Heat Transfer Coefficient Characterization for Hot Stamping of Boron Steel Blanks

Hot forming die quenching (HFDQ) is an emerging process in which sheet metal is successively stamped at high temperature and quenched by the cold tool. The microstructure and mechanical properties of ultra high strength steel parts formed by the HFDQ process are a function of the cooling rate achieved during the quenching step. Optimal energy absorption for crash performance can be obtained by a local reduction of the cooling rate in order to form softer, more ductile phases such as bainite. Accurate characterization of the heat transfer coefficient (HTC) between the sheet metal and the cold tool is required to evaluate the cooling rate and the resulting mechanical properties. Experiments were conducted in which Usibor 1500P^® boron steel blanks were austenitized at 900°C then quenched by stamping between flat tool steel dies. The HTC was calculated using an inverse heat conduction analysis for the die and a lumped capacitance approach for the blank. The heat transfer coefficients measured during the approach phase were found to be significantly lower than the values predicted by semi-empirical models based on thermal conduction through the air gap.