The segregation behavior of phosphorus in Ti and Ti plus Nb stabilized interstitial-free steels

The phenomenon of cold work embrittlement (CWE) in the P-added high strengt h interstitial-free steels has been associated with the segregation of P to the ferrite grain boundaries. This segregation by P is believed to decreas e the cohesive strength of the grain boundaries by weakening their bonding. Hence, the resistance of the steel to brittle fracture, ie., intergranular fracture, is greatly decreased. The goal of the present study was to inves tigate the segregation behavior of P during the different stages of process ing (prior to and after-coiling, and after cold rolling and annealing) in T i and Ti+Nb stabilized interstitial-free steels. It was found that a consid erable amount of segregation of P to the ferrite grain boundaries occurred during the coiling process in Ti-stabilized interstitial-free steels. Howev er, with the addition of Nb in the Ti+Nb-stabilized interstitial-free steel s, the segregation of P was decreased in the as-coiled condition. The P con tent on the ferrite grain boundaries in the final cold rolled and annealed condition was found to depend on two factors; (1) the segregation of P in t he as-coiled condition, and (2) availability of Ti to form phosphides durin g the annealing process. It was confirmed in this study that the addition o f P decreases the CWE resistance of the steel. Furthermore, it appears that the CWE resistance of the Ti+Nb-stabilized interstitial-free steels is imp roved by the presence of solute Nb on the ferrite grain boundaries.