STEEL PLATE AND WELDED STEEL TUBE EXHIBITING LOW YIELD RATIO, HIGH STRENGTH AND HIGH TOUGHNESS AND METHOD FOR PRODUCTION THEREOF

摘要

A steel plate and steel tube which comprises, in mass %, 0.03 to 0.1 % of C, 0.01 to 0.5 % of Si, 1.2 to 2.5 % of Mn, 0.08% or less of Al, and has a metal structure which substantially consists of three phases of ferrite, bainite and island martensite, wherein the area proportion of the island martensite is 3 to 20 % and a composite carbide is precipitated in the ferrite phase; and methods for producing the steel plate and steel tube. The steel plate and steel tube exhibit a low yield ratio, a high strength and a high toughness, and can be produced without the increase of a raw material cost by the addition of large amounts of alloying metals, with no deterioration of the toughness in a heat-affected zone by welding, with high efficiency, at a low cost.