Ductile performance of high-strength pipeline steel study of cooling rate impact

Abstract The laying of oil and gas pipelines passes through some areas with poor geological conditions and large temperature differences between day and night, and such a service environment puts forward higher requirements for pipeline steel, which should have high strength as well as a certain degree of toughness, to prevent pipeline cracking. This paper analyzed the impact of cooling rate on the toughness of high-grade pipeline steel by researching microstructure, grain boundary characteristics, M-A islands transformation, and fracture toughness under different cooling rates. The results showed that the microstructure of the tested steel gradually transformed from soft-phase polygonal ferrite to harder-phase lath bainite when the cooling rate changed from 0.1 °C/s to 30 °C/s. When the cooling rate was quite low, M-A islands were larger in size and smaller in number, while the number of M-A islands increased significantly to about 8%∼10% at higher cooling rates and were distributed on the prior austenite grain boundaries and bainite lath boundaries. In addition, the proportion of HAGBs increased first and then decreased with the increase of the cooling rate and reached a maximum level at 20 °C/s, and at this time, the effective grain size was smallest, 3.018 μm. A higher proportion of large-angle grain boundaries and a smaller effective grain size were quite favorable to enhance the low-temperature ductile performance of high-strength pipeline steel.