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 obtaining a fine-grained
microstructure. An experimental investigation was carried out at Salzgitter
Mannesmann Forschung GmbH (SZMF) in which the niobium content of laboratory
heats was varied between 0.02% and 0.10%. These heats were thermomechanically
rolled to a wall thickness of 25 mm and subsequently used for double-layer
submerged arc welding trials. It was found that high levels of toughness in
the heat-affected zone could be reached even at high niobium contents.