Role of heat treatments on the mechanical properties of dual-phase steel sheet

The mechanical behaviour of dual-phase (DP) steel is highly dependent upon the size, spatial distribution and volume fraction of the non-ferritic phase (e. g. martensite) as well as the ferrite grain size. To understand the effects of these parameters, a series of pre-treatments were performed on DP600 steel prior to inter-critical annealing. These pre-treatments served to change the spatial distribution of carbides before the inter-critical annealing step, thereby producing distinctly different final microstructures. The first pre-treatment involved austenitization, quenching and tempering, while the second utilized an austempering process. It was found that the final microstructure after IC annealing correlates directly with the size and spatial distribution of non-ferritic particles in the pre-treated material. Uniaxial tensile tests revealed a clear difference between the mechanical behaviour of DP steels produced with different heat treating histories. Specifically, martensite particle size and spatial distribution were found to have the most significant effect on mechanical properties.