Flow casting

A process for casting metallic strips thicker than from melt-drag processes but thinner than the inherent normal thickness of a cast melt is disclosed. The disclosed process casts molten metal, without decanting or accelerating the molten stream, onto a channel-shaped chill surface. After casting, the molten strip acquires a high thermal gradient. Following thermal shrinkage of the underside the molten top layer is squeegeed using a chill roll to uniformly distribute and crystallize the top surface.

BACKGROUND OF THE INVENTION 
This invention relates to methods of casting metallic sheets or strips from 
molten metal. More particularly, this invention relates to methods of 
casting relatively wide polycrystalline metallic strips having a thickness 
exceeding that obtainable by melt drag processes and generally thinner 
than the thickness inherent in a melt attributable to surface tension of 
the molten metal. 
This invention relates to methods of casting polycrystalline metallic 
strips having thicknesses of 20-500 mils at high quench rates and having 
top and bottom surfaces of similar and uniform crystalline microstructure. 
DESCRIPTION OF RELATED ART 
The rapid solidification of metals to form metal strip by the melt drag 
process is described in numerous patents, such as U.S. Pat. Nos. 
3,522,836; 3,605,863; 4,479,528 and 4,484,614. The process generally 
comprises forming a meniscus of molten metal at the outlet of a tundish 
nozzle, and dragging a chill surface through the meniscus. Molten metal 
thereby contacts the chill surface and solidifies thereon to form a thin 
metal strip. 
Melt drag processes involve puddling a molten stream and almost 
instantaneously accelerating the forming strip from 0 velocity to the 
velocity of the spinning wheel. This acceleration occurs in the process of 
essentially drawing the strip out of the stream puddle. Molten metal is 
left behind in this process as the strip formed is solidified and 
withdrawn as it is formed. Melt drag or melt extraction is a decantation 
type of process. 
For clarity in understanding, the present invention, unlike decantation 
processes, can be thought of similar or parallel to squirting caulking 
paste out of a tube onto a surface moving at the same rate as the paste 
exudes from the tube. The element of acceleration is eliminated. 
In the present invention, molten metal adheres to and rides with over the 
solid strip formed upon contacting the chill surface. Decantation is 
eliminated. 
The dynamics of solid metal growth are slow. Solid growth proceeds at a 
rate proportional to the square root of time. It takes 4 times as long to 
double the strip thickness. Thus, processes relying on rapidly spinning 
circular chill surfaces become impractical for forming thicker strips. 
During cooling, gravitational forces would cause molten metal running. 
Narasimhan (U.S. Pat. No. 4,142,371) discloses an apparatus for producing 
thin amorphous strip through a thin split discharge opening in a tundish, 
and depositing molten metal onto a belt-like movable chill body moving at 
a velocity of from 100 to 2000 meters per minute. Similar to other melt 
drag processes, decantation and acceleration of the melt is involved. 
Narasimhan produced thin strip of from 0.002-0.008 inch thickness. 
Also relying on decantation and acceleration of the melt is Smith U.S. Pat. 
No. 4,290,476. Smith discloses an apparatus for planar flow casting of 
metal ribbon. The Smith apparatus includes a tundish nozzle having a 
planar bottom surface which includes the leading edge of a first lip and 
side edges at the bottom of the lips; all points on the bottom of the lips 
being at least as far from the chill surface as is the first lip but no 
further from the chill surface than about 1 mm. The chill surface is 
stated to ordinarily move at a predetermined velocity at least about 200 
meters per minute. 
The present inventive process derives a process yielding thicker metal 
strips, compared to the ribbons of the prior art, and having more uniform 
surface characteristics. 
Molten metal deposited onto flat bodies has an inherent thickness due to 
surface tension of the melt. Until this invention, economic methods did 
not exist for direct casting of strip thicker than the tape-like strips of 
melt drag processes, but thinner than the inherent normal thickness of 
molten metal due to surface tension. 
By enabling direct casting of strips of selective thickness, savings are 
immediately realizable in elimination or minimizing of costly rolling and 
annealing cycles.

SUMMARY OF THE INVENTION 
The present invention discloses a process for casting strips of metal, more 
particularly strips of 20-500 mils (0.02-0.5 inch) thickness at high 
quench rates. 
