STEEL SHEET FOR HIGH STRENGTH LINE PIPE WITH EXCELLENT LOW TEMPERATURE TOUGHNESS AND STEEL PIPE FOR HIGH STRENGTH LINE PIPE

摘要

Provided is a steel sheet for high strength line pipe with excellent low temperature toughness, specifically both CTOD characteristic and DWTT characteristic. In addition to satisfying a prescribed composition, this steel sheet for high strength line pipe: satisfies, when the sheet thickness is t, the conditions of the sheet containing not more than 10/mm2 of oxides with an equivalent circle diameter of at least 2 µm at the t/2 position, the mean equivalent circle diameter of crystal grains at the t/2 position surrounded by a large angle grain boundary for which the orientation difference of two adjacent crystals is at least 15° being not more than 10 µm, and the fraction of hard structure at the t/2 position being not more than 5 area%; and satisfies the condition of the separation index SI measured at a designated temperature from a Charpy test piece fracture surface being not more than 0.15 mm/mm2.