Effect of Rolling and Coiling Temperatures on Microstructure and Mechanical Properties of Medium-Carbon Pipeline Steel

Abstract

Oil country tubular goods (OCTG) steels with a low yield ratio (yield strength/tensile strength) and excellent impact toughness have recently been demanded to ensure mining performance and safety. From this viewpoint, the optimization of the manufacturing conditions is important because they influence the microstructure and mechanical properties of the steels; in particular, in the case of OCTG steels with carbon contents greater than 0.2 wt%, the finishing mill temperature (FMT) and coiling temperature (CT) strongly affect the microstructure of the final products, which are generally composed of ferrite and pearlite phases. In this study, 0.39C-0.23Si-1.56Mn-0.11Cr steel plates were fabricated under various FMT and CT conditions and their yield strength, tensile strength, and impact energy were investigated. In addition, pipes with diameters of 244 and 508 mm were manufactured via an electric resistance welding method using two of these strips fabricated under two different optimized conditions [(1) FMT = 880 °C and CT = 630 °C and (2) FMT = 800 °C and CT = 690 °C] to analyze the change in mechanical properties induced by the work-hardening effect during the piping process. The results revealed that the FMT and CT are closely related to the volume fraction of the ferrite phase, the grain size and lamellar spacing of the pearlite phase, and the tensile and impact properties of the steel strips; the variations in the microstructure and mechanical properties with the FMT and CT were also discussed in detail.