Prediction of the micro-structure development in HHS-grades during continuous annealing and the resulting of mechanical properties

This project was dedicated to the development of submodels to predict the different phase transformations during each step of continuous annealing cycles or galvanising cycles, that means during the soaking, the primary cooling, and the overageing section. The validation of these microstructural models being performed by means of laboratory investigations (dilatometer tests, interrupted annealing tests). The main conclusions are the following. It can be summarised that useable models are available concerning the formation of austenite during heating and soaking. Concerning the growth of ferrite during cooling, the amount of ferrite available at the end of the austenite-ferrite transformation (at 600°C) could be predicted with a reasonable accuracy. The majority of difficulties was encountered for the models predicting the bainitic transformation. It is obvious that a physical model is needed in this case since the empirical model of TKS is not sufficient. The OCAS model was not validated for a wide range of TRIP steels and is only valid for carbide free bainite. Further research is required on this topic. Furthermore, a lot of difficulties were encountered in order to come to an appropriate quantification of the different phases in these grades by means of microstructural analysis. An advanced and accurate quantification of the microstructure is also of prime importance to develop models, which are correlating the microstructure to the mechanical properties for a wide range of steel compositions and processing conditions.