Developments in niobium steels for linepipe applications

Current niobium containing steels being produced for line pipe applications, developed over the last ten years, demonstrate an excellent respoNse to heavy deformation at low temperatures, which results in an optimum balance of yield strength and toughness. However, it has long been recognized that excessive use of controlled rolling involves production penalties and contributes to the characteristic anisotropy present in rolled products. Thus, changes in rolling procedures would be desirable if they minimized delays or reduced directionality thereby resulting in further improvement of secondary properties, such as through thickness ductility and hydrogen induced cracking (HIC) tendency in sour gas environments. Present steel development is focused on transformation strengthening or increased precipitation hardening. Coupled with the trend to even lower carbon (< 0.025%) and nitrogen levels (20ppm), new alloy designs including titanium have become feasible. New steel types which have been developed to embody some of the above variables are described. A significant feature of the steels is their chemistry-processing balance, comprising a critical niobium/titanium content in conjunction with possibilities of processing at relatively high finish rolling temperatures to produce a homogeneous equiaxed ferrite microstructure. (Author)