Note: Descriptions are shown in the official language in which they were submitted.
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Express Mail No. B65841760
PATENT
RL ~1380-A
METEIOD AND APPARATUS FOR DIRECT CP~STING
OF CRX'STALLINE STRIP BY RADIANTLY COOLING
BAC~GROUND OF T~IE I21VENTIQN
ThiC invention relates to method and apparatus for
direct casting of metal alloys from molten metal to continuous
strip. More particularly, it relates to feeding molten metal
through an open casting vessel outlet to solidify continuous
strip of desired thickness on a moving casting surface.
In convèntional production of metal strip, such methods
may include the steps of casting the molten metal into an ingot
or billet or slab foxm, the~ typically includes one or more
stages of hot rolling and cold rolling, as well as pickling and
annealing at any of various stages of the process in order to
produce ~he desired strip thickness and quality. The cost of
producing continuous strip, particularly in as cast gauges
ranging from 0.010 inch to 0.100 înch (0.0254 to 0.254 cm) could
be reduced by eliminating some of the processing steps of
conventional methods. The as-cast strip could be processed
conventionally, by cold rolling, pickling and annealing to final
gauges of 0.002-0.040 inch.
There are known a wide ~ariety of methods and apparatus
for the production of directly cast strip. Typical of such
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methods are those which include spraying molten metal through a
metering orifice across a gap to a rapidly moving quenching
surface such as a wheel or continuous belt; methods which
partially submerge a rotating quenching surface into a pool of
molten metal; methods which use horizontal link belts as
quenching sub-~trates upon which molten metal flows for
solidification; and methods of casting with twin casting rolls
having a pool of molten metal therebetween.
Direct casting of metals through an orifice has long
been attempted for the commercial production of strip with good
quality and structure. U.S. Patent 112,Q54 dated February 21,
1871 discloses a method of manufacturing flat solder wire from
molten metal forced through an orifice and onto a rotating
casting surface. Similarly, U.S. Patent 905,758, icsued ~ecember
1, 19~8, di~closes a method of drawing molten metal out of an
outlet at the lower end of a ves~el and onto a casting suxface.
British Patent 24,320, dated October 24, 1910, discloses a method
of producing ~heet or strip from molten metal flowing through a
tube channel having at least one side in contact with the moving
casting surface. Representative of a more recent system is U.S.
Patent 3,522,a36 - King, issued August 4, 1970, which discloses a
method of maintaining a convex meniscus projecting from a nozzle
and moving a surface past the nozzle orifice outlet to
continuously draw off material and solidify as a continuous
product. The molten material is maintained in static equilibrium
at the outlet and gravitationally maintained in continuous
contact with the moving surface. U.S. Patent 4,221,257 -
Narasimhan, issued September 9, 1980, relates to a method of
forcing molten metal under pressure through a slotted nozzle onto
the surface o a moving chill body.
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The orifice-type casting systems are generally
restricted to light gauge materials as cast usually on the order
of less than about 0.010 inch (0.0254 cm) in thickness. Such
sys~em~ appear to be gauge-limited for the moving quenching
surface appears to be limited in the material which it can
solidify and carry away as it is delivered from the nozzle
orifice. Such systems behave as a molten metal pump and transfer
excess molten metal from the orifice to the quenching surface in
a mclten state with more heat than can be extracted to provide a
suitable strip. By reducing the delivery rate of the metal
and/or by increasing the velocity o the quenching surface, such
a conditiion can be overcome, however, a reduction in gauge will
result.
When crystalline strip is attempted to be produced at
the high speeds associated with the orifice-type casting systems,
poorer guality usually results. As molten metal i5 sprayed upon
a high-speed quenching surface or is flowed out full width on a
slower-mQving hurizontal belt, it rapidly moves away from the
source of the supply in a still partially molten state. It is
this condition that leads to the deterioration in quality, for as
the strip rapidly solidifies from the quenching surface side of
the strip, shrinkage occurs which can only be moderated by a
fresh su~ply of molten metal. Without such a fresh supply of
molten metal, cracks quickly develop within the structure of the
strip and greatly reduce its physical properties. ~ttempts have
been made to improve the nozzle geometry to overcome the problems
associated with orifice-type casting as shown in U.S. Patents
4,274,473, issued June 23, 1981 and 4,290,476, issued September
22, 1981. A disadvantage of the orifice-type casting is that the
orifice meters out an amount of molten metal which, in effect,
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determines the gauge of the strip. Furthermore, relatively high
pressure heads used in order to supply enough molten metal to the
orifice and a relatively s~all standoff distance from the casting
wheel for containment of the molten metal also limits the strip
gauge.
