Note: Descriptions are shown in the official language in which they were submitted.
Express Mail No. B6584176l
P~ENT
RL-1251-80
J
MET~OD AND APPARATUS FOR
CONTINUOUS CASTING OF CRYSTALLINE STRIP
This invention relates to method and apparatus for
direct casting of me~al alloys from ~olten metal to continuous
strip. More particularly, it relates to feeding molten metal
through an open casting vessel outlet to solidify continuous
s strip of desired thickness on a moving casting surface.
In conventional production of metal strip, such methods
may include the ~teps of casting the molten metal into an ingot
or billet or slab form, then 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 the desired strip thicknes~ and quality. The cost of
producing continuous strip, particularly in as cast gauges
ranging from 0.010 inch to 0.100 inch (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.
rhere are known a wide variety of methods and apparatus
~or the production of directly cast strip. Typical of such
*~
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methods are those which include spraying molt n 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 p~ol of
molten m~tal; methods which U52 horizontal link belts as
quenching ~ubctrates upon which molten metal flows for
501idification; and methods of casting with twin casting rolls
having a p~ol of molten metal therebetween.
Direct casting of me~al.~ through an orifice has long
been attempted for the commercial production of strip with good
quality and structure. U.S. Patent 112,054 dated February 21,
1871 discloses a method of manufacturing flat solder wire from
molten metal forced through an orifice and onto a ro;ating
casting surfaca. Similarly, U.S. Patent 905,758, issued December
1, 190~, discloses a method of drawing molten metal out of an
outlet at the lower end of a vessel and onto a casting surface.
British Patent 24,320, dated October 24, 1910, discloses a method
of producing sheet or strip from molten metal flowing through a
tube channel having at least one sid~ in contact with the moving
casting surface. ~epresentative of a more recent system is U.S.
Patent 3,522,836 - 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 thP 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
forci~g molten metal under pressure through a slotted nozzle onto
the surface of 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) i~ thickness. Such
system 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 noz~le
orifice. Such systems behave as a molten metal pump and transfer
excess molten metal from the orifice to the quenching surface in
a molten state with more heat than can be extracted to provide a
suitable strip. By reducing the delivery rate of the metal
and/or by i~creasing the velocity of the quenching surface, such
a conditiion can be overcome,-how2ver, a reduction in gauge will
result .
When crystalline strip i5 attempted to be produced at
the high cpeeds associated with the orifice type casting systems,
poorer quality usually results. As molten metal is sprayed upon
a high-speed quenching surface or is flowed out full width on a
slower-moving horizontal belt, it rapidly moves away from the
source of the supply in a still partially molten state. It is
this condition that lead~ 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
resh supply 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, 1931 and 4,290,476, issued September
22, 1981. ~ disadvantage of the orifice-type casting is that the
orifice meters out an amount of molten metal which, in effect,
~ 2 4~
determines the gauge of the strip. Furthermore, relativaly high
pressure heads used in order to supply enough molten metal to the
orifice and a relatively sn~ll 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 aq by dipping a slowly rotating quenching wheel into
a static supply o molten metal to permit the solidification of a
much thicker stripO Molten metal solidifies on the surface 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 s~pply of molte~ 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 come~ from the difficulty of
keeping molten metal from solidifying upon the edges of the
slightly submerged 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 surace quality on the cast side
of the strip. Such di~ficulties 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,
pouxing of molten metal on the top of a moving casting wheel
produces strip of nonuniform gauge, poor edges and unacceptable
quality. U.S~ Patent 993,904, dated May 30, 1911, discloses an
apparatus including a molten metal first vessel with a gravity
discharge outlet opening into the lower part of a tray like
S second ve~el 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, is~ued May 7, 1968, discloses a method of forming
sheet or strip material by flowing liquid about a surface which
is wetted and bridging the distance to the mo~ing casting surface
on which it solidifies.
