STEEL SHEET FOR LOW YIELD RATIO THICK STEEL PIPE WITH 780 MPa OR ABOVE TENSILE STRENGTH, METHOD FOR PRODUCING THE SAME, AND LOW YIELD RATIO THICK STEEL PIPE WITH 780 MPa OR ABOVE TENSILE STRENGTH

摘要

PROBLEM TO BE SOLVED: To provide a steel plate for a circular steel pipe having a thick-wall thickness with a maximum plate thickness of 80 mm, achieving both a high strength of 780 MPa and a low yield ratio of 90% in strong bending work such as D/t=10-20, and further stably having satisfactory toughness even after worked in the steel pipe.SOLUTION: The steel plate has a prescribed chemical componential composition, has a hardenability index DI of 8 inch regulated by a prescribed relational formula, and satisfies the following requirements (A), (B), and (C); wherein (A) bainite has an area% of 90 in the microstructure at 1/4 portion in a plate thickness; (B) an average circle equivalent diameter in the regions surrounded by large-angle grain boundaries having an orientation difference of 15° is 4 m, in the microstructure at 1/4 portion in the plate thickness; and (C) island martensite having an average circle equivalent diameter of 0.5-3 m and having a Vickers hardness Hv of 700 is included by 3-10 area% in the microstructure at 1/4 portion in the plate thickness.