The last decades have seen a steady increase in the demand for high strength linepipe steels. These offer the most economical option to transport large gas volumes at high pressures from remote areas to the market. Since the beginning of the 1980s, high strength heavy plates, large diameter pipes and pipe bends were developed and produced at Salzgitter Mannesmann Grobblech (SMGB) and EUROPIPE in cooperation with Salzgitter Mannesmann Forschung (SZMF). While the focus was initially on tensile properties in hoop direction only, low-temperature toughness and weldability have gained growing significance recently, as gas resources are developed in increasingly hostile environments. This has spurred research activities to optimize the alloy and process design along the complete process chain, which have led to major improvements.
Modern X80 high-strength heavy plates used in the production of UOE pipes are generally produced by thermomechanical rolling followed by accelerated cooling (TMCP). This processing route results in a microstructure that consists predominantly of bainite. The combination of high strength and high toughness of these steels are a result of the microstructure realised by TMCP and are strongly influenced by the rolling and cooling conditions. The improvement of the properties of the weld is a result of an optimization of the base metal composition and the welding process. Because of the continuous development efforts devoted to the optimization of high strength large diameter pipes, they have become daily business for both SMGB and EUROPIPE. Within this paper, an overview of the improvements of low-temperature toughness of heavy plates and large diameter UOE pipes is presented.