Wha~ is needed is a ~ethod useful in commercial
productio~ ~or direct casting strip having surface guality
comparable to or better than conventionally-produced strip. The
method and apparatus of direct casting hould produce strip which
is superior to orifice-type casting, as well as other k~own
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
di~advantages 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
~2~
should have good suxface quality, edges and structure and
properties at least as good as conventio~ally cast strip.
SUM~ARY OF THE INVENTION
I~ accordance with the pre~ent invention, a method is
provided for directly casting molten metal to continuous strip of
crystalline material. The method include~ supplying molten metal
to a casting vessel having a receiving end and exit end being
adjacent a casting surface moving generally upwardly past the
exit end. The m~lten metal is fed from the receiving end to the
exit end to provide a pool of molten metal having a substantially
uniform flow and free upper urface in the exit end. The molten
metal flows from the exit end onto the casting surface such that
across the width of the exit end of the casting ves~el a
~ubsta~tially uniform flow of molten metal is pre~ented to the
ca~ting surface. The surface tension of the flowing metal
formsall the surfaces of the strip being ca~t. The top surface
tension of the free surface of molten metal pool forms the top of
the cast strip, and the surface tension of the molten metal
leaving the sides of the exit end forms the edges of the cast
s~rip. The surface tension of the molten metal leaving the bottom
of the exit end maintains a meniscus between an inside surface of
the bottom of the exit end and the casting surface to form the
bottom of the cast strip. The depth of the molten metal in the
exit end and distance between the vessel and casting surface are
con~rolled to maintain the surface tension. The as-cast strip is
removed from the casting surface.
An apparatus is also provided for directly casting
molten metal to continuous strip of crystalline material
comprising a movable casting surface, a casting vessel and a
means for supplying molten metal to the casting vessel. The
casting vessel has a receiving end, an exit end having a
generally U~shaped structure adjacent the casting surface and
having edges thereof substantinally parallel thereto and an
intermediate section to facilitate a substa~tially uniform flow
of molten metal from the receiving end to the exit end. The
U-shaped structure of the exit end has a bottom wall and
diverging inside sidewall surfaces opening upwardly and having
the width between the inside surfaces being about aq wide as the
strip to be cast. The e~it end has a fixed width along the
bottom wall between the inside surfaces and a uniform
cro ~-sectional area over a lenqth sufficient to provide a
substantially uniform flow of molten metal from the exit end~
The casting surf-ace is movable generally upwardly past the exit
end of the casting ves~el at a distance of between 0.005 to a . 060
inch (0.013 to 0.152 cm) thererom at a speed of 20 to S~0 feet
per minute.
~ continuous direct cast ~trip product made in
accordance with the present invention is also provided.
Figure 1 is a schematic of a strip casting apparatus of
the present invention.
Figure 2 i9 an elevated view in cross section of a
casting vessel of the present invention.
Figure 2a i9 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.
Flgure 3a is an end view of the casting vessel of
Figure 3.
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Figure 4 is a~ elevation view in cross section of a
preferred embodiment of a casting vessel of the present
invention.
Figure S is a top view of a preferred embodiment of the
casting vessel of Figure 4.
Figure 6 is an enlarged 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 i~vention.
Figure 8 is a photomicrogr ph of a typical Type 304
alloy csnventionally produced hot-roll band.
DETAI~E~ DESCRIPTION OF TEE PREFERRED EMBODIME~TS
Figure 1 generally illust rates casting apparatus 10
including transfer vessel 12 and feed tundish 14 for supplying
molten m~tal to casting ve~sel 18 for directly casting molten
metal on a casting surface 20 to produce continuous product in
strip or 3heet form 15. Molten metal 19 is supplied from vessel
12 to tundish 14 to casting vessel 18 in a conventional manner.
Sto~per rod 16 or other quitable means may control the flow of
molten metal to casting vessel 18 such as through spo~lt 17.
Casting vessel 18 is shown substantially horizontal having a
receiving end and an exit end disposed adjacent to the casting
surface 20.
