensuring that the oxide membrane ruptures frequently
and in a
controlled manner rather than randomly.
In particular, the inventors have found that by causing the meniscus to freely
oscillate at a controllable frequency of at least 50 Hz, the effect of oxide
layer ruptures
is controlled and reduced. It has been found that a nozzle tip designed and
used in
accordance with the present invention, e.g. as shown in Fig. 4, will allow
such regular
oscillations and oxide rupture to occur. In this tip, angle a(referred to as
the "cut-back
angle" and also as the "included angle") between the inner surface 55 and the
end
surface 56 is an acute angle less than 88 , preferably between 15 and 85
degrees, more
CA 02596473 2007-07-31
WO 2006/089419 PCT/CA2006/000267
11
preferably between 15 and 80 , more preferably between 30 and 80 degrees, and
even
more preferably between 30 to 75 . When such an angle is employed, the
meniscus 58
is free to oscillate, and, absent any outside influences, takes on a"natural"
frequency
determined by the physical properties of the molten metal, the contact
friction between
the molten metal and the moving substrate (casting belt) and the spacing of
the tip to
belt distance (spacing "S" shown in Fig. 4). The use of the acute cut-back
angle a, in
combination with a precisely defined spacing S of tip to belt, means that the
geometry
of the meniscus is reliably controlled between the casting surface 61 of the
casting belt
12 and the line of intersection 65 (referred to as the "take-off point" or
"take-off line")
between the inner surface 55 and the end surface 56 of the nozzle tip, so that
the
frequency of oscillation is stable. The use of the acute cut-back angle a
ensures that the
final point of contact between the molten metal and the nozzle is confmed to a
fixed
position on the tip, namely the line of intersection 65. At this point, the
molten metal
surface transfers from a supported condition (supported by the nozzle) to an
unsupported condition (in the form of a meniscus) and the oxide film on the
metal
surface is repetitively ruptured along this line as the meniscus oscillates.
The oxide
rupture has the same regular frequency as the frequency of oscillation and
takes place in
small and regular breaks, thus creating regular and minimal defects on the
metal surface.
If the cut-back angle a is not acute (e.g. if it is 90 degrees as in a
conventional
nozzle tip), the meniscus 58 can touch the end surface 56 of the tip during
oscillation.
This rapidly forms oxide whiskers on the end surface of the tip and this in
turn causes
adherence of the meniscus to the end surface 56 below the line of intersection
65. This
adhesion is variable and prevents regular and free oscillations of the
meniscus from
occurring. The breaks in the oxide layer are consequently irregular and
delayed and the
resulting surface defects are larger.
The amplitude of the meniscus oscillations appears to be somewhat casting
speed related, i.e. larger amplitudes are encountered at higher casting
speeds. As larger
amplitudes can result in the meniscus being more difficult to fix on the
nozzle, it is
desirable to reduce the angle a at higher casting speeds and an angle no
larger than 75
degrees is usually desirable for high casting speeds. On the other hand, if
the cut back
angle is less than 15 degrees, it becomes difficult to construct a nozzle tip
having the
requisite stiffness and strength properties.
CA 02596473 2007-07-31
WO 2006/089419 PCT/CA2006/000267
12
The spacing S (which is the distance between the "take-off point" 65 and the
casting surface 61 of the casting belt at the point where the meniscus
contacts the
surface) affects the performance of the casting process. For most alloys and
casting
operations, the spacing S should be in the range of 0.5 to 3.0 mm. The spacing
S
includes the thickness of the lower wall 53 at the tip, and so this thickness
must
necessarily also be within the range of 0.5 to 3.0 mm. The most preferred
distance for
spacing S is 1+ 0.5 mm. This can be achieved, for example, by making the
sidewall 53
from an alumina-silicate fiberboard (e.g. Fiberfrax ) having a thickness of
lmin at the
tip that is slightly compressed with a rigidizer. As the material is slightly
compressed
and changes in thickness slightly during use due to thermal expansion, the
spacing S can
be maintained at about lmm with a slight gap forms between the sidewal153 and
the
casting surface. The indicated material has sufficient strength and rigidity
despite its
narrow thickness, and desirably it also relatively low thermal conductivity.
The invention is illustrated in further detail with reference to the following
Examples, which are not intended to be limiting of the scope of the invention.
EXAMPLE 1
The effects of different feeder tip angles were investigated on a single mold
plane (water cooled) utilizing an open-topped box with a sliding bottom, to
simulate the
metal flow conditions from a stationary feeder tip onto a moving water cooled
belt. The
apparatus employed is shown schematically in Figure 5 (plan view) and Figure 6
(vertical longitudinal cross-section).
The metal was poured into a box 70 and a bottom plate 71 was pulled
horizontally at predetermined speeds and molten metal temperatures, allowing
the
metal 75 to flow from an end 72 of the moving bottom plate onto a sheet steel
mold 73,
where it solidified progressively towards the moving bottom plate. The moving
bottom
plate (forming a thin slide) was made of the same material as the feeder tips
used for
continuous casting, and the right hand end was changed in geometry as shown in
Figures 7A to 7D to study the effects of such changes on the solidified metal,
such as
meniscus break lines and other ingot surface defects. The speed of extracting
the
bottom plate was varied to simulate different metal flow rates and conditions
of the tip
to the mold surface. The geometry of Figure 7A corresponds to the present
invention,
CA 02596473 2007-07-31
WO 2006/089419 PCT/CA2006/000267
13
having a cut back angle of 75 degrees. Figure 7B has a cut back angle of 120
degrees,
i.e. outside the present invention. Figure 7C has a compound surface but the
angle at
the inner corner ("take off point") is 120 degrees. The second angle does not
affect the
meniscus because it does not form a take-off point. Figure 7D has a curved
outer
surface and there is as a result no clear inner corner or "take off point".
All conditions
in Figures 7B, 7C, and 7D resulted in undesirable oxide breaks at the
solidification
juncture, but the conditions of Fig. 7A, an undercut tip angle of 75 angle,
gave good
results. For the same design as in Fig. 7A, the undercut angle was changed to
60 and
30 , all with good results, attained by the sharp upper edge shown in Figure
7A.
EXAMPLE 2
A series of casts were performed in a pilot scale belt caster using metal
delivery
nozzles having various cut-back angles. Casts were made on copper belts using
aluminum alloy AA5754 cast 10 mm thick cast at a speed of 8 to 10
meters/minute.
The surface of the as cast strip was observed and photographed. The results
are shown
in Figures 8 to 12 and are summarized in Table 1 below.
Table 1
Cut back angle Figure Number Observation
Severe oxide banding or folds
93 degrees 8 irregularly spaced about 30 mm
a art
88 degrees 9 Oxide banding or folds irregularly
spaced about 30 mm apart
78 degrees 10 Regular fine banding about 1 mm
s acin
48 degrees 11 Regular fine banding about 1 mm
spacing
33 degrees 12 Regular fine banding about 1 mm
spacing
Cut back angles of 93 and 88 degrees are outside the range of angles of the
invention and the sheet cast using tips at such angles exhibit unacceptable
oxide folds or
banding. A spacing of 30 mm, typical of such bands, corresponds to a frequency
of
about 4 to 6 Hz. Cut back angles less than 88 degrees show an absence of such
heavy
bands, but display finer more regular surface marks with spacings of about 1
mm,
corresponding to a frequency of over 100 Hz.
