9th ECCC European Continuous Casting Conference – ECCC 2017

Process stability during continuous casting is an essential criterion for the production of high quality stainless steels. In particular the meniscus, which is the most sensitive zone of the whole strand, has to be in a steady shape and position. As the liquid core of the strand acts like a hydraulic hose, each radial movement of the strand shell is inducing a feedback on the mold level position. Hence unsteady bulging of the shell between the containment rollers is a coupled effect of the transient strand volume and meniscus position. The crucial point for this effect is that the mold level fluctuations are interacting with the generated shell in terms of thickness variations. Ferritic stainless steel grades, which show a very soft and creeping material behavior during casting, are known for their high unsteady bulging affinity. Also the roller geometry of a casting machine is influencing the unsteady bulging behavior. For instance many consecutive identical roller pitches lead to a superposition of the pumping effects of the roller pairs and an increased bulging affinity. Therefore state of the art slab casting machines have desired variations of the consecutive roller pitches in the caster segments. Slab casters with a single unit casting bow have staggered rollers between inner and outer bow due to the difference in the number of rollers. The offset between inner and outer bow rollers is reducing the bulging affinity of this machine type. At Outokumpu Tornio both machine types – segmented casting bow (caster 2) and single unit casting bow (caster 1) – are in operation. For both machines the meniscus stability has been significantly improved by the installation of an unsteady bulging compensation controller, which is suppressing periodical mold level fluctuations generated by unsteady bulging. This model based controller is realized as an add-on software package in the mold level control PLC (LevCon package).