Towards ultra-high strength dual-phase steel with excellent damage tolerance: The effect of martensite volume fraction

Ultra-high strength (> 1 GPa) dual-phase (DP) steels have been extensively investigated in literature. Yet, the damage tolerance of these DP steels remains mostly unexplored, whereas this parameter is crucial for the sheet formability and anti-crushing performance. In this work, a medium-Mn DP steel was proposed in order to develop strong but damage-resistant DP steel by refining the microstructure. Ultra-fine grained (< 1 <mu>m) DP steels were obtained by a simple cold rolling and intercritical annealing process. The developed DP steel can achieve an outstanding ultimate tensile strength (UTS) of over 1470 MPa with appreciable elongation to failure (EtF) of over 9 % when the martensite volume fraction (V-m) is 55 %. Higher V-m results in higher strength. However, the fracture strain, which is connected with damage tolerance, decreases significantly with increasing V-m, and it is even much lower than that of a full-martensite steel. Furthermore, the necking behaviour transfers from diffuse plus localized necking to diffuse necking with increasing V-m. The sufficient ferrite content in the proposed DP1470 steel enables void growth after nucleation, consequently leading to the occurrence of localized necking and higher fracture strain. Thus, a moderate V-m ranging from 45 % to 65 % is suggested for the development of a 1470 MPa grade DP steel with excellent damage tolerance.