STRUCTURE EVOLUTION DURING SEVERE WARM PLASTIC DEFORMATION OF CARBON STEEL

Abstract. By application of thermomechanical controlled rolling and accelerated cooling, the ferrite grain refinement is limited to levels of ~ 5 µm in steels. The strain assisted or strain induced transformation could be considered for the refining process. The present work, likewise, deals with grain refinement of medium carbon steel containing 0.45 wt. pct carbon. The structure refinement was conducted in two steps. Preliminary refinement has been achieved due to multistep open die forging process which provided total strain of 3-4. Uniform and fine recrystallized ferrite structure with grain size of the order of 2 µm and with nest-like pearlite colonies was obtained. The further grain refinement was accomplished during warm Equal Channel Angular Pressing (ECAP) at 400 °C of preliminary thermomechanically processed specimens. The steel was subjected to two pressing passes. Employment of this processing route resulted in exten-sively elongated ferrite grains which were sufficiently refined. The submicrometer order dynami-cally recrystallized ferrite grains enclosed by subgrains of low angle boundaries were formed within the former elongated ferrite grains. The aim of the investigation was to find an optimized relationship between mechanical properties and microstructure developed in steel. 1