Base metal and there is no 570 tensile strength superior to the toughness of the weld heat affected zone to a method of manufacturing the same and 800MPa class high strength steel plate

摘要

PROBLEM TO BE SOLVED: To provide a steel sheet excellent in toughness of a parent metal and a weld-heat affected zone and its manufacturing method.;SOLUTION: The steel contains, by mass, 0.005-0.2% C, 0.01-1% Si, 0.1-2% Mn, ≤0.02% P, ≤0.02% S, 0.0001-0.01% N, 0.0005-0.005% B, and 0.001-0.1% Al and the balance Fe with inevitable impurities. The number density of B-containing carbide, nitride and carbo-nitride whose dimensions of a circle equivalent diameter are ≥50nm and ≤10,000 nm is less than 5.0×10⁷ pieces/mm², a value obtained by dividing the number density by B % is less than 2.0×10¹⁰ pieces/(mm²×mass%), a sum of a volume fraction of bainite and martensite is ≥60%, and further an average dimension of a region from which the difference of a crystal orientation with an adjoining structure is ≥15 degrees in bainite is ≤20μm.;COPYRIGHT: (C)2004,JPO