Non-equilibrium intergranular segregation in ultra low carbon steel

Abstract

Impurity segregation to grain boundaries in ultra low carbon steel was investigated by Auger electron spectroscopy and SEM during isothermal annealing at 900°C and continuous cooling. The results of isothermal annealing at 900°C show that a concentration peak appears at different times for phosphorus, sulphur, and boron, which is contrary to the equilibrium segregation theory of McLean. The phenomena could be satisfactorily explained by the non-equilibrium segregation theory based on the impurity–vacancy complex mechanism. Under continuous cooling, the segregation concentration at the grain boundary largely depends on the cooling rate. At a low cooling rate the concentration of phosphorus and boron at the grain boundary is higher than that of sulphur, while at the higher cooling rate the concentration of sulphur is higher.