Using automated crystal-orientation mapping (ACOM) in TEM, we have shown the formation of an ultrafine grained (UFG) structure with predominantly (>90%) high angle grain boundaries in C45 steel and Armco iron after high pressure torsion (HPT) at 350°C. The UFG C45 steel exhibits a combination of high tensile strength and ductility and its mechanical properties were improved significantly compared to the analogously processed Armco iron. The higher strength of the processed C45 steel is attributed to the finer grain size, the highly fragmented cementite particles induced by the HPT deformation. The difference in ductility of the two materials is related to the distinct work hardening behaviors due to the different microstructural characteristics.