Dilatometric study of the effect of soluble boron on the continuous and isothermal austenite decomposition in 0.15C–1.6Mn steel

Abstract

Abstract The effect of soluble boron on the phase transformations during either cooling and/or isothermal holding has been studied by means of dilatometry. Significant differences in the transformation behaviour were found for all austenite phase transformation reactions. In particular, the morphologies of ferrite and bainite were strongly affected by B alloying. The kinetics was studied in detail for the austenite decomposition reactions. Soluble B was found to be very effective in suppressing the carbide formation. It was also found to interact with the Mn partitioning to the austenite. As a result, the presence of Mn-rich regions in the final microstructures decreased strongly the Ac1 temperature during reheating. Isothermal transformations in the 450–660 °C temperature range showed that the incubation times for ferrite and pearlite formations were increased. The soluble B was found to affect strongly the nucleation rate. The growth kinetics was slower due to a solute drag effect caused by the partitioning of Mn. The kinetics of bainite formation was not affected by the presence of soluble B. Upper bainite was found to be acicular ferrite in the CMnB steel as a result of the heterogeneous nucleation of ferrite on TiN precipitates.