Microstructural analysis of ductility and fracture in fine-grained and ultrafine-grained vanadium-added DP1300 steels

This paper addresses the impact of adding 0.14 wt% of vanadium to a fine-grained DP1300 steel, particularly in terms of damage and ductility. Variants with and without V were heat treated so as to obtain a similar vol% of martensite in the microstructures, and hence both steels had comparable tensile strengths. As a result of the V addition, ferrite grain size was reduced from 4.8 to 2.9 μm in rolling and normal directions in the reference steel to 1.6 and 0.8 μm in the V-added steel, respectively. Consequently, significant ductility improved achieved in the V-added steel. Through this research, we have investigated the influence of V on the mechanical properties of the ferrite and martensite phases, plastic compatibility between ferrite and martensite, and the fracture morphology. The hardness of ferrite and martensite were determined using statistical nanoindentation tests. Microstructural damage was investigated during in-situ tensile tests coupled with scanning electron microscopy followed by fractography. X-ray computed tomography was conducted to quantify microstructural damage and for further analysis of fracture. Results show that adding V to DP1300 leads to grain refinement, microstructural homogenization, enhancement of martensite ductility, and improved mechanical compatibility between ferrite and martensite. Hence, strain is distributed more homogeneously between the two phases in the V-added steel and strain localization is reduced. This helps to suppress the evolution of microstructural damage during post-uniform elongation leading to improved ductility. Dimple-based ductile fracture was dominant accompanied by some quasi-cleavage fracture in both steels and shear fracture was observed in the V-added steel.