Influence of soaking temperature on transformation behaviour and precipitate coarsening of new cold rolled weathering steel containing niobium and titanium

The continuous cooling transformation (CCT) behaviour and precipitate coarsening of a new cold rolled weathering steel containing Nb and Ti were investigated. Three different soaking temperatures (780, 820 and 860 degrees C) were applied to the steel. CCT diagrams were obtained by Formastor-F II automatic phase transformation analysis. The microstructures and second precipitated particles were examined by optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The initial sample microstructures were identified as cold rolled ferrite and pearlite structures. Results showed that increasing soaking temperatures would decrease ferrite start temperature (F-s) and increase martensite start temperature (M-s). This was mainly attributed to the changes in austenite grain size as well as the average carbon content. At the lowest cooling rate of 0.5 degrees C s(-1), a small amount of martensite structure could be observed. M-s increased smoothly with lower cooling rates and showed a dramatic increase at elevated cooling rates, at which point a coarse structure was readily obtained from transformation of coarse grained austenite and composed of bainite and martensite structures in the inner and peripheral regions respectively. Niobium/titanium precipitate coarsening took place during the soaking process: coarsening became serious as soaking temperature increased.