Investigation on thermal, mechanical and microstructural properties of quenchenable high strenght steels in hot stamping operations

Sheet metal working operations at elevated temperature have gained in the last years even more importance due to the possibility of producing parts characterized by high strength-to-mass ratio. In particular, the hot stamping of ultra high strength quenchenable steels is nowadays widely used in the automotive industry to produce body-in-white structural components with enhanced crash resistance and geometrical accuracy. The optimization of the process, where deformation takes place simultaneously with cooling, and of the final component performances requires the utilization of FE-based codes where the forming and quenching phases have to be represented by fully thermo-mechanical-metallurgical models. The accurate calibration of such models, in terms of material behaviour, tribology, heat transfer, phase transformation kinetics and formability, is therefore a strong requirement to gain reliable results from the numerical simulations and offer noticeable time and cost savings to product and process engineers. The main target of this PhD thesis is the development of an innovative approach based on the design of integrated experimental procedures and modelling tools in order to accurately investigate and describe both the mechanical and microstructural material properties and the interface phenomena due to the thermal and mechanical events that occur during the industrial press hardening process. To this aim, a new testing apparatus was developed to evaluate the influence of temperature and strain rate on the sheet metal elasto-plastic properties and to study the influence of applied stress and strain of the material phase transformation kinetics. Furthermore, an innovative experimental setup, based on the Nakazima concept, was designed and developed to evaluate sheet formability at elevated temperature by controlling the thermo-mechanical parameters of the test and reproducing the conditions that govern the microstructural evolution during press hardening. This equipment was utilized both to determine isothermal forming limit curves at high temperature and to perform a physical simulation of hot forming operations. Finally, a thermo-mechanical-metallurgical model was implemented in a commercial FE-code and accurately calibrated to perform fully coupled numerical simulations of the reference process. The material investigated in this work is the Al-Si pre-coated quenchenable steel 22MnB5, well known with the commercial name of USIBOR 1500P’®, and the developed approach proves to be suitable to proper evaluate high strength steels behaviour in terms of mechanical, thermal and microstructural properties, and to precisely calibrate coupled numerical models when they are applied to this innovative manufacturing technology. The work presented in this thesis has been carried out at DIMEG labs, University of Padova, Italy, from January 2005 to December 2007 under the supervision of Prof. Paolo F. Bariani.