Prediction and avoidance of high temperature damage in long product hot rolling

There is currently a drive towards higher contribution steels with improved machinability (in the case of free cutting steels (FCS)) and higher surface quality and consistency. This, together with the potential future implementation of the European legislation (ELVD) to promote lead substitutes in machinable steels (Bi, Te, high S) is leading to the requirement to develop a more thorough understanding of the cause of cracking during hot rolling of bloom and billets of low ductility steels. Physical understanding of the causes and mechanisms of damage initiation and growth (from micro- to macro-scale) at high temperature and relatively high strain rate has not been up to now a major focus of interest, compared to developments in room temperature brittle and ductile fracture, and creep/superplasticity failure. This paper reviews various experimental and modelling approaches (meso- to micro-scale) to develop a better understanding of the influence of thermo-mechanical conditions on damage initiation and growth for as-cast FCS steels. Particular attention is given to the development and use of new/modified mechanical tests. These include double collar and flying saucer axisymmetric tests, U-bending and revised plane strain compression tests using a Gleeble thermo-mechanical simulator to represent the triaxiality, principal stress and strain ratios experienced by the bloom and billet surface during rolling.