Steel material for line pipes and method for manufacturing same, and line pipe and method for manufacturing same.

Abstract

The purpose of the present invention is to provide: a steel material for line pipes, which is thick, has compression strength necessary for the application to a submarine pipe line, excellent low-temperature toughness and DWTT performance and also has excellent collapse resistance performance, and a method for manufacturing the steel material; and a line pipe which has a required level of compression strength, excellent low-temperature toughness and DWTT performance and also has excellent collapse resistance performance, and a method for manufacturing the line pipe. A steel material for line pipes, which has a specified component composition, and has such a property that the metal structure in a region lying at a depth corresponding to 18 / of the thickness from the steel material surface has a bainite area fraction of 85% or more, a polygonal ferrite area fraction of 10% or less and a martensitic islands area fraction of 5% or less, the 0.23% compression strength at a position corresponding to 18 / of the thickness from the steel material surface as determined in a direction vertical to the rolling direction is 340 MPa or more, and the temperature at which the percentage of ductile fracture as determined by a DWTT test becomes 85% or more is -10°C or lower.