Micromechanical modeling of Dual-Phase steels

Examensarbete för masterexamen
Master Thesis
Dual-phase (DP) steel is a high strength steel, much used by the automotive industry due to its characteristic combination of low initial yield stress, high tensile stress and easy cold working and weldability. These advantageous features are related to the microstructure of soft ferrite and hard martensite. The total mechanical behavior of such a material is primarily derived from the volume fraction of each phase, grain sizes, distribution of the phases and alloying elements. However, to be able to bene t from the prominent features of the material to an even greater extent than today, more knowledge on how the microstructural parameters are correlated to its end-properties is needed. In this thesis the mechanical behavior of DP steel in uniaxial tension is studied with regard to microstructure. A micromechanical modeling framework utilizing an axisymmetric representative volume element (RVE) is implemented in Abaqus. The RVE consists of a sphere representing the martensite, in a cylinder of ferrite. A single-phase material model describing the plastic ow is applied to each respective phase, but with different material parameters. This constitutive description is developed from a dislocation density theory where microstructural parameters control the plastic flow. More knowledge about the behavior of the DP steel, the microstructure, correlations between different parameters, modeling techniques and constitutive models suitable for DP steels were retrieved from a literature review. Microstructural characterization and mechanical testing in tensile response and hardness have previously been performed on four DP steels in varying strength classes at the Norwegian University of Science and Technology. The retrieved microstructural data was applied to the micromechanical model and the results from the mechanical testing were used to validate the predictions on tensile response from numerical simulations of the micromechanical model. Despite the simplicity of the micromechanical modelling framework it in general produces results that are in good agreement with the corresponding mechanical tests. However, deviations are seen in the modeling of the strongest steel, which has the highest volume fraction of martensite. This may indicate that the ferrite is more accurately represented than the martensitic phase. It was also shown that when the geometry of the martensitic sphere was changed in such a way that the ferrite in certain areas got more constrained, an undesired strengthening effect was obtained. This indicates that the speci c RVE used in the study has geometrical limitations and that it is more suitable for lower volume fractions of martensite. The thesis has been conducted through a collaboration between Chalmers University of Technology and Norwegian University of Science and Technology.
Maskinteknik, Materialteknik, Materialvetenskap, Produktion, Mechanical Engineering, Materials Engineering, Materials Science, Production
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