Thicker strip can be produced on a single quenching
surface such as by dipping a slowly rota~ing quenching wheel into
a ~tatic supply of molten metal to permit the solidification of a
much thic~er strip. Molten metal solidifies on the suxface of
this wheel and continues to thicken at a predictable rate until
it immerges from this bath of molten metal or it separates from
the surface. The fresh supply o~ molten metal avoids the
cracking generally associated with solidification of a finite
layer such as in orifice-type casting. Furthermore, an extremely
steep thermal gradient between this molten pool and the
solidification front also leads directly to a more uniform
internal structure and superior upper surface quality. A
drawback from such a dip system comes from the difficulty of
keeping molten metal from solidifying upon the edges of the
slightly subm2rged quenching wheel and having a tendency to cast
a channel-like structure. Furthermore, there is the added
difficulty of insuring uniform contact between the solidifying
strip and the surface of the quenching wheel as it enters the
molten pool, and results in poor surface quality on the cast side
of the strip. Such difficulties lead to spot variations in strip
gauge, wherein lighter gauge sections are produced where intimate
contact is reduced or lost.
Other direct casting processes have been proposed, but
have not developed into commercial processes. For example,
pouring of molten metai on the top of a moving casting wheel
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produces strip of nonuniform gauge, poor edges and unacceptable
quality. U.S. Patent 993,904, dated May 30, l911, discloses an
apparatus including a molten metal first vessel with a gravity
disch~rge outlet opening into the lower part of a tray-like
second vessel below the level of molten metal therein. The
molten metal passes out of the second vessel through an overflow
to deliver molten metal to a casting wheel. U.S. Patent
3,381,739, issued May 7, 1968, disclo~es a method of forming
sheet or strip material by flowing liquid about a ~urface which
is wetted and bridging the distance to the moving casting surface
on which it solidifies.
What is needed is a method useful in commercial
production for direct casting strip having surface quality
compaxable to or better than conventionally-produced strip. The
method and apparatus of direct casting ~hould produce strip which
is ~uperior to orifice-type casting, as well as other known
direct casting processes including dip-cast systems, horizontal
link belt quenching systems, and twin casting rolls. It is an
objective that the method and apparatus overcome the
disadvantages of known direct casting methods. Furthermore, what
is needed is a method and apparatus to permit the direct casting
of relatively thick strip on the order of greater than 0.010 inch
(0.0254 cm) and up to about 0.100 inch ~0.254 cm) or more. It is
desirable that the factors contributing to shrinking and cracking
of direct cast strip be minimized or eliminated in order to
provide improved surface quality and structure of strip.
Furthermore, a method and apparatus is desirable which is
suitable for commercial production of strip at reduced cost and
to facilitate production of new alloys. The direct cast strip
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should have good surface quality, edges and structure and
properties at laast as good as conventionally cast strip.
SUMMARY OF T~ _ NVENTION
In accordance with the present lnvention, a method is
provided for directly casting molten metal to continuous strip of
crystalline metal. The method includes flowing molten metal from
a generally U-shaped ~ tructure of an exit end of a casting vessel
onto an adjacent casting surface moving generally upwardly past
the exit end at a predetermined distance therefrom. The method
includes radiantly cooling the molten metal in a zone defined
1~ above the molten metal, across the width of the U-shape~
structure and adjacent the casting surface. A non-oxidizing
atmosphere may also be provided in the zone.
An apparatus is also provided comprising a moving casting
surface, a cacting ve~sel having an exit end of generally
U-shaped structure adjacent the casting surface at a
prede~ermined distance. The apparatus includes means for
radiantly cooling the molten metal in the zone. Means for
providing a non-oriented atmosphere in the zone may also be
provided.
BRIEF DESCRIPTION OF T~E DRAWINGS
Figure 1 is a schematic of a strip casting apparatus of
the present invention.
Figure 2 is an elevated view in cross section of a
casting vessel of the present invention.
Figure 2a is a detailed elevation view of Figure 2.
Figure 2b is another detailed view of Figure 2.
Figure 3 is a top view of a casting vessel of Figure 2.
Figure 3a is an end view of the casting vessel of
Figure 3.
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Figure 4 is an elevation view in cross section of a
preferred embodiment of a casting vessel of the present
invention.
