High temperature processing of line-pipe steels

It has been determined that by reducing the carbon content of line-pipe steel to about 0.03% many property improvements are achieved such as excellent toughness, ductility and ease of welding as well as reduced segregation including the slab centerline regions, necessary to guarantee resistance against sour media. Furthermore, lower carbon contents increase the niobium carbide solubility and permit the use of higher niobium contents than these traditionally used. Niobium contents up to 0.10% have recently been adopted in these low carbon pipe steels. With higher niobium contents austenite processing can be carried out at higher rolling temperatures. Additional strength increases are observed due to niobium’s role in retarding the transformation to ferrite, thus promoting a higher volume fraction of bainite, and by forming NbC precipitates in ferrite. This concept is ideally suited to produce high strength via accelerated cooling, but one has to balance the amount of alloying elements with the cooling rate to guarantee the demanded strength level for the considered plate thickness. These alloy designs are not only suitable for producing high strength sour gas resistant pipes, but the approach also allows the production of pipes with high toughness on mills not capable of withstanding high rolling forces and additionally on Steckel mills, where coiling the sheetbars is often the limiting factor during severe thermomechanical rolling. Another interesting application is for the substrate of clad pipe, where the corrosion resistance of the cladding requires high finish rolling temperatures, which would otherwise result in insufficient toughness in the base metal. (AU)