Experimental and numerical determination of relationships between process variables of thermo-mechanical material treatment and mechanical properties in graded mixed microstructures with bimodal grain size distribution

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For responsible use of resources and reduction of CO2 emissions in the transport sector, components are specifically adapted to the respective load case. For steels, this can be achieved by grading the microstructure and grain sizes. A thermo-mechanical process chain makes it possible to set different mixed microstructures and heterogeneous grain sizes, so-called bimodal grain size distributions. The latter can improve ductility, fatigue strength and corrosion properties.The overall objective of the research project is the simulation-based determination of the relationships between the process variables of thermo-mechanical material processing and the resulting graded mixed microstructure with bimodal grain size distribution. This is to be achieved for Robusal900® steel by a preliminary intercritical annealing and cold forming followed by recrystallization annealing. By extending the optical measuring method developed in the first funding period, a quantitative determination of the austenite content during intercritical annealing is to be carried out. In the field of numerical investigations, the phase field method is intended to be used to develop a material model for simulating the microstructural transformations. For this purpose, a concept is applied which has already been used for phase transformations. The comparison between experimental and numerical results shall be done by integral quantities. Through the simulation, load-adapted combinations of strength and ductility will be determined based on the quantitative compositions of the bimodal microstructure.