The present invention provides a new and improved process for casting strip 
materially, particularly polycrystalline strip material. Such process 
comprises providing a flat chill surface movable relative to a tundish 
having an orifice for receiving and holding molten metal and having a 
discharge opening through which molten metal is deliverable to the chill 
surface. Between the tundish and chill surface, a channel-shaped 
cross-sectional space serves to confine the cast molten metal for a time 
sufficient for surface solidification to occur such that the molten metal 
occupies the volume of the channel shape and forms a thin bar product. The 
full volume of the channel-shaped cross-sectional space is defined by the 
tundish and chill surface as the tundish moves relative to the length of 
the chill surface. 
DETAILED DESCRIPTION 
Referring particularly to the drawings, FIGS. 2 and 5 generally illustrated 
the process of the present invention of casting molten metal into a 
channel-shaped space. By restricting two directions of flow of the melt, 
the molten metal is substantially confined to a uniform bar shaped volume. 
In FIG. 5 the tundish 1 is shown positioned on chill surface 2. Tundish 1 
has molten metal discharge opening 3 through which molten metal contacts 
the chill surface and fills channel-shaped space 4S. 
FIGS. 2 and 5 in particular illustrate molten metal 5 being discharged 
through discharge opening 3. Molten metal 5A prior to squeegeeing is 
rounded by surface tension and shown as uniformly smooth metal 5B after 
squeegeeing with chill roll 7. 
In FIGS. 1 through 5 the channel-shaped space 4S is located in the chill 
surface. FIG. 5 depicts the channel shaped chill surface 2 as a segmented 
belt. Tundish 1 rides on flanges 2A. 
The present invention allows an improvement over melt drag processes in 
that thicker and shaped polycrystalline strips can be cast. 
It has been found particularly advantageous to provide a squeegee 
preferably in the form of a chill roll 7 to immediately smoothen or 
squeegee the melt in the channel-shaped space. The roller, preferably 
resting and riding on the raised sides of the chill surface, hastens 
quenching and equalizes the top side in terms of polycrystallinity with 
the polycrystalline surface of the side of the melt contacting the flat 
chill surface. 
The chill surface can be a flat or channel-shaped length of metal or can be 
made into a belt, for example, composed of small composite segments. 
Copper is preferred as the chill surface through other heat conducting 
materials can be used. The chill surface must be able to absorb the heat 
from contact with molten metal. With more continuous operations, cooling 
by conduction can be augmented by using fluid, namely water, cooling 
through or to the underside of the chill surface. Refrigerated fluids or 
gases can also be advantageously used. As would be evident, such cooling 
can be applied to all chill surfaces described herein including the 
cooling squeegee or roll. 
The chill surface is moved relative the tundish at a rate preferably about 
1 meter/second and up to 2.5 meters per second. The ideal rate of movement 
is the rate the melt is leaving the tundish. 
This process enables manufacture of strip of a lesser thickness than that 
dictated by the surface tension of the metal. Molten metal has an inherent 
thickness due to surface tension of the melt; however, the strip formed 
from the melt by this invention has a solid undersurface layer formed upon 
contact with the chill surface. Over the solid layer a molten layer is 
carried along wetted to the solidified underlayer. The molten layer is 
immediately hot rolled, actually squeegeed so as to cool, thin, smooth and 
solidify the top surface. Such two sided cooling enables obtaining a 
smoother strip whose surfaces are of relatively uniform microstructure. 
This hot rolling is enabled because the cast metal has a strong thermal 
gradient, more specifically a wet or molten top surface but a solidified 
undersurface. Normally, hot rolling of just-cast hot metal would ruin the 
casting. 
Hot rolling or using double roll systems previously had been problematic 
and has not been widely practiced in the industry. This process makes hot 
rolling useful in a more simple but effective manner to yield a more 
uniform product having substantially similar top and bottom surfaces. 
In practicing this invention, the channel--shaped area is formed in the 
substrate. This can be conveniently accomplished by use of a one piece 
chill surface with carved channel or assembled from a flat bar plus edges, 
shims or flanges 2A on either side of the chill surface 2 and on which the 
tundish rides. Advantageously the chill surface can be a copper segment 
belt with two copper shimming belts defining a channel-shaped chill 
surface between them. A particularly efficient way of placing the shimming 
belts is around three rollers external to the copper segment chill surface 
belt. The tundish then can be placed riding on the shims but within the 
circuitous, triangular, path traveled by the shimming belts around and 
over the tundish. The shimming belts would lift off the chill surface 
after strip solidification. In this manner shorter shimming belts can be 
used. 
To change strip metal thickness, a different thickness shim or flange can 
be applied. 
In the preferred embodiment, the tundish discharge opening is selected such 
that when the molten metal is cast into a strip the strip's edges do not 
actually make substantial contact with the shimming material until after 
rolling or squeegeeing. This procedure can avoid some materials problems 
associated with intense heat transfer including shrinkage, warpage, and 
the like. Simple copper strapping material can be made into a useful 
shimming belt. 
