PRECIPITATION AND SURFACE SEGREGATION IN LOW-CARBON STEELS

Surface segregation in ultralow carbon interstitial-free (IF) steels was investigated by Auger electron spectroscopy taking into account the precipitation phenomena in the bulk. Transmission electron microscopy and scanning transmission electron microscopy studies of precipitates were performed on two steels: LF titanium and IF titanium-niobium alloys. The results are discussed in terms of a thermodynamic analysis of the precipitation sequence during industrial treatment. The surface segregation kinetics realized at 700 and 800 degrees C are interpreted using the thermodynamic formalism developed for binary and ternary systems. It is emphasized that the surface segregation free enthalpies are strongly modified by precipitation which alters the amount of free elements in the solid solution. The desegregation of phosphorus observed in IF Ti steel can be attributed to the formation of Ti-P clusters in the bulk. In contrast, in the IF Ti-Nb steel, phosphorus desegregation is mainly due to the S-P repulsive interaction. The study also reveals the large ability of sulphur to segregate in spite of the very low bulk content of free sulphur in these steels.