As-cast austenite grain size (d_γ) in low carbon steels has been studied at a cooling rate from 0.1 to 40 K/s. An analytical model has been proposed to predict d_γ. The model is based on the classical grain growth theory of Burke-Turnbull, by assuming a normal grain growth with a parabolic law in the temperature range of a γ single phase. The proposed equation is expressed as follows:
d²_γ-d²₀=K_g[g(T_rg)-g(T_f)]/T,
where K_g is a material constant; T_rg and T_f are starting and finishing temperatures of the rapid growth, and g(T) is the dimensionless number indicating the square growth amount below T. The square growth amount of the as-cast γ grain is proportional to the inverse of the cooling rate, T^-1. The square growth amount decreases to 20% by phosphorus addition, which decreases K_g and T_rg. The prediction agrees well with the experimental data of the continuously cast 100 mm-thick slab and the 2 mm-thick strip.