This subject was focused on: study of the fracture mechanics and toughening mechanism of high-strength pipeline steel under the low temperature and high pressure service conditions; clarify the principles of the physical metallurgy to control ductile fracture and arrest under the low temperature conditions; establish a theoretical system for steel used in the low-temperature and super-large-volume pipelines. After 4 years of research and development, the main scientific problem of "the weld and hot zone fracture mechanism and toughness crack arrest metallurgy principle of the low temperature X80 pipeline steel" had been solved. The two common key technologies of "composition design and phase transformation control technology of the low temperature thick X80 steel plate/strip" and " toughening control technology of the welded joint of the low temperature thick wall X80 steel pipe" were broken through. Four main contents of research “metallurgical principles of low temperature fracture toughness control of X80 pipeline steel”, “fine control technology of ultra-wide thick steel plate/belt composition design, phase change dynamics and microstructure”, “the relationship among fine structure characteristics of steel pipe submerged arc weld, hot zone and low temperature toughness”, “toughening control technology of X80 steel pipe girth weld” were carried out. The main conclusions are as follows: The correlation between the grain size control of the deformed austenite in pipeline steel and the performance of DWTT was first proposed in the world, which provided a theoretical foundation to solve the problem of the DWTT performance control with thick specifications. The innovative technology of "low temperature and fast rough rolling" was developed, which realized a major technological breakthrough in the production of thick-size pipeline steel in our country. A new generation of manufacturing technology of high-grade pipeline steel with "ultra-fine ferrite + acicular ferrite" was developed. The cause of the self-shielded welds was discovered. The characteristics of the strengthening and toughening in each zone of the multi-layer and multi-pass welds were clarified. The high-strength X80 self-shielded welding wire of high Ni series was developed. The welding wire had higher adaptability to the chemical composition of the base metal compared with the original composition system. The composition design of the low temperature X80 pipeline steel based on good weldability was proposed for the first time in China. This component design effectively guided the formulation of technical conditions for the China-Russia eastern natural gas pipeline engineering.
