Through-process modelling of microstructure evolution in hot forming of steels

The objective of the paper is presentation of the complex thermal-mechanical-microstructural model, which simulates the whole process of hot forming, starting from heating of the stock material through the forming operations up to the controlled cooling after forming. The thermal-mechanical finite element solution is the basis of the approach and it is coupled with the closed form equations describing microstructural phenomena. The microstructural equations are solved in each node of the finite element mesh and at each time step, what makes the feedback between the models possible. Three stages of modelling are presented. The first concerns heating of the stock before forming. The model predicts temperature field, stresses and grain size during heating. The second stage uses thermal-mechanical-microstructural model of forming operations. Beyond the conventional closed for equation describing microstructure development, the models based on the internal variable approach are presented. Particular emphasis is put on identification of parameters of the models. The inverse analysis with the multi criterion objective function is applied. Simulation of the controlled cooling of products after hot forming is the third objective of the modeling. The models couple finite-element solution of the thermal problem with equations describing kinetics of phase transformations and properties of materials in the room temperature. Identification of the phase transformation models is based on the inverse analysis combined with the experimental TTT diagrams. Several examples of simulations and experimental validation, which are presented in the paper, confirm good predictive capabilities of the model.