As a research for developing fine-grained high strength low carbon steels, the effects of austenitization temperature and hot deformation on microstructure was investigated in 0.15 wt.% carbon steels with microalloying elements such as Nb and Ti. When the steels were reheated at $1250^{circ}C$, Nb containing steel showed very coarse austenite grain size of $200{mu}m$ whereas Nb-Ti steel did fine one of $70{mu}m$ because Ti carbonitrides could suppress the austenite grain growth. In case of 50% reduction at $850^{circ}C$, the austenite grains in the Nb steel partially recrystallized while those in the Nb-Ti steel fully recrystallized probably due to finer prior austenite grains.For the Nb-Ti steel, ferrite grain size was not sensitively changed with austenitization temperature and compression strain and, severe deformation of 80% reduction was not essentially necessary to refine ferrite grains to about $3{mu}m$ which could be obtained through lighter deformation of 40% reduction.