The shim material is preferably loosely held against the chill surface 
rather than bolted or screwed to the chill surface. The heat absorbed from 
the molten metal tends to buckle and warp the shim material if bolted, 
therefore, less rigid adherence is preferred, the optimal amount of 
securing being readily ascertainable. 
A revolving channel-shaped belt as the chill surface would be preferred. 
The belt would move at less than 2.5 m/sec, preferably about 1 
meter/second. 
In FIG. 5, the tundish floor has an orifice serving as a discharge opening 
3 substantially centrally located and toward the forward end of the 
tundish. The longitudinal extent of discharge opening 3 approximates the 
approximate width of the strip to be cast. Uniform flow of metal through 
the discharge opening is provided by maintaining a quantity of molten 
metal in the tundish to exert a metallostatic head pressure sufficient to 
cause flow out discharge opening 3 as the tundish or chill surface is 
moved. 
The tundish is advantageously constructed of heat insulating material such 
as firebrick. Other molten metal resistant materials can also be employed 
including by way of illustrations graphites, carbides such as silicon 
carbide, alumina, or zirconia. 
The process of the present invention yields a thicker bar product than the 
strips of the prior art. This bar product is polycrystalline and can be 
rolled to sheet products with less rolling and less energy expenditure 
than the currently practiced mill rolling operation. 
The method for casting metal strip from a melt according to this invention 
comprises the steps of: providing a flat chill surface; providing a 
tundish having an orifice for receiving and holding molten metal and 
having a discharge opening through which molten metal is deliverable to 
the chill surface as the tundish is moved relative the chill surface; and 
providing a channel-shaped cross-sectional space whose volume is defined 
by the chill surface and tundish as it moves relative the length of the 
chill surface. Then, a quantity of molten metal is introduced into the 
tundish, the molten metal having a surface tension such that the metal 
flows from the tundish through the discharge opening and into the 
channel-shaped space as the tundish is moved relative the length of the 
chill surface. Finally, after metal introduction, the tundish is moved 
relative the chill surface such that a thin bar strip of metal is cast 
within the volume of the channel-shaped space. 
Since the motion of the tundish is relative the chill surface, of course 
either any one or both can be moved to provide relative motion. 
Squeegeeing of the cast strip can be accomplished using a roll having a 
chill surface. Such rolling or squeegeeing should be accomplished at the 
point the cast melt undergoes thermal shrinkage and unsticks from the 
underlying chill surface or belt substrate. 
The above process for casting strip material can be practiced by providing 
a channel-shaped chill surface comprising a flat length of metal having 
raised sides defining a channel therebetween. A tundish for receiving and 
holding molten metal having a discharge opening therein can be provided 
through which molten metal is deliverable to the chill surface as the 
tundish is moved relative the chill surface. A reservoir of molten metal 
should be provided in the tundish at a gas overpressure or metallostatic 
head pressure sufficient to cause melt flow from the tundish. At least 
one-quarter pound per square inch as the discharge opening within one 
second after pouring is initiated would be sufficient head pressure. 
Additional molten metal should be poured into the tundish at a rate 
sufficient to maintain a substantially constant pressure at the discharge 
opening through the casting operation. 
Alternatively, one can provide a channel-shaped chill surface comprising a 
flat length of heat conducting material preferably in belt form having 
raised sides defining a channel therebetween such as exemplified in FIG. 
5. A tundish for receiving and holding molten metal having a discharge 
opening therein can be provided through which molten metal is deliverable 
to the chill surface as the tundish is moved relative the chill surface. 
It would be advantageous to additionally provide a squeegee 7 or chilled 
roll resting and riding on the raised sides of the chill surface spanning 
the channel of the chill surface. A quantity of molten metal can then be 
introduced into the tundish. After metal introduction, the tundish can be 
moved relative the chill surface such that a thin strip of metal, 
preferably of 0.02-0.5 inches thickness, is cast within the channel of the 
channel-shaped chill surface. The cast strip next is rolled so as to 
squeegee the molten top surface of the metal cast within the channel of 
the channel-shaped chill surface. 
The principles, preferred embodiments, and modes of operation of the 
present invention have been described in the foregoing specification. The 
invention which is intended to be protected herein, however, is not to be 
construed as limited to the particular forms disclosed, since these are to 
be regarded as illustrative rather than restrictive. Variations and 
changes can be made by those skilled in the art without departing from the 
spirit of the invention.