Niobium in structural steels

Since the widespread application of niobium (Nb) to linepipe steels started with the Trans Alaska Pipeline System (TAPS) project in the 1970’s, the Nb-bearing technology has rippled across all the Thermo-Mechanical Control Processed (TMCP) steels due to its best-suited austenite conditioning effect. However, recent applications for steels have required not only ferrite-pearlite type microstructures but also bainite or martensite type high strength structural steels. The Nb-added 950MPa tensile strength TMCP steel was supplied, for the first time, for the construction of the Kannagawa hydraulic power plant in 2002. Here the Nb was the key to increase strength without deteriorating the toughness of the martensitic structure. On the other hand, some ferrite (and bainite) based steel, such as offshore structure steels and LPG carrier hull steels, have contained Nb in order to increase low temperature toughness with decreased carbon content. The diverse applications of Nb-metallurgy for structural steels are presented in this paper from both mother plate and weldment points of view with actual project examples. (AU)