Role of martensite morphology on mechanical response of dual-phase steel produced by partial reversion from martensite

This work focused on the effects of martensite morphology on the mechanical properties of the dual phase (DP) steels produced via partial reversion from martensite, which is different from the traditional DP steels. A commercial DP780 steel was used, and the needle and blocky DP steels were produced by controlled heat-treatments. Combinations of SEM, EBSD and tensile tests were performed for understanding the property and damage mechanisms. The needle DP steel has higher ductility while lower strength than that of blocky DP steel. Much higher density micro-voids were uniformly formed in the needle DP steel than that of blocky case, and the coalescence of micro-voids into microcracks is strongly suppressed, due to the high-density ferrite/martensite interfaces and uniform local plastic strain. On the other hand, the blocky martensite islands distribution leads to the non-uniform local plastic strain, which caused the lower density micro-voids formation and the generation of micro-cracks, thus the resultant lower ductility. In addition, the higher strength of the blocky DP steel is due to strengthening of the sub-micron sized needle martensite.