Figure 5 is a top view of a preferred embodiment of ~he
casting vessel of Figure 4.
Figure 6 is an enlarg d elevated view of a preferred
embodiment of the exit end of a casting vessel of the present
invention.
Figure 7 is a photomicrograph of typical Type 304 alloy
as-cast strip of the present invention.
Figure 8 i5 a photomicrograph of a typical Type 304
alloy conYentionally produced hot-roll band.
DETAILED DESCRIPTION OF THE PREF~ED EMBODIMENTS
Figure 1 generally illustrates casting apparatus 10
including tran~fer vessel 12 and f~!ed tundish 14 for supplying
molten metal to casting ve~sel 18 for directly ca~ting molten
metal on a casting surface 20 to produce continuous product in
strip or sheet form 15. Molten metal 19 is supplied from vessel
12 to tundish 14 to casting vessel 18 in a conventional manner.
Stopper rod 16 or other suitable means may control the flow of
molten metal to casting vessel 18 such as through spout 17.
Casting vessel 18 is shown substantially horizontal having a
receiving end and an exit end disposed adjacent to the casting
surface 2~.
The supply of molten metal 19 through the casting
vessel 18 may be accomplished by any suitable conventional
methods and apparatus of vessels, tundishes, or molten metal
pumps, for example. Vessel 12 and feed tundish 14 may be of
known design and should be suitable for supplying an adequate
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amount of molten metal to casting vessel 18 for strip generation
at the quenching wheel.
Casting surface 20 may also be conventional and may
take the form of a continuous belt, or a casting wheel.
Preferably a casting wheel is used. The compo~ition of th~
casting surface does not appear to be critical to the present
invention, although some surfaces may provide better results than
others. The method and apparatuC of the pre~ent invention have
been used with casting surfaces of copper, carbon steel and
stainless steel. It is important that the casting surface be
movable pa~t the casting vessel at controlled speeds and be abl
to provide desired quenching rates to extract sufficient heat for
solidifying the molten metal into strip form. The casting
surface 20 is movable past casting vessel 18 at speeds which may
range from 20 to 500 feet per minute, preferably S0 to 300 feet
per minute (FPM), which is suitable for commercial production of
crystalline matPrial. The casting surface 20 should be
sufficiently cool in order to provide a quenching of the molten
metal to extract heat from the molten metal for solidification of
strip of crystalline form. The quench rates provided by casting
surface 20 of apparatus 10 are less than lO,OOO~C per second and
typically preferably less than 2000C per second.
Two important aspects of the casting surface are that
it have a direction of movement generally upwardly past the exit
end of vessel 18 and a free surface molten metal pool in exit end
2~ 26. The free surface of the molten metal pool in exit end 26 is
essential to development of good top surface quality of the cast
strip. By "free", it is meant that the top surface is unconfined
by structure, i.e., not in contact with vessel structure and free
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to seek its own level between receivi.ng section 22 and exit end
26. Generally, the path is oriented at an included angle
~ from about 0 to 135 from the horizontal and in the
direction of metal flow as measured between the direction of
metal flow at the free surface of molten metal in the exit end
and the direction of movement of the casting surface at the free
surface in the exit end of casting vessel 18. For a casting
whe~l, the path of the casting surface is tangent to the free
surface at the exit end of ve~sel 18. Preferably, the angle is
between 0 and 45 from the horizontal. For a casting wheel,
preferably, the vessel is adjacent a position in an upper
quadrant of the wheel when the free surface of molten metal is
near the crown of the casting wheel, the angle is at about the 0
position.
Casting vessel 18 is essential to the method and
apparatui 10 of the present invention and is better shown in
Figure 2 which i3 an elevation view of the vessel 18. Casting
vessel 18 is disposed adjacent casting surface 20, preferably ls
substantially horizontal r and is composed of heat insulative and
refractory material described below. This arrangement is
necessary for providing the required uniform and fully-developed
flow of molten metal to the casting surface 20. Vessel 18
includes a receiving end 22 at a rearward section and an exit end
26. Preferably, receiving end 22 and exit end 26 have
substantially the same cross-sectional area or exit end 26 has a
greater cross-sectional area as measured perpendicular to the
direction of metal flow from the receiving end 22 to exit end 26.
Receiving end 22 is shown deeper than exit end 26 which
facilitates receivirlg molten metal 19 such as from supply spout
17 and for developing a flow of molten metal to exit end 26.
