In-situ conditioning of a strip casting roll

A strip caster (10) for producing a continuous strip (24) has a tundish (12) for containing a melt (14) and a pair of horizontally disposed water cooled casting rolls (22). The casting rolls are juxtaposed relative to one another for forming a pouring basin (18) for receiving the melt through a teeming tube (16) thereby establishing a meniscus (20) between the rolls for forming a strip (24). The melt is protected from the outside air by a non-oxidizing gas passed through a supply line (28) to a sealing chamber (26). Devices (29) for conditioning the outer peripheral chill surfaces of the casting rolls includes grit blasting nozzles (30A, 30B, 30C, 30D), a collection trough (32) for gathering the grit, a line (34) for recycling the grit to a bag house (36), a feeder (38) and a pressurized distributor (40) for delivering the grit to the nozzles. The conditioning nozzles remove dirt, metal oxides and surface imperfections providing a clean surface readily wetted by the melt.

BACKGROUND OF THE INVENTION 
This invention relates to a method and an apparatus for conditioning the 
chill surface of a strip casting roll. More particularly, the invention 
relates to cleaning the peripheral surface of the casting roll by in-situ 
grit blasting to remove dirt, metal oxides and surface imperfections that 
accumulate or form on the chill surface during casting of metal strip. 
It is known to use brushes, buffers, grinders, flapper type devices and the 
like for conditioning the peripheral or circumferential surface of a 
casting roll used to solidify a metal melt into continuous metal strip. 
These mechanical conditioning devices may be ineffective for completely 
removing tightly adherent metal oxide. If metal oxide is not completely 
removed, the molten metal may not uniformly wet the roll chill surface and 
the solidified strip may not be adherent to the surface of the roll. These 
conditioning devices also tend to cut or otherwise mark the surface of the 
casting rolls. Scratch or grinding lines left on the surface tend to form 
surface defects on the strip. These conditioning devices also have a 
tendency to vibrate the casting roll which may affect the meniscus causing 
blemishes and cracks on the strip surface. These conditions also are 
undesirable because for strips having wide widths the devices must be 
segmented causing inhomogeneous surface conditions such as forming lines 
at the joints between the segments. These lines or seams may cause 
differences in the cast strip due to the effect on heat extraction. Also, 
there is a tendency to remove desirable surface finish of the casting 
roll. 
Accordingly, there remains a need for removing metal oxides from the chill 
surface of a casting roll without leaving grinding lines or gouges. There 
also remains a need for conditioning the chill surface without vibrating 
the casting roll. 
BRIEF SUMMARY OF THE INVENTION 
A principal object of the invention is to remove metal oxide from the 
peripheral surface of a casting roll. 
Other objects include improving wetting of molten metal to the surface of a 
casting roll thereby providing uniform strip adhesion across the width of 
the roll surface, elimination of surface defects to the strip surface, 
elimination of vibration to the casting roll, providing a uniform casting 
roll surface and maintaining a fiat strip surface during casting. 
The invention relates to conditioning the surface of a strip casting roll. 
The invention is for a method and an apparatus therefor for casting a 
metal melt into a continuous strip including providing a melt of the metal 
to be cast, providing a casting roll having a peripheral chill surface 
having a width at least as wide as the width of the strip, rotating the 
casting roll, blasting the entire width of the chill surface in-situ with 
grit for removing dirt, metal oxides, surface imperfections, and the like 
thereby providing a clean surface readily wetted by the melt, casting the 
melt onto the clean surface forming the continuous strip and removing the 
strip from the surface of the casting roll. 
Another feature of the invention is to surround the cleaned roll surface 
with a non-oxidizing atmosphere and maintaining the cleaned surface in the 
non-oxidizing atmosphere until the dean surface is wetted by the melt. 
Another feature of the invention includes providing a plurality of evenly 
spaced nozzles for jetting the grit against the chill surface of the 
casting roll. 
Another feature of the invention includes providing means for traversing 
the nozzles across the full width of the chill surface. 
Advantages of the invention include improved wetting of a metal melt to the 
peripheral surface of a casting roll, improved adhesion of the 
as-solidified strip to the roll surface, uniform adhesion across the width 
of the roll surface, elimination of vibration to the casting roll, 
elimination of surface defects on the surface of the as-cast strip, 
uniform cleaning of the chill surface of the casting roll and formation of 
a variety of surface finishes on the casting roll. 
The above and other objects, features and advantages of the invention will 
become apparent upon consideration of the detailed description and 
appended drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
This invention relates to a method and an apparatus for in-situ 
conditioning the outer or peripheral chill surface of a casting roll by 
grit blasting. The chill surface is conditioned by removing dirt, metal 
oxides, surface imperfections and other non-uniformities that otherwise 
tend to accumulate or be formed on the chill surface during casting of 
metal strip. During casting, the chill surface of a casting roll will 
normally have a temperature of at least 300.degree. C. When casting steel, 
the chill surface temperature may approach 600.degree. C. At a temperature 
near 600.degree. C., constituents contained in the melt such as aluminum, 
silicon and chromium tend to form oxides which tenaciously become adhered 
to the chill surface of the casting roll. These oxides decrease wetting of 
the molten metal to the chill surface and must be completely removed prior 
to contacting the melt to the casting roll. Impaired wetting of the melt 
to the casting roll can result in tearing of the strip and strip stickers. 
Various other defects to the strip surface such as blow holes can form 
from gases evolving when an oxide layer is permitted to buildup on the 
chill surface. Other problems associated with surface oxides include 
reduced thickness of the cast strip after heavy build up of oxide and 
retarding heat extraction from the strip. 
