Extending an empirical and a fundamental bainite start model to a continuously cooled microalloyed steel

The empirical model proposed by Lee and a fundamental model for the isothermal bainite start temperature (Bs) are applied in this work to a microalloyed steel undergoing continuous cooling transformation. The fundamental model for Bs is based on displacive transformation of bainite. The method proposed in this paper is termed the Bainite Intercept Temperature method (BIT). Using this method it is possible to determine the volume fraction of the phase that forms before bainite, i.e., quasi-polygonal ferrite. BIT analysis is applied to continuous cooling where a two phase microstructure is formed of quasi-polygonal ferrite and mixed bainitic ferrite and bainite/martensite. The estimated intrinsic hardness of the components based on BIT is compared with isothermal transformation results and the hardness is calculated based on the contribution of strengthening mechanisms.