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Exit end 26 of ves~el 18 has a generally U-shaped
structure defined by a bottom wall portion 28 and sidewalls 30,
as is shown in Figure 3. Sidewalls 30 may have vertical inside
wall inside surfaces 31, but preferably, the surfaces 31 of
sidewalls 30 of the U-sbaped structure diverge to open upwardly
S to facilitate metal flow. The slight taper tends to improve
metal flow from exit end 26, but too great a taper may cause a
loss of surface tension control and flooding of molten metal. A
taper of less than 10 per side and preferably 1-5 is provided.
Exit end 26 includes bottom wall 28 which has a
generally planar inside porti~n having a length sufficient to
provide a substantially uniform flow of molten metal from the
exit. Preferably, the length of the planar wall portion as
measured in the direction of metal flow is at least equal to the
depth of molten metal pool to be contained in exit end 26. More
preferably, the ratio of length to depth is at least 1:1 or
greater. Exit end 26 preferably has fixed or uniform dimensioins
of width and height throughout the length of the planar inside
surfaca of bottom wall 28 to define a uniform cross-sectional
are~ in exit end 26. The width of the exit end 26 as measured
between the inside surfaces 31 of sidewalls 30 along the free
surface of molten metal pool is about as wide as the strip to be
cast. Preferably, exit end 26 is positioned adjacent casting
surface 20 with the ends or edges of the sidewalls 30 and bottom
wall 28 defining the U-shaped structure being substantially
parallel to the casting surface.
To facilitate transition flow between receiving section
22 and exit end 26, an intermediate section 24 communicating
between the receiving end 22 and the exit end 26 should be
provided in order to have a substantially uniform flow at exit
end 26. Preferably, intermediate section 24 maintains
substantially uniform cross-sectional area throughout its length
for receiving section 22 to exit end 26. Intermediate section 24
shown in Figure 3 has a gradually increasing width from the
receiving end 22 to exit end 26 and, as shown in Figure 2, a
gradually decrea~ing depth so as to maintain a substàntially
uniform cro~s-sectional area throughout it length. Intermediate
section 24 may be provided with a tapered bottom wall 32 which
gradually decreases the depth of the vessel 18 from the receiving
end 22 to the exit end 26. Similarly, intermediate section 24
may have at least one sidewall 34 which fans outwardly in order
to provide a gradually incxeasing width from the narrower
receiving end 22 to the wider exit end 26. Figure 2 is a top
view of casting vessel 18 illustrating the widening of sidewall
34 of intermediate section 24.
Figure 2 also illustrates that weirs or weir plates 36
may be used in casting vessel 18 such as in an intermediate
section 24 or near where section 24 merges into exit end 26 in
order to further facilitate development of uniform flow. Weir
plates 36 should be made of a refractory ox heat-resistant
material which is also resistant to corrosion by molten metal.
Raowool refractory board, treated with a diluted colloidal silica
suspension haq provan satisfactory. Weirs 36 may extend across
the entire width or a portion of the width of casting vessel 18.
As shown in Figuxe 2, preferably, the molten metal level in the
receiving end 22 of casting vessel 18 is at about the same level
as the molten metal in exit end 26. Weirs 36 are useful for
baffling or dampening the flow in order to facllitate development
of a uniform fully-developed flow and to restrain movement of
surface oxides and slag.
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Figures 2a and 2b illustrate the use of surface tension
of the flowing molten metal to form the surfaces of the strip
being cast. Figure 2a is a detailed elevation view in partial
cro~s section of exit end 26 adjacent casting surface 20. Molten
metal flowing from the exit end 26 forms and maintains a meniscus
35 between the inside surface of bottom wall 28 of the U-shaped
structure and the casting qurface. The surface tension forming
meniscus 35 forms the bottom of the strip 15 being cast~ The
urface tension of the free surface of the molten metal pool in
exit end 26 forms a curvilinear portion 39 on the top of the
molten metal in the U-shaped ~tructure as it forms the strip
product.
Figure 2b illustrates exit end 26 adjacent casting
surface 20 showing solidifying metal 19 therebetween in a view
from under exit end 26. The surface tension of the molten metal
19 forms the convex surfaces or meniscus 37 between exit end 26
and ca~ting s1lrface 20 at the inside surface 31 of sidewalls 30
near bottom wall 28.
~ preferred embodiment of casting vessel 18 is shown in
the elevation and top views of Figures 4 and 5, respectively.
