DEVELOPMENT OF API 5L X70 GRADE STEEL THROUGH THIN SLAB CASTING ROLLING PROCESS

摘要

API 5L grade steel is mainly used for oil and gas transportation. The economy of gas transportation via pipeline demands for high operating pressures and large pipe diameters in order to improve transportation capacity which requires heavy thickness and/or high grade of the steel. This pushed the steelmakers to develop high strength steels (HSS) with superior metallurgical and mechanical (strength, toughness and ductility) properties in order to allow exploitation in hostile environments. The technology of production of API 5L grade through conventional thick slab process is matured enough as it gives flexibility of using higher %C lower casting speed, high slab thickness (200-250 mm), higher reheating temperature and time, high reduction etc. However due to slower cooling rate during liquid to y transformation, possibilities of centerline segregation defect increases. Thin slab technology (TSCR), on the other side allows a reduction in energy consumption (because of lower slab thickness and elimination of reheating process), with consequent benefits in terms of production costs and pollution reductions. But producing API X65 and above though TSCR route with subzero impact and DWTT is a challenge because of the difficulties in achieving a refined and homogeneous microstructure due to lower reduction ratio from slab to finish sheet thickness. This paper aims to give an overview of recent developments of high strength pipe steel grades as API 5L X70 through TSCR route. Information regarding the metallurgy and processing, such as chemical composition, microstructural design, thermo-mechanical controlled process (TMCP) and accelerated cooling process (AcC), to achieve the target strength, ductility and toughness properties are discussed. Mechanical properties are well above the requirement of X70 at HR stage as well as after pipe formation. Excellent Impact and DWTT is achieved up to -40°C.