Effect of Reduction Ratio below Austenite Recrystallization Stop Temperature on Mechanical Properties of an API X70M PSL2 Line Pipe Steel

Abstract

Steel grades having high toughness and high strength are required for line pipes since gas and oil should be transported through them at high pressures. Thermo-mechanically controlled rolling processes are used for increasing both toughness and strength at low temperatures since the line pipes are exposed to harsh climatic conditions at full length and rather severe service conditions. Various test methods such as Charpy, DWTT, CTOA have been used for measurement of toughness values of these steels and the results have been evaluated considering various criteria. In this study, thermo-mechanical rolling trials were performed at the temperature below the recrystallization temperature of austenite on an API X70M PSL2 grade steel, which is large scale used in the oil and natural gas line pipes, to increase the strength without sacrificing the toughness. Different reduction ratios were utilized and the effect of reduction ratios on mechanical properties and microstructures were investigated during the trials. It was observed that final grain size decreased and strength and toughness increased with increasing reduction ratio.