Effects of Phase Transformation in Hot Stamping of 22MnB5 High Strength Steel

In automotive industry, hot forming of high strength steels offers the possibility to obtain significant reduction of weight without affecting the structural performances of the car-body-in-white. The sheets used in the hot stamping process are heated above the austenitization temperature, and, then, formed and quenched in cooled dies by using a minimum cooling rate of 27°C/s in order to obtain the direct martensitic transformation from the initial austenite. However, improper transfer times from the furnaces to the dies may be unacceptable with regard to the phase changes to be realized during the quenching. Furthermore, strain induced effects influence the phase transformations, so making the process difficult to design and control in case of new part shapes: variations of the applied strains determine changes in the Continuous Cooling Transformation (CCT) curves and, consequently, the cooling rate becomes fundamental to control the material phase transformations, in order to obtain the desired mechanical properties. The paper investigates the effects of the soaking time and cooling rate in the nose area of the CCT curves, where the time represents a critical aspect to control the phase changes in the microstructure. It was found that the phase transformations at the critical temperature of 550°C determine an increase of the flow stress that depends on the specific cooling rate during the dies quenching and on the percentage of bainitic transformation.