Design of V containing line-pipe grades for improved strength and weldability

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There is a strong need for new large-diameter, high-performance and durable advanced high strength steel (AHSS) line pipes in Canada that can withstand the harsh Canadian conditions. Low-temperature toughness, good stress corrosion properties and excellent field weldability are some of the main requirements. Microalloying with Ti, Nb and V, controlled rolling and accelerated cooling are commonly employed processing techniques for AHSS pipe. Whilst the individual effects of each of these microalloying elements on properties are well established, inconsistent results have been reported when two or more species are present. Here we will first focus on the effect of vanadium in isolation. Vanadium is of strong interest as it has been shown to have a strengthening effect on bainite, one of the main microstructural phases present in high strength line pipe steels. The origin of this strengthening is disputed; some authors claim that it is due to refinement of the bainitic microstructure, others claim that precipitation or clustering is involved, and still others believe that vanadium suppresses dynamic recovery. We will carry out trials on laboratory cast and hot-rolled specimens to determine the validity of each of these hypotheses and to develop a more accurate structure-properties model for V containing bainitic steels. Subsequently, a more complex alloy containing both V and Nb additions will be studied to check for antagonistic/synergistic effects. This new knowledge will lead to improvements in pipeline strength, reliability and costs through a better optimisation of the composition and process parameters.