Effect of Coiling Temperature on Microstructure and Mechanical Properties of X80 Pipeline Steel with a Design Factor of 0.8

The mechanical properties and microstructures of X80 pipeline steels with a design factor of 0.8 produced at different coiling temperatures were investigated by means of tensile test, impact test, scanning electron microscopy, laser scanning confocal microscopy and transmission electron microscopy. The results show that the microstructures of experimental steels mainly consisted of acicular ferrite. With the increase of the coiling temperature, the volume fraction of M/A islands decreased, the acicular ferrite content increased and lath bainite disappeared gradually. When the coiling temperature increased to 480 ℃, the quasi-polygonal ferrite percentage increased and the grain size enlarged gradually. The influence of the coiling temperature on precipitate phase was little, and the precipitates were composed of typical (Nb,Ti)(C,N) composite grains. With the increase of the coiling temperature, the yield strength and percentage elongation of the experimental steel first increased then decreased, while the tensile strength showed a decreasing trend. The influence of the coiling temperature on the low temperature impact toughness was little. And the optimized microstructures and mechanical properties of the experimental were obtained after coiling at 370 ℃.