Grain boundary segregation in ultra-low carbon steel

Segregation of phosphorus and sulfur to grain boundaries in ultra low carbon steel (IF-steel) was investigated by Auger electron spectroscopy (AES) and scanning electron microscopy (SEM). The segregation kinetics of phosphorus and sulfur while holding at 600°C have the characteristics of non-equilibrium segregation and a maximum peak was presented for each element in the initial hour, which could be reasonably explained by diffusion of phosphorus-vacancy complexes and sulfur-vacancy complexes, respectively. With longer holding at 600°C, the concentration of phosphorus would increase and then approached an equilibrium value, whereas the concentration of sulfur almost remained at a low level. Temperature dependence of phosphorus and sulfur concentration on grain boundaries showed that the concentration of phosphorus decreased continuously with increasing temperature; however, the concentration of sulfur increased slightly, which could be explained by site competition and chemical interaction between phosphorus and sulfur. Interaction between phosphorus and sulfur was evaluated using the Guttmann model of a regular solution with size competition for the ternary system. It clearly reveals that the repulsive interaction between phosphorus and sulfur in grain boundaries is very strong and that the interaction coefficient αPS decreases with increasing temperature. From the interactive coefficient, the free energies of segregation of sulfur at 600 and 700°C were approximately estimated.