A mathematical method is presented to quantify solute redistribution during solidification with δ/γ transformation; new methods to solve the equation of diffusion numerically and to estimate the equilibrium phase diagram of multi-component low alloyed steel are described. Thermal analyses of low alloyed steels were made to verify the mathematical method described. It has been shown that the unequilibrium phase diagram of low alloyed steel for a given cooling rate can be predicted by this method.