Effect of post-rolling cooling process on microstructure of large-transmission X80M pipeline steel

In order to ensure the high strength of large-transmission X80M pipeline steel and further improve its low temperature toughness, and achieve a good match of strength and toughness, Gleeble-3800 thermal simulation tester was used to simulate rolling. The dynamic CCT curve was established by thermal expansion method combined with microstructure analysis, and the effects of cooling rate and final cooling temperature on the microstructure and hardness were studied by OM, SEM, hardness testing and other analytical methods. The results show that with the increase of cooling rate, the microstructure produces the transition of PF+P→GB→AF→BF, and the Vickers microhardness gradually increases. The ideal microstructure consisting of fine uniform AF and diffusely distributed M/A islands can be obtained when the cooling rate is in the range of 15-25 ℃/s. Final cooling temperature has an obvious effect on its phase composition. As the final cooling temperature decreases, the M/A island size becomes smaller, the number increases and its microstructure gradually refines. Combining experimental research with production practice, the final industrialized TMCP parameters are set to final rolling temperature of 780 ℃+final cooling temperature of 360 ℃+cooling speed of 20 ℃/s. The obtained X80M pipeline steel coil has high strength and excellent low temperature toughness, which meets its engineering and technical requirements, and is successfully applied in the major pipeline project of West-East Gas Transmission Line IV. This study provides technical reference for the research and development of high-strength and high-toughness pipeline steel and strongly supports the construction of national major pipeline projects.