The Role of Non-Ferritic Phase in the Micro-Void Damage Accumulation and Failure of Dual-Phase Steels

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Sloan, Andrew
dual-phase steel , void damage , banding , X-ray micro-computed tomography
Dual-phase (DP) sheet steels are a class of advanced high strength steels which boast a desirable combination of properties for the forming of automotive components, including: high strength, continuous yielding behaviour, and a high initial work hardening rate. The higher strength of DP steels relative to predecessors used to form automotive components allows for a reduction in part gauge, translating to the potential for reduced automobile weight, emissions, and fuel consumption. However, a form of premature failure during component forming known as `shear fracture' has become a prominent challenge to manufacturers' adoption of DP steels. Martensite particles in DP steel microstructures act as nucleation sites for the development of void damage during deformation, resulting in a deleterious effect upon formability and thought to contribute to the observed shear fractures. This dissertation contributes to the overall goal of offering guidance for the improvement of DP steel microstructures for more desirable fracture behaviour. Specifically, the role of non-ferritic phase/constituent (NFP) volume percent and spatial distribution in the accumulation of void damage in DP steels was investigated. Void damage accumulation in ten DP steel microstructural variants tested to failure under near plane-strain deformation was qualified and quantified in three dimensions using an X-ray micro-computed tomography technique. These results were correlated to the microstructural parameters of the variants, which clearly indicated the detrimental effects of NFP banding in DP steels. It was observed that DP microstructures with increased severity of NFP banding (generally aligned in the sheet rolling direction) incurred a reduced strain to failure. Often, microstructural variants with NFP bands aligned transverse to the major loading direction incurred a reduced strain to failure, accumulated a greater number of voids, and exhibited a larger void volume percent than a specimen with oppositely oriented NFP bands. Void damage spatial distribution was generally reflective of the spatial distribution of the most coarse NFP bands through the sheet thickness. In microstructural variants with NFP bands aligned transverse to the major loading direction, accumulated void damage was often observed to be highly elongated in the direction of NFP banding.
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