The supply of molten metal 19 through the casting
vessel 18 may be accomplished by any suitable conventional
methods and apparatus o~ 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
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 composition of the
ca~ting ~urface doe not appear to be critical to the present
invention, although some ~urfaces may provide better results than
others. The method and apparatus of the present invention have
been u~ed with casting surfaces of copper, carbon steel and
stainless steel. It is important that the casting surface be
movable past the casting vessel at co~trolled speeds and be able
to provide desired quenching rates to extract sufficient heat for
solidifyin~ the molten metal into strip form. The casting
surface 20 is mo~able past casting vessel 18 at speeds which may
range from 20 to S00 feet per minute, preferably 50 to 300 feet
per minute ~FPM), which is suitable for commercial production of
crystalline material. 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 apparatu~ 10 are less than 10,000C 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
26. The free surface o the molten metal pool in exit end 26 is
essential to development of good top surface quality of the cast
strip. By "freet', it is meant that the top surface is unconfined
by structure, i.e., not in contact with vessel structure and free
~24~ 1~7B
to seek its own level between receiving 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 ~urface of molten metal in the exit end
and the direction of movemen of the casting surface at the free
surface in the exit end of casting vessel 18. For a casting
wheel, the path of the casting surface is tangent to the free
surface at ~he exit end of vessel 18. Preferably, the a~gle is
between 0 and 45 from the horizontal. For a -asting wheel,
preferably, the ve~sel 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
positio~.
Casting ve~sel 18 is essential to the method and
apparatus 10 of the present invention and i5 better shown in
Figure 2 which iq an elevation view of the vessel 18. Casting
vessel 18 is disposed adjacent casting surface 20, preferably is
substantially horizontal, and is composed of heat insulative and
refractory material described below. This arrangement is
necessary for providing the re~uired 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 receiving molten metal 19 such as from supply spout
17 and for developing a flow of molten metal to exit end 26.
~2~7~
Exit end 26 of vessel 18 has a generally U-shaped
structure defined by a bottom wall portion 2~ and sidewalls 30,
as is shown in Figure 3. Sidewalls 30 may have vertical inside
wall inside surface~ 31, but preferably, the surfaceq 31 of
sidewalls 30 of the U-~haped structure diverge to open upwardly
to facilitate metal flow. The slight taper tends to improve
metal flow from exit end 26, but too great a taper ~ay cause a
lo~s of surface te~sion control and flooding of molten metal. A
taper of less than 10 psr side and preferably 1-5 is provided.
Exit e~d 26 includes bottom wall 28 which has a
generally planar inside portion having a length sufficient to
provide a substantially uniform flow of molten metal from the
e~it. Preferably, the length of the pianar wall portion as
measured in the direction of metal flow is at least equ~l to the
depth of molten metal pool to be contained in e~it end 26. More
preferably, the ratio of length to depth is at least l:l or
greater. Exit end 26 preferably has fixed or uniform dimensioins
o width and height throughout the length of the planar inside
surface of bottom wall 28 to define a uniform cross-sectional
area 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 re~eiving end 22 and the exit end 26 should be
provided in order to have a substantially uniorm 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 decreasing depth ~o as to maintain a substantially
uniform cross-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 xeceiving
end 22 to the exit end 26. Similarly, intermediate section 24
may have at least one ~idewall 34 which fans outwardly in order
to provide a gradually increasing 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 al o illustrates that weirs or weir plates 36
may be used in casting vessel 18 such as in an intermediate
section 24 or near where ~ection 24 merges into exit end 26 in
order to further acilitate development of uniform flow. Weir
plates 36 should be made of a refractory or heat-resistant
material which is also resistant to corrosion by molten metal.
Kaowool refractory board, treated with a diluted colloidal silica
suspension has proven satisfactory. Weirs 36 may extend across
the entire width or a portion of the width of casting vessel 18.
~s shown in Figure 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 facilltate development
of a uniform fully-developed flow and to restrain movement of
surface oxides and slag.
