Steel plate for high-strength line pipe with excellent low temperature toughness and steel pipe for high strength line pipe

摘要

A steel sheet for a high strength line pipe excellent in both low temperature toughness, particularly CTOD characteristics and DWTT characteristics, is provided. When a high strength line pipe steel sheet of the present invention, with the satisfaction of a predetermined component, t the plate thickness, the position of t / 2, and an oxide or more of circle equivalent diameter of 2μm 10 gae / mm ² or less, the average equivalent diameter of the crystal grains surrounded by the diagonal grain boundary having the azimuthal difference of the adjacent two crystals at the position of t / 2 is 15 占 or more is 10 占 퐉 or less, the fraction of the hard tissue at the position of t / 2 is 5% And the separation index SI measured from the Charpy test piece wave front at the specified temperature satisfies 0.15 mm / mm ² or less.