New secondary cooling patterns for peritectic and microalloyed steel

Surface cracking in continuously cast semis of aluminium grain refined and microalloyed steel grades, especially those in the peritectic carbon range, is still a major concern for many caster operators, albeit that significant work has been, and is being, undertaken to improve the quality off the caster and through the initial stages of the process route. The demands of reduced surface rectification (scarfing or grinding at either the as-cast or intermediate rolled stage) and an increasing demand for direct charging are driving the need for reduced surface cracking. This is the case for most continuously cast semis but especially for those grades known to be 'surface critical', where the section and process route do not allow development of a stable crack resistant surface and sub-surface metallurgical structure and the near-net shape technologies where the technologies are being expanded to the crack sensitive grades. Surface cracking may be initiated on the caster, at cut-off and as the semi is handled off the caster, through cooling for inspection, transport to the mills and either reheating or temperature equalisation for rolling. Transverse cracking is initiated and propagated on the caster. With increasing demands for its elimination across all sections and steel grades, the traditional methods for its control are no longer sufficient on their own and the possibility of alleviating the problem by careful choice of the steel chemistry is limited. The methods for control of transverse cracking have included control of the oscillation mark (oscillation practice and mould powder selection), minimisation of bulging, control of reheat both within and at the end of secondary cooling and, most significantly, by avoiding straightening in the ductility trough associated with peritectic and microalloyed steel grades The objective of this project was to investigate the possibility of realising a fine grain size and small grain boundary precipitate within the surface and sub-surface regions by use of an alternative secondary cooing practice and evaluate the improvement of hot ductility achieved and the resulting improved surface quality. The grain refinement was intended to be achieved by applying an intense cooling for rapidly cooling the skin of the cast product below the transformation temperature and followed by a self-reheating before straightening. The initial objective of Corus UK in the project was the development of an intense cooling system for one of the long product slab casters at Corns Teesside Works. A spray layout for intense cooling was developed but before this could be implemented the application to conventional slab casting was placed on hold due to the 2001 restructuring of Corus an d the requirement of Teesside to make slab for Port Talbot and Llanwern. The problem was compounded by the Port Talbot blast furnace incident, also in 2001. To aid the design of an intense cooling system, the Teesside Technology Centre's 1-D numerical model had to be modified to accommodate the effect of intense cooling of heat transfer. This was done using an inhouse developed Leidenfrost curve and also a method whereby the heat transfer was multiplied by a factor, termed the 'factor method'. Two series of trials were undertaken on the intermediate thickness slab caster at Corus Tuscaloosa.