The casting roll is water cooled and normally fabricated from a highly 
conductive metal such as copper. To enhance adherence of the as-cast strip 
to the surface of the roll and to improve heat and wear resistance of roll 
surface, the peripheral or chill surface of the roll may be electroplated 
with an oxide resistant coating having a thickness of about 0.2-1.0 mm 
such as nickel. 
In this invention, by in-situ conditioning of the casting roll will be 
understood to mean cleaning of the roll surface during the strip casting 
operation. That is, the chill surface of the casting roll is cleaned on a 
continuous basis so that a clean surface is always available for the melt. 
By grit blasting will be understood to mean shot peening, bead blasting, 
jetting, toughening or cleaning of the outer or circumferential annular 
face of the casting roll by hard particulate type materials. Particles 
satisfactory for this purpose include such materials as alumina (Al.sub.2 
O.sub.3), iron oxide (Fe.sub.2 O.sub.3) and glass beads (SiO.sub.2). The 
continuous strip of the invention can be formed from a variety of ferrous 
and non-ferrous molten metals such as stainless steel, alloy steel, low 
carbon steel, aluminum and aluminum alloys, copper and copper alloys and 
amorphous metals. 
Referring to FIG. 1, reference numeral 10 denotes a caster for producing a 
continuous strip. Caster 10 includes a tundish 12 for containing a melt 
14, a pair of horizontally disposed water cooled casting rolls 22 and 
means (not shown) for rotating rolls 22 toward one another as indicated by 
arrows 23. Casting rolls 22 are juxtaposed relative to one another for 
forming a pouring basin 18 for receiving melt 14 thereby establishing a 
meniscus 20 between the rolls. Melt 14 is delivered to pouring basin 18 
through a teeming tube 16. As molten metal 20 is withdrawn from between 
rolls 22, a solidified strip 24 is formed. Preferably, melt 14 is 
protected from the outside air by a non-oxidizing gas within a sealing 
chamber 26 through a gas supply line 28. Suitable protective gas that may 
be used include nitrogen, argon, helium, hydrogen, carbon monoxide, carbon 
dioxide and ammonia. Means 29 for conditioning the outer peripheral chill 
surfaces of casting rolls 22 includes one or more grit blasting nozzles 
30, a collection trough 32 for gathering the grit after impacting the 
chill surface of the casting roll, a line 34 for recycling the grit to a 
bag house 36, a feeder 38 and a pressurized distributor 40 for delivering 
the grit to nozzle 30. It may be desirable to have more than one bag house 
for storing more than one type grit. For example, a second type grit may 
be used to provide a different type texture finish to the chill surface of 
the casting roll. 
The type strip caster illustrated in FIG. 1 is commonly referred to a twin 
roll or dual drum caster. It will be understood conditioning means 29 of 
the invention also could be used with a single roll caster as well. Unlike 
the twin roll caster of FIG. 1 wherein the strip is withdrawn from below 
an opposing pair of rolls, a strip is formed by being pulled over the top 
of the casting roll in a single roll caster. 
FIG. 2 is an enlarged view of the caster with sealing chamber 26 removed. 
Pouring basin 18 is formed between the meniscus between rolls 22 by a pair 
of side dams 42. In the embodiment illustrated, the conditioning means 
includes one row of four nozzles 30A, 30B, 30C and 30D evenly spaced from 
one another and positioned a short distance away from a chill surface 44 
of each casting roll 22. Nozzles 30A, 30B, 30C and 30D are evenly spaced 
across the entire width W of chill surface 44 to uniformly and completely 
clean the entire width. It may be desirable to provide more than one row 
of the nozzles to even out any cleaning irregularity by the cleaning 
action of the nozzles. 
FIG. 3 is an enlarged view illustrating the spacing of nozzles 30A, 30B, 
30C and 30D between each other and chill surface 44 of casting roll 22. In 
this embodiment, distributor 40 includes means 45 for traversing nozzles 
30A, 30B, 30C and 30D across the width W of chill surface 44. Traversing 
means 45 includes an air cylinder 46, a piston 54 and a traversing arm 48 
connected to the piston by a bolt 52. Nozzles 30A, 30B, 30C and 30D are 
mounted onto traversing arm 48. Traversing means 45 allows the nozzles to 
oscillate in a direction indicated by an arrow 50 to insure complete and 
uniform cleaning by grit 56 of dirt, metal oxides, surface imperfections 
and any non-uniformities that may occur on chill surface 44 of casting 
roll 22. Traversing means 45 also tends to even out any irregularity in 
the cleaning action of the nozzles. 
In addition to protecting the melt from atmospheric oxidation, it also may 
be desirable to protect cleaned surface 44 of roll 22 from oxidation from 
the outside air as well, particularly when casting metals such as steel. 
When casting steel, the chill surface temperature of the casting roll 
typically approaches 600.degree. C. At this temperature, the chill surface 
may re-oxidize prior to being contacted by the melt thereby diminishing 
adhesion of the steel strip to the chill surface. In this case, it may be 
desirable to surround the cleaned portion of the casting roll with a 
non-oxidizing atmosphere similar to those recommended for sealing chamber 
26. The clean chill surface would be protected by the non-oxidizing gas 
until the clean surface is rotated to and becomes covered by the melt pool 
in the pouring basin. 
It will be understood various modifications may be made to the invention 
without departing from the spirit and scope of it. Therefore, the limits 
of the invention should be determined from the appended claims.