High temperature ductile damage during rolling of free cutting steels

Abstract

The drive towards higher contribution free cutting steels with improved machinability and higher surface quality and consistency, together with the potential future implementation of the European legislation to promote lead substitutes (Bi, Te and high S), is leading to a requirement for improved understanding and control of as rolled surface quality, most critically for steels which have low ductility and a narrow temperature range for defect free rolling. Ductility breakup for the range of steel investigated (free cutting steel steels) is intrinsically complex with many interactions existing between casting, reheating and rolling. This is especially important during the transition from as cast to wrought structure, i.e. for thermomechanical and microstructural conditions acting in cogging/roughing mills, where the intrinsic reheated cast ductility of some machining steels can be as low as 10%. This paper builds upon previous studies for developing a more physical approach to predict the mechanisms of high temperature ductile damage acting at surface/subsurface and across the length scale of continuously cast free cutting steel billet feedstock during rolling. A methodology combining mechanical testing, modelling and characterisation has been developed to study effect of thermomechanical conditions such as triaxiality including inversion, principal stress, strain, strain path, strain rate and temperature as well as accounting for local microstructural heterogeneities such as MnS inclusions. This methodology has been used to refine the science of nucleation, growth and coalescence acting at high temperature for this range of low ductility steels during rolling. This knowledge has been put into practice with the establishment of regime maps and guidelines to minimise propensity of cracking during industrial rolling as well as being able to design effective schedules and grooves. Issues and future directions in terms of models and experimental technique development will also be highlighted.