Steel material for line pipes, manufacturing method thereof, and manufacturing method of line pipes

摘要

It is an object of the present invention to provide a steel material for a line pipe having a thickness of 30 mm or more and having high strength for application to a submarine pipeline and excellent in low temperature toughness and DWTT performance, a method of manufacturing the same, and a method of manufacturing a line pipe. Steel having a specific component composition is heated to a temperature of 1000 to 1200°C, and the cumulative reduction ratio in the non-recrystallization temperature range is 60% or more, and the cumulative reduction ratio in the temperature range below (rolling end temperature + 20°C) is 50 % Or more, after hot rolling with an Ar ₃ transformation point or more and 790°C or less as a steel sheet average temperature, the cooling stop temperature at a cooling rate of 10°C/s or more from a temperature above the Ar ₃ transformation point is 200 to 450°C as the average steel sheet temperature Accelerated cooling is carried out to, and then reheating is performed so that the surface temperature of the steel sheet is 350 to 550°C, and the central temperature of the steel sheet is less than 550°C, and the tensile strength is 570 MPa or more, compressive strength 440 MPa or more, and plate thickness 30 mm. The above method of manufacturing a steel material for a line pipe.