UBC Theses and Dissertations
Deformation and fracture behaviour of a low-carbon dual-phase steel Mazinani, Mohammad
The primary goal of this study was to evaluate the effect of martensite plasticity on the deformation and fracture behaviour of an intercritically annealed commercial low carbon (0.06 wt.%) dual phase steel. The volume fraction and the morphology (banded and almost equiaxed) of the martensite phase were systematically varied by control of the intercritical annealing temperature and the heating rate to this temperature. It was observed that the yield and tensile strengths were dependent on the martensite content but not on the martensite morphology. On the other hand, the true uniform strain, fracture strain and fracture stress were found to have a significant dependence on martensite morphology. An Eshelby based model, which allowed for the calculation of the stress in the martensite islands, was employed in order to rationalize the tensile properties of the dual-phase steel samples with different martensite contents and morphologies. In addition, by comparing the calculated stress in the martensite with an estimate of yield stress, it was possible to examine the conditions under which martensite plasticity occurs. The work hardening behaviour and the fracture properties of the steel samples were rationalized by the implications of martensite plasticity. For the cases where martensite showed significant plasticity (or co-deformed with the ferrite matrix), the void nucleation rate during post-necking deformation decreased considerably and hence, the final fracture properties were dramatically improved. The deformation of martensite in different dual-phase steel samples was examined both qualitatively (using optical micrographs of the undeformed and deformed sections of fractured tensile samples) and quantitatively (through image analysis of the microstructures before and after tensile deformation). The tensile stress-strain responses of different dual-phase steel samples were modeled using the modified Eshelby method. This approach was found appropriate for modelling the stress-strain behaviour of the steels with equiaxed morphology and martensite contents below approximately 30%. In the case of banded morphology, the stress-strain behaviour of the steel sample with 17% martensite was successfully predicted by the model. However, the model overestimated the flow stress of the steel with 30% martensite. For the martensite contents greater than 30%, the overestimation of the flow stress of the steel samples with banded morphology was greater than that for the equiaxed samples. Finally, the void formation process during tensile deformation was examined quantitatively through image analysis of the fracture surface of the steels. The experimental results showed very little void growth during ductile fracture of the steel samples with 17% and 41% martensite. Modelling the void formation process in these steels assuming no void growth stage resulted in the same observation. This confirmed the quantitative observation that void nucleation is the dominant effect during ductile fracture of these steels.
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