Modelling of Microstructure Evolution during Hot Rolling of a 780 MPa High Strength Steel

The microstructure evolution during thermo-mechanical processing of a state-of-the-art Ti-Nb HSLA steel with a tensile strength of 780 MPa has been investigated with laboratory investigations. The entire hot strip rolling process was simulated with hot torsion tests. Using the Gleeble 1500 thermomechanical simulator, constitutive behaviour, static recrystallization and austenite decompositon were quantified with single and double hit tests as well as continuous cooling transformation (CCT) tests. Precipitation strengthening was studied using aging tests. Based on the experimental results a microstructure model is proposed for hot rolling, run-out table cooling and coiling of the investigated steel. The model has been applied to predict the microstructure of industrially processed coils. The prediction of the ferrite grain size is seen to be in good agreement with that obtained during industrial hot strip rolling.