Characterization and physical based modeling of the bake-hardening effect in dual-phase steels: Development of a through-process modeling approach for the material behavior

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The automotive industry is driven by the continuous challenge to improve safety and fuel economy requirements for future vehicles. This implies, however, the need for improved steel grades to meet higher standards for automotive structural and safety parts. Dual-phase (DP) steels are widely used in automotive applications because they combine high yield strength and good formability at moderate production costs. Recently, it has been proposed to use the bake-hardening (BH) effect as an additional strengthening mechanism in DP steels to benefit from increased yield strength. This strengthening occurs during paint baking of the car body after sheet forming operations and car body assembly. In automotive engineering, computer simulations of material behavior have become an integral part in the product and process development. For the applicability of these simulations in industrial practice it is essential to have models provided which are able to predict the material behavior during each step of processing. With this through-process modeling approach, computer simulations can be used to adjust the process parameters in each process step in order to achieve the final mechanical properties. The project aims at developing new physical-based approaches for the BH effect in DP steels. This will be achieved through a series of advances in experimental characterization of the BH effect in DP steels combined with theoretical modeling. The experimental investigations will focus on the different factors affecting the BH effect in DP steels. Explicitly, on industrially produced DP steels different microstructure characteristics (e.g. grain size, martensite volume fraction and morphology), pre-loading conditions (uni-axial, bi-axial and plane strain), and ageing conditions will be investigated down to the nanoscopic scale. The experimental results from the planned experiments are to be used in the theoretical modeling of the BH effect in DP steels which will be accomplished in cooperation with TU Wien. The theoretical modeling will cover the range from the nanoscopic to the macroscopic scale. For the physically-based modeling of the static strain-ageing kinetic in ferrite and the tempering effects in martensite, a new theoretical approach will be employed. The physical-based internal-state-variable (ISV) approach will be utilized for the modeling of microstructure evolution as well as for the flow-stress modeling. Moreover, a model for stress-strain curves describing the yield-point phenomenon will be developed. Finally, the integrated outcome of the proposed work will result in an advanced software tool for through-process modeling of the BH effect in DP steels.