Role of Niobium on the Segregation Behavior of Phosphorus and Manganese in High Strength IF Sheet Steels for Automotive Use

摘要

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 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 the surface property of the steel. In this work, high strength IF steels contained with different level 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) and SEM were used to achieve this goal. Experimental results show that manganese competes with phosphorus for the sites in GB of the high manganese IF steels. Site competition between boron and manganese also occurs in the boron-added high strength IF steels containing high level of manganese and phosphorus. Part of the improvement of the SWE resistance of the steel is believed to be attributed to the decrease of GB manganese as well. Niobium also segregates to GB and the surface of the steel by the annealing. Segregation of niobium decreases the GB concentration of phosphorus and the enrichment of manganese on the surface of the steel. The SWE resistance and the surface property of the steel can be improved with the addition of niobium accordingly.