HAZ Properties of Nb Microalloyed High Strength Pipeline Steels

The combination of high strength and excellent low-temperature toughness of modern linepipe steels is a direct result of the thermo-mechanically controlled process (TMCP), which leads to a reduced grain size after rolling. Addition of niobium is an effective measure to inhibit recrystallization during finish rolling, which is the key to obtain a fine-grained microstructure. An experimental investigation is carried out in which the niobium content of low carbon laboratory heats is varied between 0.02 and 0.105%. These heats are thermo-mechanically rolled to a wall thickness of 25 mm and subsequently used for double submerged arc welding (DSAW) trials. It is found that high levels of toughness in the heat affected zone (HAZ) could be reached regardless of the niobium content up to high values of 0.105%.