Abstract The effects of phosphorus addition on the phase transformation behavior of low-carbon steels were analyzed using a computational simulation focusing on the micro-segregation behavior of phosphorus during solidification and subsequent cooling processes. Based on the results of the simulation, a fully theoretical approach to modeling austenite grain growth was suggested for the continuous cooling processes. A previous model [Yoshida N, Umezawa O, Nagai K. ISIJ Int 2004;44:547; Yoshida N, Kobayashi Y, Nagai K. Tetsu-to-Hagane 2004;90:198] for the prior austenite grain size prediction was also refined based on the newly suggested approach by considering the liquid/solid interfacial energy reduction due to phosphorus addition. The prior austenite grain size that was predicted with the refined model showed good agreement with experimental data in the range 0–0.2% phosphorus.