Enhancing plasticity by increasing tempered martensite in ultra-strong ferrite-martensite dual-phase steel

The present study focuses on the influence of tempered martensite (TM) on the mechanical properties and fracture mechanisms of an ultra-strong dual-phase (DP) steel. The steel was subjected to intercritical annealing combined with an aging process. The high fraction of martensite (above ∼70 vol%) results in a high strength level above 1300 MPa, and the presence of TM ensures a good ductility up to ∼10% total elongation. The microstructures of the tested steels were analyzed by the scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD). Results showed that a higher fraction of TM significantly improves ductility, mainly due to its beneficial effects on damage tolerance. More specific, the damage behavior alters from martensite cracking to ferrite-martensite interface decohesion, with increasing TM fractions. This results in higher post uniform elongation (PUE) values. With the increase of intercritical annealing temperature, the yield strength (YS) increases, but both the ultimate tensile strength (UTS) and strain hardening rate decrease. The strain hardening rate was discussed based on the influence of carbide precipitates and decreasing geometrically necessary dislocation (GNDs). Besides, we found that the aging process could significantly increase the volume fraction of TM and improve the plasticity.