Microstructure characteristics and impact fracture mechanisms of Nb and V–Ti micro-alloyed offshore platform steels

Abstract

Nb, V, and Ti are alloying elements that are often added to Marine steel to improve the mechanical properties. In this paper, the microstructure characteristics and impact fracture mechanisms of Nb and V–Ti micro-alloyed offshore platform steels were investigated. The results show that Nb plays a better role in prior austenite grain refinement and precipitation behavior, while V/Ti is better in bainite/martensite laths refinement and stabilizing retained austenite. In terms of mechanical properties, Nb steel exhibits better yield strength, ultimate tensile strength, and Vickers hardness, while V–Ti steel performs better in total elongation and low-temperature impact toughness. The fracture surface of V–Ti steel exhibited a mixed fracture mode with the ductile and cleavage region while that of Nb steel reveal a cleavage failure without any ductile features. The ductile fractures were deduced from retained austenite deformation and the cleavage fracture areas were mainly correlated with martensite and bainite deformation. When the impact load was applied, the cracks first nucleated between lathy martensite and lathy bainite and then propagated along with the interface. Some cracks terminated after encountering M/A constituents and produced the secondary cracks, and some would encounter grain boundaries and formed microcracks.