Microstructural evolution of IF-steel during cold rolling

The microstructural evolution during cold rolling of IF-steel up to reductions of 90% has been investigated using scanning and transmission electron microscope techniques. The deformation microstructures consist of similar features to those already identified in several FCC metals, namely cell blocks showing a pattern of subdivision on two levels. An important difference compared to FCC metals is an increased prevalence of strain localization, seen most directly by highly localized micro-shearing. An orientation dependence of the deformed microstructure is observed in that nearly all the extended dislocation boundaries and micro-shear traces are crystallographic, coincident either with {1 1 0} or {1 1 2} slip planes. A sharp fall in ductility seen between rolling reductions of 30% and 50% is attributed to the onset of localized micro-shearing. A flow stress calculation, based on linearly additive contributions from dislocation and grain boundary hardening, leads to flow stress values and a strain hardening behavior in good agreement with experimentally determined values.