Balancing metallurgy and production of high strength steels

Abstract

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 limited loads imposed on the mill during hot rolling. The ultralow carbon bainitic steels developed to meet customers' demands have chemical compositions that extensively retard static recrystallisation. Therefore a double hit compression technique was used to quantify the static softening behaviour. High speed plane strain compression (PSC) experiments were performed to simulate actual multistand rolling schedules at semi-industrial speeds in combination with a controlled coiling simulation. The sample size allows determination of mechanical properties, microstructure, or crystallographic texture. The force–displacement curves permit more accurate tuning of the rolling models. Hence, control of thickness at the head end of the strip during hot rolling can be improved. The transformation behaviour was measured using dilatometry and thermal analysis. In combination with the mechanical properties obtained from the PSC samples, these data allowed conclusions to be drawn regarding optimum cooling rate and coiling temperature. This subsequently allowed high productivity industrial production of high strength steels having good mechanical properties, while reducing loads in the mill to acceptable levels. The work has demonstrated that the combination of metallurgical models and experiment is a powerful tool in rapid implementation of product developments.