12
Figures 2a and 2b illustrate ~he use of surface tension
of the flowing molten metal to form the surfaces of the s~rip
being cast. Figure 2a is a detailed elevation view in partial
cross section of exit end 26 adjacent casting surface 20. Molten
metal flowing from the exit end 26 forms and maintains a meniscus
35 be~ween the inside surface of bottom wall 28 of the U-shaped
structure and the casting surface. The surface tension forming
meniscus 35 form~ the bottom of the strip 15 being cast. The
surface tension of the free surface of the molten metal pool in
exit end 26 forms a curvilin2ar portion 39 on the top of the
molten metal in the U ~haped structure as it forms the strip
product.
Figure 2b illustrates exi~ end 26 adjacent casting
surface 20 showing solidifying metal 19 therebetween in a view
lS 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 casting surface 20 at the inside surface 31 of sidewalls 30
near bottom wall 28.
A preferred embodiment of casting vesse} 18 is shown in
the elevation and top views of Figures 4 and 5, respectively.
Vessel 18 is shown having an outer metal support shell 38, a
re~ractory 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
13
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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 betwee~ the casting surface
and the vessel assembly and rubbing tbe ~aper against the vessel
18 to make the edges parallel to the wheel~ The ront surface 33
of the casting vessel 18 may then be brush coated with zirconia
cement and allowed to dry be~ore casting.
Figures 4 and 5 illustrate a preferred embodiment of
the casting vessel 18 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 used depending upon the type of material used for the
insulation layer 40. Insulation layer 40 may be a foamed-ceramic
cement insulation which would need an external support such as a
metal support shell 38. In the alternative, if a standard
refractory bricX or block i9 used and cemented together into the
desired shapes and then carved to achieve the desired inner and
outer dimension~, then the outer shell 38 is not necessary.
The vessel 18 m2y also be a monolithic shape formed from castable
ceramic material. Liner 42 on the internal surface of casting
ve~sel 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 also 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 heisht of the overflow element 44 determines the maximum
14
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depth of molten metal that may be contained in the receiving end
22 and, accordingly, the depth of the molten metal in the exit
end 26 of casting ves el 18. Overflow element 44 facilitates
control of the molten metal level in the casting vessei 18 which
is essential to gauge and quality control of the ca t strip.
Also shown in Figure 4 iq a casting vessel 18 which may
optionally include a cover a~sembly 46 in the vicinity of
intermediate section 24 of casting vessel 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 ge~erally composed of
a refractory insulative material resistant to molten metal.
Co~er 46 may comprise a liner 42, a refractory insulation layer
40 and an outer metal shell 3~ having a similar manner of
construction as is casting vessel 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 ~urface 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 illustrate~ 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 th~ U-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 combination.
~2~ 8
Means for providing a non-oxidizing atmosphere provides
a protective cover or blanket of inert or reducing gases in a
zone about the molten metal 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 ca~t into the cast strip. The non-oxidizing
atmosphere may be sta~ic, or a recirculating atmosphere.
Preferably, a non-contacti~g cover over the zone above the molten
metal pool at the exit end 26 of casting vessel 18 and at least
~ 10 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 i~ introduced qo that it impinges
in the zone on the top of the molten metal liguid pool where the
strip is emerging. The embodiment m3y provide a protective cover
for ~ealing the zone over the molten metal pool containing a
blanket of inert or reducing gases directed into streams of gases
to push any oxide away from the forming of the strip. The series
of narrow gas nozzles 56 i~ positioned along the width of the
casting strip so that streams or jets of gas impact the zone
wherein the ~trip emerges from the liquid pool. Nozzles 56 are
directed counter to the casting o the strip at an angle to the
plane of the formed stxip, preferably about 20~-30. The gas
blanket may be a gas selected from the group consi~ting of
hydrogen, argon, helium, and nitrogen in order to minimize the
oxides that may be formed duxing 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 and result in damage to the cast strip.
