Effect of niobium and molybdenum on phase transformations in advanced low-carbon steels

Advanced high strength steels are frequently microalloyed with Nb and/or Mo. Both elements are known to have a tremendous effect on the austenite-ferrite transformation, which constitutes the key metallurgical tool to tailor the properties. The effects of Nb on the continuous cooling transformation behavior are quantified for two linepipe steels by separately controlling the austenite grain size and the content of Nb in solution. One of the investigated steels is an X80 grade that also contains Mo and the transformation studies are used to quantify the synergistic role of Nb and Mo. Bringing Nb into solution decreases the transformation temperature by up to 100 °C for accelerated cooling conditions. The experimental results are described in the framework of phenomenological models for the austenite-to-ferrite transformation. In particular the effect of Nb and Mo on the apparent austenite-ferrite interface mobility is analyzed. (AU) Copyright © 2014 Companhia Brasileira de Metalurgia e Mineração (CBMM) All rights reserved