| The effects of martensite morphologies on the microfracture mechanism of a dual phase HSLA steel are investigated in this study. In the intermediate quenching process(IQ : repeating to the two phase range after quenching from austenite), very fine fibrous martensite is well distributed in the ferrite matrix. On the other hand, in the step quenching process(SQ ; two phase annealing before quenching), bulk martensite is mixed with ferrite. After heat treatment, the IQ specimen shows high strength with relatively good ductility, but the SQ results in very poor ductility. In-situ SEM fracture test reveals that microcrack initiation occurs mostly in the ferrite region where strain is localized into shear bands, rather than in the martensite region. In the IQ case, a crack tends to propagate in the uniform manner throughout the whole ferrite/martensite microstructure, yielding good elongation with high strength. However, in the SQ case, deformation is concentrated into the ferrite and a local crack propagates just along the ferrite regions, so that bulk martensite cannot effectively control the crack propagation behavior, resulting in poor ductility. |
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