Vessel 18 is shown having an outer metal support sheil 38, a
refractory insulation 40, and a liner 42 which defines the
internal surface of the casting vessel 18 and which is in contact
with molten metal during casting. The construction of vessel 18
should be made from refractory material which is heat insulative
and resistant to molten metal corrosion. The casting vessel may
be secured to some suitable table or means to orient and position
the vessel at the desired casting position on the casting surface
or wheel 20. The exit end 26 of casting vessel 18 should have
the front face or edges 33 of sidewalls 30 and bottom wall 28
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which define and form the U-shaped structure contoured to the
casting surface. This can be done simply by using 60 or 100-grit
silicon carbide grinding papers held between the oasting surface
and thne vessel assembly and rubbing the paper against the vessel
18 to make the edges parallel to the wheel. The front surface 33
S of the ca~ting v2ssel 18 may then be brush coated with zirconia
cement and allowed to dry before casting.
Figures 4 and 5 illustrate a preferred embodiment of
the casting vessel 19 of the present invention which is useful
for casting strips of 4 inches, and up to about 13 inches and may
be useful up to 48 inches wide. The metal support shell 38 may
be u ed depending upon the type of material used for the
insulation layer 40. Insulation layer 40 may be a foamed ceramic
cement in~ulation which would need an external support such as a
metal support shell 38. In the alternative, if a standard
refractory bric~ or block is used and cemented together into the
desired shapes and then carved to achieve the desired inner and
outer dimensions, then the outer shell 38 is not necessary.
The vessel 18 may al50 be a monolithic shape formed from castable
oeramic material. Liner 42 on the internal surface of casting
vessel 18 is also made of an insulating refractory molten metal
resistant material. It has been found that an insulating blanket
of a high alumina fiber-silicate composition is useful, such as
Fiberfrax brand material, that has been saturated in a diluted
colloidal silica suspension and contoured within the casting
vessel 18 and then dried prior to actual use.
Figures 4 and 5 al50 show a rear overflow element 44
including a rearwardly-sloping surface 45 extending from the
inner surface of casting vessel 18 to the outer walls of vessel
18. The height of the overflow element 44 determines the maximum
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depth of molten metal tha~ may be contained in the receiving end
22 and, accordingly, the depth of the molten metal in the exit
end 26 of casting vessel 18. Overflow element 44 facilitates
control of the molten metal level in the casting ves~el 18 which
is essential to gauge and quality control of the cast strip.
Also shown in Figure 4 is a casting vessel 18 which may
optionally include a cover assembly 46 in the vicinity of
intermediate cection 24 of casting ves~el 18. Cover 46 includes
downwardly-extending walls 48 and 50 joined by a bottom surface
52. The downwardly-extending walls 48 and 50 are similar to the
weir plates shown in Figure 2. Cover 46 is generally composed of
a refractory insulative material resistaQt to molten metal.
Cover 46 may comprise a liner 42, a refractory insulation layer
40 and an outer metal shell 38 having a similar manner of
construction as is casting ve~sel 18. The cover 46 may extend
across the entire width or part of the width of casting vessel 18
in the vicinity of intermediate section 24. It is important that
the presence of a cover 46, which is useful for retaining the
heat in the molten metal in the casting vessel 18, does not
contact the molten metal in the receiving end 22 and exit end 26
in order to maintain the free surface in the pool in exit end 26.
The cover also can extend over portions or all of rear receiving
section 22 to contain a protective atmosphere therein.
Figure 6 illu~trates another embodiment wherein the
exit end 26 of vessel 18 is provided with a means for providing
a non-oxidizing atmosphere in a zone defined above the molten
metal across the width of the ~shaped structure of exit end
adjacent to casting surface 20 together with a means for
radiantly cooling the molten metal in that zone. The two
features may be present separately or in combinatio~.
14
Means for providing a non-oxidizing atmosphere provides
a protective cover or blanket of inert or reducing gases in a
zone about the molten me~al in the U-shaped structure of exit end
26. The gases minimize or prevent the buildup or formation of
slag and oxides on the top surface of the molten metal, which
oxide could be cast into the cast qtrip. The non-oxidizing
atmosphere may be static, or a recirculating atmo~phere.
