Different Dual Phase steels were processed with ferrite grain size between 0.7 µm and 4 µm and martensite islands dispersed along the grain boundaries of the ferrite phase. Improvement of both strength and elongation is obtained when non radial large strain paths are imposed as cold deformation prior to the thermal annealing. In the present work, specimens were deformed by cold swaging, which is an unusual but very efficient method to apply non radial compressive loading path at room temperature. Local probing of the mechanical properties of the ferrite phase was conducted by nanoindentation. By changing the indentation depth with respect to the size of the microstructure, separation is made between the various contributions to the strengthening : nanoindentation size effect, natural hardening of ferrite, grain size effect and reinforcement by the martensite. A simple model which relates the strengthening mechanisms to their characteristic sizes supports the interpretation of the experimental results and highlights the relationship existing between the different length scales.
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