Means for radiantly cooling the molten metal in the
30 zone may include providing a coolant in the vicinity of the zone
16
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~cooled tubes 54 ~ealed to the top of the casting vessel
18 with refractory m~terial ~nd cement. Radiant cooling of the
top surface of molten metal as it flows from the U-shaped
structure of exit end 26 on~o casting surface improves the heat
extraction from the top surface of the solidifying moltesl metal
to improve as-cast strip top surface quality and structur~ by
controlling the growth o dendritic structure in the strip.
Preferably, means for providing a non-oxidi~ing
atmosphere and the means for radiantly cooling are used in
combination. ~ non-contacting cover for sealing th~ zone over
the molten metal at the exit end 26 incllldes 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 tubes 54 and a series of gas nozzles 56. The inert
gases in this embodiment are cooled by the tubes 54 which further
facilitate removal of the radlant heat. The cover containing the
cooling tubes 54 seals the zone 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 temperatures prior to introducing
molten metal into the casting vessel 18 for the production of
strip material. ~ny conventional heating means should be
suitable and may be used.~ ~n air-acetylene or air-natural gas
heating lance positioned in the receiving end 22, as well as
~2a¢~71~3
providing a preheat front cover for the front edges of the
casting vessel U-shaped structure which will be placed adjacent
the ~asting surfac~ 20. Normal preheating temperatures for
c~asting molten stainless steel may be on the order of
1900-2000F. After the minimum preheat levels desired are
reached, the heating lances are removed ancl the vessel 18 is
positioned adjacent the casting surface at a preset standoff
distance ~uch as between 5 and 20 mils.
In commencing 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 feed tundish 14 and
thereafter to the casting vessel 18 which is oriented
~ubstantially horizonta~ly. The flow of molten metal from feed
tundish 14 to casting vessel 18 may be controlled and regulated
by valve mean~ such as stopper rod 16 and through spout 17 into
the rear feed section or receiving end 22 of casting vessel 18.
~s vessel 18 begins to fill with molten metal, the molten metal
begins to flow in a dire~tion toward the exit end of the vessel
and flows through an intermediate section 24 and the exit end 26
as qhown in Figure 2. Casting vessel 18 permits the molten metal
to flow 50 as to feed the molten metal to the exit end 26 of
vessel 18. Casting ves~el 18 may include weirs 36 such as shown
in Figure 2 to dampen and ba~fle 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
~he exit end 26. Generally exit end 26 is wider than the
receiving end 22 and the ~-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
7~
section. Casting vessel 18 is designed to prevent cross flows of
molten me~al within the vessel while developing a uniform
turbulent flow from exi~ end 26 across the width of the U-shaped
tructure il~ end 26 ~uch that the fully-developed flow has the
bulk of the velocities in the direction of flow from the
receiving end 22 to the exit end 26. The level of molten metal
in exit end 26 is about the same as the level in receiving end
22, although the depth of the molten metal will be less in exit
end 26. The molten metal continues to flow from the exit end 25
onto the moving casting surface 20 such that across the width of
the U-~haped structure of exit end, a substantially uniform flow
of molten metal is presented to the casting surface 20. The
molten metal in exit end 26 h~s a top surface tension and the
molten metal leaving the opening has edge surface tension which
form, in part, the top and edges~ 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 the
U-~haped structure and the ca~ting 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 vessel 18 commences with the molten metal contacting
the castiny 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 molten 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
19
~2~ 7~
molten metal solidified as it was pulled out of the vessel 18
adjacent the top curvilinear surface tension portion 39. It i9
estimated that more than 70% and probably more than about 80% of
the strip thickness results from the pool of molten metal
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 o~ the opening. The
po ition 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 vessel 18 is positianed,
preferably t 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 abili~y
to cast desired gauges of metal strip ranging from a . ol to 0.06
inch with good surface guality 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 the
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.
