Influence of Temperature and Deformation on Phase Transformation and Vickers Hardness in Tailored Tempering Process: Numerical and Experimental Verifications

Hot stamping of quenchenable ultra high strength steels currently represents a promising forming technology for the manufacturing of safety and crash relevant parts. For some applications, such as B-pillars which may undergo impact loading, it may be desirable to create regions of the part with softer and more ductile microstructure. In the article, a laboratory-scale hot stamped U-channel was produced with segmented die, which was heated by cartridge heaters and cooled by chilled water recirculation independently. It can be concluded that in order to satisfy tailored mechanical properties by introducing regions, which have an increased elongation for improved energy absorption, the minimum die temperature should be no less than 450 °C. Optical micrographs were used to verify the microstructure of the as-quenched phases with respect to the heated die temperatures. For the cooled die region, the microstructure was predominantly martensite for all the die temperatures interested. With the increase of heated die temperature, there was a decrease of Vickers hardness in the heated region due to the increasing volume fractions of bainite. The finite element (FE) model was developed to capture the overall hardness trends that were observed in the experiments. The trends between the simulations and experiments were very similar, with acceptable differences in the magnitude of Vickers hardness. The transition widths were measured and simulated and there was a quite good agreement between experiment and simulation with almost the same value of 10 mm by taking heat conduction into account.