Sheet caster cooling

A more uniform cooling system for direct strip forming processes on chilled drums wherein the drum 1 is hollow and is partially filled with a coolant pool 6. A central tube 7 provides coolant spray 8 to the portions of the drum interior when not covered with the coolant pool. Patterned cooling for thickness control is practiced by changing the heat conduction through the drum wall at selected locations.

TECHNICAL FIELD 
The invention relates to casting metal sheet directly from the melt onto a 
cylindrical casting drum. Presently, a layer of molten metal is metered to 
the outer surface of the rotating drum. The drum is generally cooled by 
means of a liquid coolant circulated through a labyrinth of cooling 
channels on the inside surface. The cooling effect to the outside casting 
surface is highly localized around the cooling channels and leads to 
relatively warmer and cooler areas, differential cooling and non-uniform 
sheet thickness. 
On way to avoid the localized cooling is to provide a hollow drum having a 
pool of coolant therein free to contact the inside surface below the pool. 
Such design is shown in the August 1986 issue of I&SM at page 29, and in 
French Pat. No. 1,364,717 and U.S. Pat. No. 2,561,636. As shown in the 
latter, the pool covers the lower part of the drum inside surface when the 
drum is rotating at low speeds. The patent proposes a nozzle to spray the 
inside surface above the pool. 
SUMMARY OF THE INVENTION 
It is an object of the invention to provide a method and apparatus for 
casting uniform sheet directly from the melt. 
It is also an object to provide a method and apparatus for casting sheet 
with a preselected thickness profile. 
It is further an object to provide such a sheet casting method and 
apparatus by controlled cooling of the casting surface. 
In accordance with the objectives, the invention is a method and apparatus 
for casting metal sheet with a selected thickness profile directly from 
the melt. The apparatus comprises a hollow, cylindrical casting drum 
having an outer casting surface, means for rotating the casting drum about 
the cylinder axis, tundish means for delivering a layer of molten metal to 
the casting surface, a liquid coolant pool in the interior of the hollow 
casting drum, means for maintaining a coolant level below about the 
cylinder axis, means for spraying a coolant liquid inside the casting drum 
against substantially the entire inside surface of the casting drum above 
the liquid level, and means for changing the heat transfer through the 
drum wall in selected locations. The heat transfer may be decreased by 
making the drum wall thicker, by insulating the wall or by changing the 
temperature of the spray at selected location. Higher temperature of the 
casting surface produces thinner sheet. 
The method for sheet casting generally comprises rotating a hollow, 
cylindrical casting drum having an outer casting surface about a generally 
horizontal cylinder axis, controlling the heat transfer through the drum 
wall in a preselected manner, delivering a molten metal layer to the outer 
casting surface, cooling the interior of the casting drum by admitting a 
liquid coolant therein, maintaining a coolant liquid level below about the 
cylinder axis and spraying a liquid coolant inside the casting drum onto 
substantially the entire surface of the drum above the coolant liquid 
level.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The invention relates to direct metal forming of sheet on a chill surface. 
Such processes attempt to directly produce net shape or near net shape 
castings, thus avoiding costly rolling operations. 
As shown in FIGS. 2 and 3, a cylindrical drum 1 may be used to cast metal 
sheet 5. Molten metal 4 is held in a tundish 3 in close proximity to the 
casting drum. The molten metal typically contacts the outer casting 
surface 15 of the drum and solidifies to a rigid sheet before removal from 
the drum. The casting drum is rotated along the cylinder axis by 
conventional means. It is cooled by a pool of coolant 6 inside the drum. 
The coolant is unconstrained by cooling coils, for example, in the drum. 
Fresh coolant is typically constantly added and warm coolant removed. A 
perforated spray conduit 7 runs along the cylinder axis of the drum. 
Coolant is passed through the conduit and is sprayed through the 
perforations 11 onto substantially the entire inside surface 13 of the 
drum above the liquid level of the coolant pool 6. A second coolant may be 
simultaneously added through conduits 16 and 17 and perforations 18 and 
sprayed against selected portions (the drum ends in this case) of the 
inside surface. The second coolant may have a different temperature or 
composition, for example, to change the cooling characteristics across the 
drum surface. As shown best in FIG. 2, the coolant pool 6 may be expanded 
by additions through conduit 9 or through the perforated conduit 7. 
Coolant may be removed through conduit 10. These conduits may enter and 
exit the drum through fixed bearings 12. The drum may then rotate around 
the fixed bearing. 
The liquid level of the coolant is preferably maintained at slightly below 
the cylinder axis and the perforated conduit 7 so that the perforations 
are open and able to spray coolant on the exposed drum inner surface. At 
high rotational speeds, centrifugal force will push the coolant against 
the entire inside surface. At slower speeds, however, the coolant tends to 
stay in the bottom portion of the drum. The coolant spray then continues 
to cool the upper surface area which is not covered by the coolant pool at 
slower speeds. It has been found that at slower speeds generally used for 
making sheet and in the absence of spraying over substantially the entire 
inside surface, nucleate boiling can occur on the uncovered surface. This 
causes adherent bubbles which, when rotated into the coolant pool, can 
prevent good coolant contact. 
The invention allows good control over the cooling rate of the sheet. It 
also allows the patterned cooling of the wheel and thickness control of 
the sheet. For example, the heat transfer characteristics of the drum 
surface are altered locally to cause a change in the thickness of the 
sheet. In FIG. 3, segments 22 have been added to increase the thickness of 
the wall and to lower the heat transfer rate. The casting surface is 
therefore hotter in these areas and the sheet will be thinner. The design 
shown in FIG. 3 would be useful for casting a sheet with a thicker center 
strip or would at least compensate for the normally lower temperatures 
near the edges of the drum. The segments 22 can be conductive or 
insulating materials. If the former, the drum could be machined in that 
manner during manufacture. Or separate pieces of conductive or insulating 
materials could be selectively joined to the inside surface in selection 
location. 
There are other ways to selectively change the heat transfer of the drum 
wall. One way is to spray only a portion of the inner wall, but it is 
preferred to provide at least two different temperature "coolant" sprays 
in the interior of the drum at selected locations. A higher temperature 
spray would still prevent the deleterious bubble formation, but would 
reduce the heat transfer at such locations compared with the lower 
temperature spray. 
Two different coolant compositions could also be used to selectively change 
the heat transfer characteristics. Different coolants or different 
temperature coolants may be added in any known manner, for example, as 
shown in FIG. 2.