Development of high strength if-steel by equal channel angular extrusion and application of the method to IF-steel sheets

In this study, an IF-steel in bulk and sheet forms was successfully processed by equal-channel angular extrusion/pressing (ECAE/P) following different routes and pass numbers. After these processes, the effect of microstructural alterations on the strength, ductility, recrystallization behavior, formability, toughness and ductile-to-brittle transition behavior was investigated, and the ?process-structure-property relationship? was established. In general, the ECAE processing refined the microstructure of coarse-grained (CG) steel and transformed it into ultra-fine grained (UFG) structure consisting of grains in the range of 420 nm-820 nm. A characteristic crystallographic orientation formed by a simple shear emerged in the UFG microstructure. The UFG formation along with high dislocation density brought about 6-7 times higher yield strength than that of CG steel. But, the ductility decreased after the processes. A microstructure having higher strength with moderate ductility was produced by appropriate heat treatments aiming to establish a balance between strength and ductility. The formability of CG IF-steel was affected depending on applied strain paths and, in general, a slight decrease in formability was observed after UFG formation. However, this behavior was improved again by appropriate heat treatment following the conditions that gave a good balance between strength and ductility. Furthermore, orange peel effect appeared on the surface of deformed CG steel diminished considerably after both UFG formation and appropriate annealing. Ductile-to-brittle transition temperature of the CG IF-steel decreased from -40°C to about -90°C by the formation of UFG structure. Also, the UFG formation flattened the transition curve, and caused the ductile-to-brittle transition to occur in a wider temperature range compared to that of CG counterpart.