The Onset Temperatures of γ to α-Phase Transformation in Hot Deformed and Non-deformed Nb Micro-alloyed Steels

In the present paper, continuous cooling transformation behaviors in Nb micro-alloyed steels were systematically investigated by the thermal dilatation method, during which the effects of Nb contents and hot deformation of austenite on phase transformation behaviors, especially the transformation start temperatures, were studied in detail. The tests were carried out with the samples having been reheated+hot deformed or reheated+non-deformed prior to the dilation measurements. It was found that the Ar3 temperatures measured from the dilatometric curves changed with increasing Nb content in parabolic ways under both hot deformed and non-deformed conditions. It is believed that dissolved Nb in austenite may have had the Nb solute drag effect that could delay austenite to ferrite transformation. On the other hand, Nb precipitates in austenite could retard the growth of austenite grains and act as potential nucleation sites, both of which could enhance the transformation kinetics. Also, the interaction of strain, precipitation and temperature was estimated by using the Sellars model, which predicted that the strain induced precipitation had occurred in hot deformed Nb steels before phase transformation started, and in non-deformed steels with Nb content greater than 0.023 mass%, precipitation was also likely to have occurred under slow cooling rate before phase transformation started, which could have played an important role in determining Ar3. These factors worked together to make Ar3 changing with Nb content in the parabolic way. Based on the experimental results, a mathematical model for the Ar3 calculation for Nb and C–Mn steels were developed, which exhibited a good accuracy in predicting the Ar3 of the steels with and without hot deformation.