Preferably, a non-contacting cover over the zone above the molten
metal pool at the exit end 26 of casting vessel 18 and at least
one gas nozzle or a series of nozzles 56 provides a continuous
flow of inert or reducing gas counter to the direction of the
cast strip. Preferably the gas is introduced so that it impinges
in the Sone on the top of the molten metal liquid pool where the
strip is emerging. The embodiment may provide a protective cover
for sealing the zone over the molten metal pool containing a
blanket of inert or reducing gases directed into streams of gases
to pu3h any oxide away from the forming of the strip. The series
of narrow gas nozzles 56 is positioned along the width of the
casting strip so that streams or jets of gas impact the zone
wherein the strip emerges from the liquid pool. Nozzles 56 are
directed counter to the casting of the strip at an angle to the
plane of the formed strip, preferably about 20-30. The gas
blanket may be a gas selected from the ~roup consisting of
hydrogen, argon, helium, and nitrogen in order to minimize the
oxides that may be formed during casting. The velocity of the
gases from nozzle 56 should be quite low, for higher velocities
may cause a disturbance in the upper surface of the molten metal
pool ~nd result in dama~e to the cast strip.
Means for radiantly cooling the molten metal in the
zone may inclu~e providing a coolant in the vicinity of the zone
to facilitate extraction of heat from the top surface of the
molten metal. The coolant may be provided by a panel of tubes or
pipes 54 located above the molten liquid to remove radiated heat
from the molten metal. Water or other fluid may be used as a
coolant. Preferably, a cover is provided which includes a series
of water-c~oled tubes 54 sealed to the top of the casting vessel
18 with refractory material and cement. Radiant cooling of the
top surface of molten metal as it flows from the U-shaped
structure of exit end 26 onto casting surface improves the heat
extraction from the top surface of the solidifying molten metal
I0 to improve a~-cast ~trip top surface quality and structure by
controlling the growth of dendritic structure in the strip.
Preferably, means for providing a non-oxidizing
atmosphere and the mean-C for radiantly cooling are used in
combination. A non-contacting cover for sealing the zone over
the molten metal at the exit end 26 includes a cooling means to
remove radiated heat from the molten metal and a non oxidizing
atmosphere means. Preferably, the cover includes a series of
water-cooled tube~ 54 and a serie~ of gas nozzles 56. The inert
gase~ in this embodiment are cooled by the tubes 54 which further
facilitate removal of the radiant hea~. The cover containing the
cooling tubes 54 seals the ~one to reduce oxide or slag formation
which could be deposited on the strip product.
In the operation of the casting apparatus of the
present invention, vessel 12, tundish 14 and casting vessel 18
are preheated to operating temperature~ prior to introducing
molten metal into the casting vessel 18 for the production of
strip material. Any conventional heating means should be
suitable and may be used. An air-acetylene or air-natural gas
heating lance positioned in the receiving end 22, as well as
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providing a preheat front cover for ~he front edges of ~he
casting vessel U-shaped structure which will be placed adjacent
the casting surface 20. Normal preheating tempera~ures for
casting molten stainless steel may be on the order of
1900-2000F. After the minimum preheat levels desired are
reached, the heating lances are removed and the vessel 18 is
positioned adjacent the casting surface at a preset standoff
distance such as between S and 20 mils.
In Gommencing the method of directly casting alloy from
molten metal to continuous strip, molten metal 19 is supplied
from a bulk transfer ladle or vessel 12 to a $eed tundish 14 and
thereafter to the ca~ting ves~el 18 which is oriented
sub~tantially horizontally. The flow of molten metal from feed
tundish 14 to casting vessel 18 may be controlled and regulated
by valve means such as stopper rod 16 and through spout 17 into
the rear feed section or receiving end 22 of casting vessel 18.
As vessel 18 begins to fill wi~h molten metal, the molten metal
begins to flow in a direction toward the exit end of the vessel
and flows through an intermediate section 24 and the exit end 26
as shown in Figure 2. Casting vessel 18 permits the molten metal
to flow so as to feed the molten metal to the exit end 26 of
vessel 18. Casting vessel 18 may include weirs 36 such as shown
in Figur~ 2 to dampen and baffle the flow of molten metal 19 in
order to facilitate a uniform fully-developed flow in exit end 26.
