Role of niobium on the segregation behavior of phosphorus and manganese in cold-rolled and annealed high strength IF steels

Phosphorus and manganese are commonly used to increase the strength of interstitial free (IF) steels for automotive use. However, both phosphorus and manganese will segregate to the grain boundary (GB) of the steel during the annealing process and lead to the problem of secondary work embrittlement (SWE). Surface segregation of manganese also takes place in the annealing process and deteriorates surface property of the steel. In this study, high strength IF steels containing different levels of manganese, phosphorus, boron and niobium were used to investigate the role of niobium on the segregation behavior of manganese and phosphorus in the steel. Atom probe field ion microscopy (APFIM), glow discharge optical emission spectrometry (GD-OES), SEM and XRD were used to achieve this goal. Experimental results showed that manganese competed with phosphorus for the sites in GB of the high manganese IF steels. Site competition between boron and manganese also occurred in the boron-added high strength IF steels containing high level of manganese and phosphorus. The improvement of the SWE resistance of the steel is believed to be partially attributed to the decrease of GB manganese as well. Niobium also segregated to the GB and surface of the steel during the annealing process. Segregation of niobium decreased the GB concentration of phosphorus and the enrichment of manganese on the surface of the steel. The SWE resistance and surface property of the steel could be improved with the addition of niobium accordingly. (AU)