Characterization and physical-based modeling of the BH effect in DP steels

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 will develop 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.

In the automotive industry, there is a strong need for efficient processing of the individual exterior parts. On the one hand, a good ductility is mandatory to form the parts; on the other hand, high strength is desirable for the drivers safety. This is achieved by the Bake- Hardening (BH) effect, where the steel is hardened after shaping and during the paint baking process of the car body. In detail, the deformation of the material generates dislocations. Baking the material afterwards leads to two hardening processes. One is the segregation of carbon atoms into those dislocations, the other is the precipitation of carbides which further strengthens the material. The goal of this project is to get a better understanding of the overall BH process for dual-phase steels. To achieve this, the work was split between the project partner TU Clausthal in Germany, who were mostly responsible for the experimental part, and TU Wien, where the model development and simulation took place. In the course of this project, recently developed models were implemented in the software MatCalc, which is renowned for simulating microstructural properties in steels and other metals used for industrial purposes. More precisely, the segregation process of carbon, which was mostly described using an empirical formulation until now, was modelled with a thermodynamic approach in collaboration with F.D. Fischer from the University of Leoben and J. Svoboda from the Academy of Sciences in Brno. This links the processes of carbon segregation and carbide precipitation on a physical level in which each effect has a direct influence on the other one and marks a new milestone in the modelling of the BH process. Furthermore, dual-phase steels have shown tendencies of a reduction in strength for extended baking times. This has been thoroughly investigated on an atomic level with the help of nanoindentation and transmission electron microscopy, done at TU Wien, and atom probe tomography, done at the University of Leoben. For specimens which were baked at a prolongated time, a reduction of the total carbon concentration was observed in the ferritic phase, leading to a reduction of strength in the aforementioned phase. This has been successfully reproduced in an extensive diffusion simulation between both phases of the steel. Additionally, a new thermodynamic description of the coarsening process between carbides along dislocations was formulated, which has a further impact on the loss of strength. Coupling all developed models with existing models showed a good agreement between simulations and experiments.