The molten metal preferably maintains a substantially uniform
cross-sectional area of flow from the receiving end 22 through
the exit end 26. Generally exit end 26 is wider than the
receiving end 22 and the U-shaped structure has a width which
approximates the width of the strip to be cast. Casting vessel
18 has a casting volume having tapered and fanned intermediate
17
section. Casting vessel 18 is designed to prevent cross flows of
molten metal within the vessel while developing a uniform
turbulent flow from exit end 26 across the width of the U-shaped
structure in end 26 such that the fully-developed flow has the
bulk of the velocities in the direction of flow from the
5 receiving end 22 to the exit end 26. The level of molten metal
in exit end 26 i5 about the same as the level iQ receiving end
22, although the depth of the molten metal will be less in exit
end 26. The molten m~tal continues to flow from the exit end 26
onto the moving casting surface ~0 such that across the width of
the U-shaped structure of exit end, a substantially uniform flow
of molten metal is pre~ented to the casting surface 20. The
molten me~al in exit end 26 has a top surface tension and the
molten metal leaving the opening has edge surface tension which
form, in part, the top and edge~, respectively, of the cast strip
15~ The bottom surface is formed from surface tension in the
form of a meniscus between the bottom inside surface of ~he
U-shaped structure and the casting surface.
Though there is no intent to be bound by theory, it
appears that the solidification of the molten metal leaving the
exit end of ve~sel 18 commences with the molten metal contacting
the casting surface as it leaves the bottom of the U-shaped
opening of exit end 26 of vessel 18. The strip is solidified
from the pool of moiten metal available to the casting surface at
the exit end of vessel 18 and forms a thickness wherein the
solidifying strip is continually presented with an oversupply of
molten metal until leaving the exit end 26 of vessel 18. Such a
pool of molten metal is believed to form a substantial part of
the strip thickness as it contacts the moving casting surface 20
with only a minor portion of the strip thickness resulting from
18
~,,2~7~zt7~3
molten metal solidified as it was pulled out of the vessel 18
adjacent the top curvilinear surface tension portion 39. It is
estimated that more than 70% and probably more than about 80~ of
the strip thickness results from the pool of molten metal
S provided adjacent the meniscus 35. The molten metal solidifies
from the bottom of molten metal pool provided to the casting
surface from the bottom of the U-shaped structure of exit end 26
of vessel 18.
Casting surface 20 moves past casting vessel 18 in a
generally upward direction from the bottom of the U-shaped
opening of of exit end 26 to the open top of the opening. The
position of vessel 18 on the casting surface 20 and the speed of
the casting surface are predetermined factors in order to achieve
the quality and gauge of the cast strip. If the casting surface
20 is a casting wheel, then the veqsel 18 i5 positioned,
preferably, on an upper quadrant of the casting wheel.
By the method of the present invention, there is an
important control of several factors which results in the ability
to cast desired gauges of metal strip ranging from 0.01 to 0.06
inch with good surface quality and, edges and structure. The
control of molten metal flow onto the casting surface, the speed
of the casting surface, the solidification from the bottom of ~he
molten metal pool, and the controlled depth of molten metal in
the pool and standoff distance from the casting surface to
maintain the surface tension of the molten metal are important
interrelating factors.
In order to better understand the present invention,
the following examples are presented.
Example I
A casting vessel having the structure generally as
19
shown in the Figure 2 but having only one weir plate 36 near the
exit end 26 was constructed from hardened blocks of Raowool
refractory, which is an alumina-silica composition material. It
was treated by soaking it with a colloidal silica suspensi~n
dried overnight at 250F and then fired Eor 1 hour at 2000F in
air. ~fter the blocks were cut and shaped, they were coated with
a thin layer of ~aowool cement. The vessel was shaped to the
contour of the wheel and then the U-shaped structure ends were
coated with a thin layer of a zirconia cement. ~ weir of similar
composition was used. The casting vessel was then heated with
air-acetylene lances. The ~essel 18 was about 8.75 inches long
from the receiving end 22 to the exit end 26 and was about 6.5
inches wide at the receiving end 22 and about 4 inches wide at
bottom wall 28 at exit end 26. Molten metal of Type 304 alloy
was tapped at 1580C, supplied to the vessel 18 and maintained at
a level of about 1.7S inches deep in the receiving end 22 and khe
molten metal was about 0.75 inches deep in the U-shaped structure
in ~he exit end 26 of vessel 18. ~ casting surface was a copper
casting wheel having a width of 7 inches and a diameter of about
36 inches which provided cooling on the order of less than
2000C/sec. The casting wheel was rotated at a speed of about
250 to 300 feet per minute past the exit end of vessel 18 and
spaced about 40 mils therefrom at an angle ~ of about 40.
The U-shaped structure of the vessel had diverging or tapered
inside surface 31 of sidewalls 30 of exit end 26 opening upwardly.
~he taper was on the order of about 3 per inside surface. Run
25 of about 100 pounds was cast according to the present
invention and resulted in successful production of strip having a
wid~h of about 4 inches and a uniform thickness of from 16 to 18
- 20
mils having smooth and uniform upper and lower surfaces as-cast
and flat edges showing no signs of raggedness or curls.
