This invention relates to the casting of metal strip. It has particular application to the casting of metal strip by continuous casting in a twin roll caster.
In a twin roll caster molten metal is introduced between a pair of contra-rotated horizontal casting rolls which are cooled so that metal shells solidify on the moving roll surfaces and are brought together at the nip between them to produce a solidified strip product delivered downwardly from the nip between the rolls. The term “nip” is used herein to refer to the general region at which the rolls are closest together. The molten metal may be poured from a ladle into a smaller vessel or series of smaller vessels from which it flows through a metal delivery nozzle located above the nip so as to direct it into the nip between the rolls, so forming a casting pool of molten metal supported on the casting surfaces of the rolls immediately above the nip and extending along the length of the nip. This casting pool is usually confined between side plates or dams held in sliding engagement with end surfaces of the rolls so as to dam the two ends of the casting pool against outflow, although alternative means such as electromagnetic barriers have also been proposed.
The setting up and adjustment of the casting rolls in a twin roll caster is a significant problem. The rolls must be accurately set to properly define an appropriate width for the nip, generally of the order of a few millimeters or less, and there must also be some means for allowing at least one of the rolls to move outwardly against a biasing force to accommodate fluctuations in strip thickness particularly during start up.
Usually, one of the rolls is mounted in fixed journals and the other is rotatably mounted on supports which can move against the action of biasing means to enable that roll to move laterally to accommodate fluctuations in strip thickness. The biasing means may be in the form of helical compression springs or alternatively, may comprise a pair of pressure fluid cylinder units.
A strip caster with spring biasing of the laterally moveable roll is disclosed in U.S. Pat. No. 6,167,943 to Fish et al. In that case the biasing springs act between the roll supports and a pair of thrust reaction structures, the positions of which can be set by operation of a pair of powered mechanical jacks to enable the initial compression of the springs to be adjusted to set initial compression forces which are equal at both ends of the roll. The positions of the roll supports need to be set and subsequently adjusted after commencement of casting so that the gap between the rolls is constant across the width of the nip in order to produce a strip of constant profile. However, as casting continues the profile of the strip will inevitably vary due to eccentricities in the rolls and dynamic changes due to variable heat expansion and other dynamic effects. Previously, there has been no means to provide dynamic wedge or profile control to suppress strip profile fluctuations during casting. By the present invention, it is possible to provide a very effective means for such dynamic profile control.
A related problem dealing with variations due to eccentricities in the casting rolls where changes in the casting speed causes variation in strip thickness. There is a need to provide a means to maintain a substantially constant force by the rolls against the strip irrespective of the variation in thickness of the strip during production. By the present invention, it is possible provide an effective means for providing a substantially constant force by the rolls on the strip during casting with variation in the strip thickness.
The present invention is an improvement in an apparatus for continuously casting metal strip where a pair of parallel casting rolls form a nip between them, a metal delivery system delivers molten metal into the nip between the rolls to form a casting pool of molten metal supported on casting roll surfaces immediately above the nip confined against outflow adjacent the ends of the nip, and a casting roll drive system drives the casting rolls in counter-rotational directions to produce a solidified strip of metal delivered downwardly from the nip.
The improvement of the present invention provides for controlling thickness of the strip against variation during casting and comprises sensors positioned downstream of the nip capable of sensing the strip thickness at a plurality of locations across the strip, said sensor capable of producing electrical signals indicative of the strip thickness sensed at the sensor positions, at least one of the casting rolls supported on a roll carrier capable of allowing one of the casting rolls to move laterally toward and away from the other casting roll, carrier drives capable of moving roll carriers and in turn varying the strip thickness of the strip across the strip at the nip, and a control system capable of controlling the carrier drive's response to electrical signals from the sensors to vary the thickness of the strip at the nip to at least partially correct for variations in the strip thickness sensed by the sensors.
The roll carriers may be positioned adjacent to each end of the moveable casting roll and capable of moving independently of each other. The carrier drives may be comprised of servo mechanisms capable of independently moving the roll carriers so as to vary the strip thickness across the width of the strip at the nip, or comprised of roll biasing units each acting on the roll carrier at each end of the casting rolls to bias the casting roll bodily toward the other casting roll so as to vary the strip thickness across the strip at the nip.
