Steel for Pipes of Strength Class K60 (X70) with Ferrite, Martensite and M/A-Component Microstructure for Sections of Main Pipelines Crossing Active Tectonic Disturbance Zones

Formation of the microstructure and properties of pipe steel grade K60 (X70) are studied using physical modeling of thermomechanical processing in a DUO 300 laboratory mill and industrial experiment. Steel microstructure is investigated and temperature ranges for formation of various phases and structural components (ferrite, bainite, martensite, M/A-component) with accelerated cooling are determined. It is found that formation of martensitic regions and M/A-component as secondary structural constituents in a ferrite matrix instead of cementite-containing secondary structures facilitates an increase in steel strength and improvement of the σ y /σ f ratio without a significant reduction in cold resistance. The chemical composition of steel grade K60 (X70) is developed on the basis of laboratory and industrial experiments, as well as a new industrial process for producing rolled product for pipes for the Sila Sibiri gas pipeline sections crossing active tectonic disturbance zones. Sheet and pipes have a low σ y /σ f ratio, high values of relative and uniform elongation, impact strength and proportion of ductile component in a fracture after DWTT testing at – 20°C.