A Novel Alloying Concept for Thermo-Mechanical Hot-Rolled Strip for Large Diameter HTS (Helical Two Step) Line Pipe

One of the major priorities of the research and development department of the Salzgitter Group is placed on the product and process optimisation of both approved conventional steel grades for line pipe application and novel high strength alloying concepts. With respect to reduced wall-thicknesses and higher operating pressures for gas transportation pipelines, the requirements for hot wide strip material are steadily increasing. Material strength can be increased either by grain refinement of the ferritic-pearlitic phase in combination with precipitation hardening or by replacing the ferrite-pearlite by an intermediate microstructure, so-called acicular ferrite. A low carbon content supports the formation of the microstructure desired and results in an improvement of the carbon equivalent. This acicular ferrite does not only raise the material strength but also improve low temperature toughness, weldability and workability. New processing routes based on the demands of novel alloying concepts have been developed in order to achieve the intermediate microstructure. In the following paper, two different alloying and processing strategies and the resulting mechanical properties and microstructures are described and illustrated.