Example I~
A casting vessel havins a structure generally as shown
in Figure 4 was construc~ed having a Koawool refractory and
alumina bubble refractory insulation 40 in a metal shell 38. ~he
li~er 42 was made of Fiberfrax material, 0.5 inch (1~27 cm) thick
at eight pounds per cubic foot which was saturated with a diluted
colloidal silica suspension and then dried prior to usa. The
vesnel 18 outside dimensions were about 15 inches long and 18
inches wide at the exit end vessel 18 had a slight increasing
cross-sectional area to exit end 26. Weir plate 36 was made
and positioned similar to Example $ and cemented between
sidewalls of vessel 1~. The inside surfaces 31 of sidewalls 30
were also tapered or diverging on the order of about 3 per
~urface. The casting vessel was set at a standoff distance of
about 35 mils at an angle of about 0 for the free surface of the
molten metal was near the crown of the casting wheel. A
500-pound Run 84-97 of molten metal of Type 304 was cast
according to the present inventiion on a casting surface of a low
carbon steel seamless pipe having a 12.75-inch outside diameter,
a 0.375-inch wall thickness, 48 inches wide and internally spray
water cooled. The casting wheel was rotated at about 200 FPM at
the start of the casting for 10-15 seconds to facilitate flushing
of the initial metal flow and then slowed to 100 FPM for the
duration of the Run. The molten metal maintained a depth of
about 2 inches (5.08 cm) in the exit end 26 and 2.75 inches (6.98
cm) in the receiving end 22.
~ he vessel 18 also included a cover having a means for
radiantly cooling and means for providing a helium atmosphere as
~.2 ~
shown in Figure 6. The cooling was effected by circulated water
at about 3 gallons per mi~ute through copper tubing having a
0.375-inch ou~side diameter.
The as-cast strip was about 13 inches wide, and having
a uniform thickness of about 45 mils and having good upper
surface quality which was uniform, ~mooth and crack freeO The
a~-cast qtrip was then conventionally processed by pickling in a
nitric/hypofluroic acid, cold rolling about 50% reduction,
annealing at 1950 for 5 minutes, pic~ling again in a similar
manner, and then cold rolling to 5 mils and annealed. The room
temperature mechanical properties of the annealed as-cast samples
are shown below in comparison to typical properties of
conventionally produced Type 304 annealed hot-roll band.
Table
0.2~ Elongation in
15 Samples Tensile Strength Yield Strength 2 inch.
(RSI) (RSI) (%)
1 10~.6 44.6 52.0
2 100.8 40.8 50.0
3 100.8 40.8 49.0
20 4 100.0 40.0 52.S
7 192.8 42.0 55.Q
8 102.0 42.0 57.5
9 103.6 44.0 52.0
105.2 44.0 54.5
Type 304 alloy conventionally produced may have typical
or average room temperature mechanical properties of annealed
hot-roll band of 101.1 KSI tensile strength, 43.8 KSI yield
strength and 57% elongation in 2 inches.
22
Figure 7 is a photomicrograph of as-cast strip of the
present invention showing the typical internal structure from Run
84-52. The Type 304 alloy, shown at lOOX magnification,
illustrates the typical as-cast structure of small columnar cells
5~ oriented in the direction of strip thickness, i.e., top to bottom
surfaces. This direction generally conforms to the direction o
heat extraction from the strip as it solidifies. The method and
apparatus of the present invention controls the growth of the
dendritic structure in the strip to produce an as-cast strip
which can be conventîonally processed into finished strip
having properties comparable to or better than conventionally
produced strip product.
Figure 8 illustrates a typical structure of a
conventionally produced hot-roll band of Type 304 alloy at lOOX
magnification.
It is observed that the method and apparatus of the
present invention results in even better strip structure and
quality as the gauge of the strip product i~crease~ and as the
width of the strip increases. The tendency of edge curl in the
strip product cast in 4 to 6-inch widths appears to no longer
be present in the wider widths up to 13 inches. The method and
apparatus of the present invention provides an uncomplicated and
direct method for casting crystalline metal strip or sheet from
molten metal to continuous strip. The shrinking and cracking
problems of finite film solidification are eliminated and a
relatively thick strip of quality comparable to or better than
conventional production methods is provided.
The methods and apparatus appear useful for various
metals and alloys, including stainless steels and silicon steels.
