We examined the microstructure development in low carbon steel (0.15% C) during heavy warm deformation (HWD) using field emission scanning electron microscopy (FESEM) and electron back-scattering diffraction (EBSD). Plane strain compression tests have been conducted in the temperature range of 773-923 K at strain rates of 0.01 s(-1) and 1 s(-1) with the specimens deformed to 25% of their original thickness. We summarize the strain rate and temperature into the Zener-Hollomon parameter and investigate its variation with plastic strain on the basis of the evolved microstructures and grain boundary character with a view to understanding the critical conditions for forming ultrafine grains and classifying them. Once established, these compressive strain-Z parameter plots simplify the selection of processmg parameters (such as strain, strain rate, and temperature), towards achievmg tailor-made microstructures in industrial components.