Balancing metallurgy and production of high strength steels

The ever increasing demands for strip with high strength and strain values and tolerances stretch the capabilities of production sites to their limits , because the high strength at room temperature has to be combined with lim ited loads imposed on the mill during hot rolling. The ultralow carbon bain itic steels developed to meet customers' demands have chemical compositions that extensively retard static recrystallisation. Therefore a double hit c ompression technique was used to quantify the static softening behaviour. H igh speed plane strain compression (PSC) experiments were performed to simu late actual multistand rolling schedules at semi-industrial speeds in combi nation with a controlled coiling simulation. The sample size allows determi nation of mechanical properties, microstructure, or crystallographic textur e. The force-displacement curves permit more accurate tuning of the rolling models. Hence, control of thickness at the head end of the strip during ho t rolling can be improved. The transformation behaviour was measured using dilatometry and thermal analysis. In combination with the mechanical proper ties obtained from the PSC samples, these data allowed conclusions to be dr awn regarding optimum cooling rate and coiling temperature. This subsequent ly allowed high productivity industrial production of high strength steels having good mechanical properties, while reducing loads in the mill to acce ptable levels. The work has demonstrated that the combination of metallurgi cal models and experiment is a powerful tool in rapid implementation of pro duct developments. (C) 2001 loM Communications Ltd.