Numerical modelling of the tailored tempering process applied to 22MnB5 sheets

In order to enhance the crash characteristics and geometrical accuracy, components hot formed in a fully martensitic state have gained in the last few years more and more importance. However, the very high strength exhibited by these components makes subsequent operations such as cutting difficult due to the high process forces and associated high wear of the cutting tools. Moreover, for some applications, such as B-pillars and other automotive components that may undergo impact loading, it may be desirable to create regions of the part with softer and more ductile microstructures. The novel process called the tailored tempering process allows doing this by suppressing the martensitic transformation in those zones of the sheet located under heated parts of the tools. In the paper, a numerical model of the tailored tempering process was developed, accurately calibrated and validated through a laboratory-scale hot forming process. Using the commercial FE code Forge™ a fully coupled thermo-mechanical-metallurgical model of the process was set up. The influence of the phase transformation kinetics was taken into account by implementing in the model phase transformation data, namely the shift of the TTT curves due to the applied stress and the transformation plasticity coefficients, gained from an extensive dilatometric experimental campaign and analysis. A laboratory-scale hot-formed U-channel was produced using segmented tools with heated and cooled zones so that the cooling rate of the blank can be locally controlled during the hot forming process. The part Vickers hardness distribution and microstructural evolution predicted by FORGE™ were then compared with the experimental results, proving the validation of the numerical model by taking into account the influence of the transformation plasticity and deformation history on the phase transformation kinetics.