Due to the increasing demand for natural gas, the construction of long-distance pipelines through seismically active regions or arctic regions with ground movement caused by permafrost phenomena will become more and more necessary. To guarantee the safe operation of those pipelines, the pipe material has to fulfil strain-based design requirements. Hence in longitudinal direction low yield-to-tensile ratios, high uniform elongation values and a roundhouse shape of the stress-strain curve combined with sufficient strength values in transverse direction are essential. Moreover, a satisfactory low temperature toughness has to be guaranteed.
An adequate plate metallurgical design is fundamental for appropriate pipe properties achievement. As far as concerns the plate design the understanding and the control of microstructure are the key factors, achieved by an adequate steel chemical composition and proper process parameters.
In the framework of a co-operation between Baosteel and Centro Sviluppo Materiali (CSM), a project has been started aimed at manufacturing X80 strain based designed pipes. As a starting point pilot trials have been carried out at Baosteel Research Center in order to produce different microstructures. Besides the steel chemical composition, the cooling process has the most significant influence on the formation of the microstructure: in order to assess the effect of the cooling process, the same rolling schedule was adopted for producing the different test materials, obtained varying the start cooling and finish cooling temperatures.
The microstructure and mechanical properties of the different test materials were assessed and the best microstructure for the plates for X80 pipes with enhanced strain capacity has been identified.