Experimental investigation and modelling of the heat transfer during hot stamping

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The growing relevance of ecological aspects as well as governmental regulations regarding CO2 emissions are only two factors for the increasing importance of lightweight design in the automotive industry during the last decade. Especially hot stamping of ultra-high-strength boron-manganese steels has developed to a state-of-the-art process for manufacturing safety-relevant car-body parts. The process of hot stamping starts with a full austenitization of the semi-finished parts above the alloy specific AC3 temperature. After the heat treatment, the hot sheets are directly transferred to the press, where they are subsequently formed and in-die quenched. As long as the cooling rate exceeds a specific value, the austenite completely transforms into martensite. Due to this phase transformation, the hot stamped parts exhibit an ultimate tensile strength of at least 1500 MPa. The temperature evolution along the process chain and in particular during in-die quenching has a significant influence on the final mechanical properties. For a precise numerical process design, exact modelling of the heat transfer in the contact area between workpiece and tool is important. In the finite-element method, a corresponding heat transfer coefficient is used for modelling, either in the form of a constant value for the whole process or as a function of the contact pressure. This leads to significant uncertainties in terms of the numerical process-modelling results. Although more complex models are available in the literature, these models are primarily valid for a constrained series of experiments. Therefore, the present research project aims to acquire a fundamental understanding of the heat transfer mechanisms involved in the hot stamping process and to model them for the numerical process design. The analysis of the heat transfer during hot stamping is realized through a representative series of experiments under systematical variation of the process parameters. During the experiments, the temperature evolution of the workpiece and the tool is recorded and used for the calculation of time-dependent heat transfer coefficients. This experimental data as well as additional measurement results on the evolution of the surface topography are the basis for the development of a physically-based model, which has the ability to determine time-dependent heat transfer coefficients in the contact area between workpiece and tool as a function of the process parameters. Since the heat transfer also depends on the respective contact situation, the different conditions of contact will be modelled for the numerical simulation as well. By combining the heat transfer model with a submodel for the continuous evaluation of the true contact area between workpiece and tool, the prediction quality of the numerical process design will be improved.