Development Of X70 Ultra - Thick Submarine Pipeline Steel Production Technology

Submarine pipeline transportation is the main transportation mode of offshore oil and gas because of continuity, speed, large transportation volume and so on. With the rapid development of the offshore oil industry, development of the submarine pipeline steel with high properties has been a popular study point. The service conditions of submarine pipeline are much different from the traditional pipeline, and the quality requirement of offshore pipeline is much stricter than that of onshore pipeline due to harsh offshore environment. Comparing to onshore pipeline steel, submarine pipelines require high.strength, high toughness, anti-fatigue, low diameter-thickness ratio and high elliptic precision. So, it is very difficult to development and produce the submarine pipeline steel. The technology of microstructure homogenization and refinement are studied in this thesis which supported by National High Technology Research and Development Program "Technology Research of Deep-sea and High-pressure Oil&Gas Pipe with high strength and wall thickness"(NO.2013AA09A219), which offers many experimental data and theoretical foundation for production of X70submarine pipeline steel with heavy wall thickness.In this paper, the chemical composition and controlled rolling and cooling process parameters of X70submarine pipeline steel with heavy wall thickness are discussed and designed. Recrystallization behavior of austenite and the effects of deformation amount of non-recrystallization region, deformation temperature and initial coiling temperature on ferrite transformation are studied by thermal simulating test. Meanwhile, the effects of cooling rate and final cooling temperature on microstructure and microhardness are analyzed. By analyzing trial-production results, chemical composition and technology parameters of X70submarine pipeline steel with heavy wall thickness are further optimized. The results show that recrystallization fraction of the austenite increases and grain size decreases with the increase of deformation. The deformation of tested steels which happen partial dynamic recrystallization decrease with the increase of deformation temperature, when the temperature is980℃, the deformation of partial dynamic recrystallization was30%, when temperature is1000℃, the deformation for partial dynamic recrystallization was25%. and the deformation for partial dynamic recrystallization was20%at1020℃. During the process of holding temperature after deformation, the static recrystallization fraction increases with the extension of holding time, and austenitic grains size which reach35μm. During the double-hit hot compression process, the static recrystallization fraction increases with the increase of deformation temperature and deformation, the austenitic grains size decreases with increase of the deformation. And the grains would be excessively coarse when the deformation temperature is raised. In austenite non-recrystallization region, the density of ferrite nucleation site would increase when the strain temperature is780℃and deformation>50%, which can speed up ferrite nucleation rate and refine polygonal ferrite grain. The fraction of ferrite transformation can grow up when slow cooling rate is1.0℃/s and start cooling temperature≤660℃. The effects of cooling rate and final cooling temperature after rolling on microstructure and microhardness are obvious. Microstructure refines, and content of granular bainite and hardness increase with the increase of cooling rate and the decrease of final cooling temperature.Industrial trial-production results show fine and homogeneous "F+B" are obtained when the finish rolling temperature is780℃,and final cooling temperature is400℃. At this time, strength and toughness can meet the requirements of submarine pipeline steel, the dual phase scale is deviations at the thickness direction owing to high thickness of steel pipes, but DWTT performance can meet the requirements of submarine pipeline steel.