Until recently X70 steel has been the generalized grade for linepipe steel. However, there is a need to use higher grade steels such as X80 or X100 for high transmission efficiency and lower construction costs.As X80 steel is manufactured by higher alloy addition and severe rolling reduction, weldability of X80 steel seems to be a more important issue than with previously-used lower grade steels. Several mill trials have been performed to optimize the production processes for X80 steel at POSCO. Field manufactured API-X80 grade plates with a thickness of 15.6mm were found to have very high strength and toughness. It is thought that the high strength and enhanced low temperature toughness of developed plates are attributed to the acicular ferrite and very fine polygonal ferrite microstructure of the material. The yield strength of API-X80 grade plates with acicular ferrite microstructure increased after pipe forming. X80 plates have high impact toughness in seam weld heat affected zones (HAZ) and sufficient strength without cold cracking in girth weld joints.
The material quality requirements of the gas and oil transmission pipelines have steadily increased over the past few decades. The gas and oil industry demands more economical transportation due to longer transportation lines and active exploitation in areas of severe weather conditions.This transport efficiency can be achieved by constructing pipelines with high strength steels, allowing higher operating pressures and gas transmission rates. API-X80 steels have been developed and applied to the several pipeline projects during the last decade. Further, the market needs for the more economical and safe transportation of oil and gas in hostile environments require that X80 steel has better toughness than the previously developed steels. Recently, large-scale natural gas pipeline projects are being constructed and planned in North America and Northeast Asia (Klatt, 2000; Asakura, 2000).