xample_I
A casting vessel having the structure generally as
.7~31
show~ in the Figure 2 but having only one weir plate 36 near the
f..'`~`. exit end 26 was constructed from hardened blocks of Kaowool~
refractory, which is an alumina-silica composition material. It
was treated by soaking it with a colloidal silica suspension
S dried overnight at 250F and then fired for 1 hour at 2000F in
air. After the blocks were cut and shaped, they were coated with
a thin layer of Xaowool 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. A weir of similar
composition was used. The casting vessel was then heated with
alr-acetylene lances. The vessel 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 1.75 inches deep in the receiving end 22 and the
molten metal was about 0.75 inch~s deep in the U-shaped structure
in the exit end 26 of vessel 18. A 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.
The 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
width of about 4 inches and a uniform thickness of from 16 to 18
rf c~ ~ a r k .
21
~2~ 7~3
mils having smooth and uniform upper and lower surfaces as-cast
and flat edges ~howing no signs of raygedness or curls.
~2~
~ casting vessel having a structure generally as shown
in Figure 4 was constructed having a RoawooL refractory and
alumina bubble refractory insulation 40 in a metal shell 38. The
liner 42 was made of Fiber~rax 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 use. The
ves el 18 outside dimPnsions wexe about 15 inches long and 18
inches wide at the exit end vessel 18 had a sligh~ increasing
cro~-sectional area to exit end 26. Weir plate 36 was made
and positioned similar to Example I and cemented between
; 15 sidewalls of ve3sel 18. The inside surfaces 31 of sid~walls 30
were also tapered or diverging on the order of about 3 per
surface. The casting ve~sel was set at a standoff distance of
about 35 mil~ at an angle of about 0 for the free surface of the
molten metal wa~ near the crown of the casting wheel.
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 thicknes~, 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 l6.98
cm) in the receiving end 22.
The vessel 18 also included a cover having a means for
radiantly cooling and means for providing a helium atmosphere as
shown in Figure 6. The cooling was effected by circulated water
at about 3 gallons per minute through copper tubing having a
0.375-inch outside diameter.
The as-cast strip wa abou~ 13 inches wide~ and having
5' a uniform thickness of about 45 mils and having good upper
~urface quality which wa~ uniform, smooth and crack free. The
as-cast strip was then conventionally processed by pickling in a
nitric/hypofluxoic acid, cold rolling about 50% reduction,
annealing at 1950 for 5 mi~utes, pickli~g again in a similar
manner, and then cold rolling to 5 mils and annealed. The room
temperature mechanical propertie~ of the annealed as-cast sa~ples
are shown below in comparison to typical propertie~ of
conventionally produced Type 304 annsaled hot-roll ban~.
Table
0.2~Elongation in
Samples Ten~ile Strength Yield Strength 2 inch.
(KSI) (RSI) (~)
_ _ ___
1 104.6 44.6 52.0
2 100.~ 40.8 50.0
3 100.3 40.8 49.0
4 100.0 40.0 52.5
7 102.8 42.0 55.0
8 102.0 42.0 57.5
~ 103.6 44.0 52.0
105.2 44.0 s4.s
Type 304 alloy conventionally produced may have typical
or average room temperature mechanical properties of annealed
hot-roll band of 101.1 RSI tensile strength, 43.8 KSI yield
strength and 57~ elongation in 2 inches.
23
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 ~agnification,
illustrates the typical as-cast structure of small columnar cells
oriented in the direction of strip thickness, i.e., top to bottom
surfaces. This direction generally conformc; to the direction of
heat extraction from the strip as it solidifie~ 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 conventionally processed into finished strip
having properties comparable to or better than conventionally
pxoduced strip product.
Figure 8 illu~trates a typical structure of a
lS conventionally produced hot-roll band of Type 304 alloy at lOOX
magnification.
lt is observed that the ~ethod and apparat~s of the
present invention results in even better strip structure and
quality as the gauge of the strip product increases and as the
width of the strip increases. The tendency of edge curl in the
~trip 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
metal- and alloys, including stainless steels and silicon steels.
2~
.
