Influence of Partitioning Temperature on the Microstructure and Mechanical Properties of (B+M/A) X80 High Deformability Pipeline Steel

In this work, the application of the novel heat on-line partitioning (HOP) process to a traditional X80 pipeline steel by using Gleeble-3500 thermo mechanical simulator has led to the development of a new kind of duplex microstructure comprising bainite matrix and martensite-austenite constituent (MA) as a second phase. The effects of partitioning temperature on microstructure evolution, mechanical properties and retained austenite content of (B+M/A) X80 pipeline steel are researched by means of mechanical property test, microscopic analysis and X-ray diffraction. The study results suggest that the strength level of the steel subjected to the HOP process increases at first and subsequently decreases, while the elongation change in the opposite trend of the trail Steels with the increase of partitioning temperature. This is due to the synergistic effect of the increase in the retained austenite fraction, the decrease in carbon supersaturation in bainite, the change in the dislocation density in bainite matrix, and the formation of transition carbide. Meanwhile. in the early stages of the low-temperature partitioning process, carbon partitioning from bainite to austenite plays a dominant role in the stability and content of retained austenite. With the increase of partitioning temperature, the content of retained austenite presents the pink distribution, as does the effect on the plasticity. Therefore, the elongation is sensitive to partitioning temperature.