Where the carrier roll system is comprised of roll biasing units, each roll biasing unit may be comprised of thrust transmission structures connected to the roll carriers at each end of the casting rolls, compression springs acting against the thrust structure to exert force on the thrust transmission structure and in turn the roll carriers at each end of the casting roll, and a thrust reaction setting device operable to vary the lengths of the compression springs. In this embodiment, the control system may control operation of the thrust reaction setting device such that movement of the thrust transmission structure moves the roll carriers and in turn varies the strip thickness across the strip at the nip. More specifically, each roll biasing unit may in addition comprise a thrust reaction structure abutting the compression spring and moveable by the thrust reaction setting device such that the control system varies the position of the thrust reaction structure to exert force against the compression spring and through the thrust transmission structure to vary the strip thickness across the strip at the nip.
Irrespective of the embodiment, the carrier drives may be disconnectable from the roll carriers to enable a module comprised of the casting roll and the roll carrier to be moveable without removing or dismantling the carrier drives.
In an alternative or supplement to the above-described invention, the improvement of controlling thickness of the strip against variation during casting may comprise at least one of the casting rolls mounted on roll carriers capable of allowing one of the casting rolls to move laterally toward and away from the other casting roll, roll biasing units each acting on the roll carrier at each end of the one casting roll to bias the one casting roll bodily toward the other casting roll, each roll biasing unit comprising thrust transmission structures connected to the roll carriers at each end of the casting rolls, compression springs acting against the thrust transmission structure to exert force on the thrust transmission structure and in turn the roll carriers at each end of the casting roll, a thrust reaction structure capable of compressing the compression spring and moveable axially of the compression spring, a thrust reaction structure setting device operable to vary the position of the thrust reaction structure relative to the compression spring, and a control system capable of controlling operation of the setting thrust reaction device such that movements of the thrust reaction structure replicate movements of the thrust transmission structure whereby movements of the thrust reaction structure do not significantly affect the biasing force exerted on the roll carrier and casting roll by the compression spring.
In a preferred illustrative embodiment of the present invention, at least one of the casting rolls may be mounted on a pair of moveable roll carriers which allow one roll to move bodily toward and away from the other roll, and there may be a pair of roll biasing units acting one on each of the pair of moveable roll carriers to bias said one roll bodily toward the other roll. Each roll biasing unit may comprise a thrust transmission structure connected to the respective roll carrier, a thrust reaction structure, and a compression spring acting between spring abutments on the thrust reaction structure and the thrust transmission structure to exert a thrust on the thrust transmission structure and the respective roll carrier. A thrust reaction structure setting device is operable to vary the position of the thrust reaction structure, and a control system is provided to control operation of the setting device such that movements of the thrust transmission structure are replicated as movements of the thrust reaction structure whereby movements of the thrust transmission structure do not significantly affect the biasing force imposed thereon by the compression spring.
Preferably, the thrust reaction structure setting device is a pressure fluid actuable device acting between the thrust reaction structure and a fixed structure. The pressure fluid actuable device may be provided by a fluid cylinder and piston unit connected at one end to a fixed structure, the other end of the piston unit either forming or being connected with the thrust reaction structure.
To provide dynamic wedge control, the sensors are positioned at a plurality of locations across the width of the strip, and the control system is capable of controlling the carrier drives responsive to the electrical signals from the sensors to vary thickness of the strip across its width at the nip to at least partially correct for variations in the strip thickness sensed by the sensors.
Alternatively, to maintain a substantially constant force on the strip, the control system controls operation of the setting thrust reaction device such that movements of the thrust reaction structure replicate movements of the thrust transmission structure.
The control system is capable of controlling operation of the setting thrust reaction device such that movements of the thrust reaction structure replicate movements of the thrust transmission structure whereby movements of the thrust transmission structure do not significantly affect the biasing force imposed on the roll carrier and casting roll by the compression spring. The control system may comprise a first position sensor to sense the position of the thrust transmission structure, and to operate the fluid pressure actuable device such that a movement sensed by the sensor is replicated by a movement of the thrust reaction structure.
The roll carriers may comprise a pair of roll end support structures for each of the casting rolls disposed generally beneath the ends of the respective casting roll. Each pair of roll end support structures may carry journal bearings mounting the respective roll ends for rotation about a central roll axis.
The casting rolls and roll carriers may be mounted in a roll module installed in and removable from the caster as a unit. In that case, the thrust transmission structure of each carrier drive may be disconnectable from the respective roll carrier to enable the module to be removed without removing or dismantling the carrier drives.
In an apparatus in accordance with the invention both of the roll carriers and supported casting rolls may be moved laterally by respective pairs of carrier drives. Alternatively, one of the rolls may be restrained against lateral bodily movement and the other allowed to move laterally against forces in accordance with the invention.