Banded structure in Dual Phase steels in relation with the austenite-to-ferrite transformation mechanisms

New experimental data are reported relative to the development of banded structures in Mn-segregated Dual Phase steels during the austenite-to-ferrite transformation stage of their final heat treatment. Two types of ferrite are detected to grow simultaneously, i.e. allotriomorph and acicular ferrite, with distinct consequences on the topology of the final (ferrite + martensite) microstructure: allotriomorph ferrite sharpens the banded structure of ferrite and martensite between Mn-poor and Mn-enriched regions, respectively, whereas acicular ferrite favours more isotropic microstructures. Owing to an original image treatment procedure, the sharpness of banded structure is quantitatively analyzed for different heat treatment conditions. Results confirm those of previous works, but we propose to rather explain them by considering that a change in a heat treatment parameter results in a variation in the respective amounts of both ferrite types.