STEEL PLATE FOR LOW YIELD RATIO THICK-WALLED CIRCULAR STEEL PIPE HAVING TENSILE STRENGTH OF 780 MPA OR MORE, METHOD FOR MANUFACTURING THE SAME, AND LOW YIELD RATIO THICK-WALLED CIRCULAR STEEL PIPE HAVING TENSILE STRENGTH OF 780 MPA OR MORE

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.