High-strength steels for the automotive sector: A simple austenite transformation model for continuous annealing and galvanizing lines

In continuous annealing and hot-dip galvanizing processes, the final steel structures heavily depend on the process parameters (e.g. times and temperatures), which, thus, must be accurately tuned. In particular, the short-term permanence in a two-phase ferrite + austenite field and the following cooling process have a strong impact and can be very articulated, according to the plant type. Therefore, it is extremely useful to have practical and simple models estimating the austenite transformation based on the undergone thermal cycle. Actually, the process speed, especially for the cooling stage, implies that standard CCT curves are not applicable for determining the final structure. This work presents a solution exploiting a simple metallurgical-mathematical model. Such model, on the basis of experimental tests and starting from the chemical composition and the thermal cycle, estimates both the final structure and the mechanical properties of Advanced High Strength Steels, which are of utmost interest in the automotive sector.