The present work, deals with grain refinement of medium carbon steel AISI 1045 (0.45% C), having different initial ferrite–pearlite microstructure resulted from thermal and thermomechanical treatment (TM). The purpose of TM steel processing was to refine ferrite and modify pearlite lamellae structure. The final grain refinement of steel structure was then accomplished during warm Equal Channel Angular Pressing (ECAP) at 400°C. Employment of this processing route, in dependence of the applied effective strain ϵef, resulted in extensive deformation of ferrite grains and cementite lamellae fragmentation. When applying higher shear stress (ϵef = 4) the mixed structure of subgrains and ultrafine grains was formed within ferrite phase, regardless the initial steel structure morphology. In pearlite grains, modification of cementite lamellae due to shearing, bending, twisting, and breaking was found efficient as straining increased. Processes of dynamic polygonization and recrystallization in deformed structure also contributed to submicrocrystalline grains formation in deformed structure. Comparing results the course of lamellae cementite spheroidization was then more efficient in prior TM treated steel. The tensile deformation results confirmed the strength increase, however deformation behavior and strain hardening generally for different initial structural conditions of steel, showed diversity.