Abstract: Taking the large diameter (OD1422 mm) with heavy wall thickness (38.5 mm) pipeline made of X80 grade hot-rolled steel sheet as the research object, the microstructure and fracture morphologies of -20 ℃ low temperature DWTT of the steel were analyzed by means of OM and SEM, and the relationship between fracture and microstructure was discussed. The results show that band structure and uneven grain size at different position in wall, mixed structure of granular bainite, degraded pearlite, quasi-polygonal ferrite, martensite-autenite island, ect. will cause the initiation and propagation of cracks and the appearance of cleavage fracture, which reduces low temperature toughness. However, the martensite-austenite structure with a size of 3 μm or less and the uniformly distributed bainite ferrite hinder the crack propagation, and the fine martensite-austenite structure and bainitic ferrite can improve the fracture toughness of the steel plate, which is effective for controlling the low-temperature fracture of the steel.

Key words: pipeline steel, DWTT, structure, crack, fracture