Investigation of 22MnB5 mechanical and phase transformation behaviour at high temperature

One of the main challenges of the automotive industry is represented by the weight reduction of structural components achieved through the great potentiality of enhanced strength to mass ratio of high and ultra-high strength steels. The unacceptable loads on tool and springback phenomena obtained with traditional sheet metal forming operation of such materials at room temperature force industries to shift even more their attention to new forming technologies at elevated temperatures. The hot stamping of quenchable high strength steels offers in fact the possibility to reduce the body-in-white weight maintaining the crash requirements together with increased formability of sheets. However, the optimization of this innovative process chain through reliable FE simulations requires a deep knowledge of material behaviour in terms of microstructural evolution and resistance to plastic deformation in temperature. A new experimental set-up was developed in order to reproduce the thermo-mechanical conditions that material undergoes during the different phases of the industrial process and thus to overcome the absence in the technical- scientific literature of accurate data on material mechanical properties at elevated temperatures. The influence of temperature and strain rate on 22MnB5 elastic and plastic properties was evaluated through uniaxial tensile tests, considering the effective strain after necking by means of an optical strain measurement system. An extensive dilatometric analysis perfomed through a modified axial extensometer, both with and without applied stress, was employed to evaluate the shift of the TTT curves due to stress as well as the transformation plasticity coefficients.