HIGH STRENGTH LOW YIELD RATIO STEEL WITH HIGHLY UNIFORM ELONGATION CHARACTERISTIC, METHOD OF PRODUCING THE SAME AND HIGH STRENGTH LOW YIELD RATIO WELDED STEEL PIPE

摘要

PROBLEM TO BE SOLVED: To provide a steel which has a TS (Tensile Strength) of ≥550 MPa and has a low yield ratio and highly uniform elongation, that is, has an YR (Yield Ratio) of ≤80% YP and a product of the TS and the uniform elongation of ≥7,500 MPa×%, to provide a method of producing the steel and to provide a high strength low yield ratio welded steel pipe.;SOLUTION: The steel contains, by mass%, 0.03 to 0.14% C, 0.01-0.5% Si, 1.0-4.0% Mn, 0.003-0.08% Al, 0.01-0.08% Nb, 0.005-0.025% Ti and further, one or more elements among Cu, Ni, Mo, Cr, V, B, Ca, REM, Zr and Mg, and the steel has microscopic structure including: a host phase mainly comprising ferrite or bainite; and MA (Martensite-Austenite constituents) which disperses in the host phase and has an average aspect ratio of ≤2.0 and an area ratio of 5-20%, wherein 90% or more of the MA is present at a ferritic granular boundary or a prior austenitic granular boundary. The method of producing the high strength low yield ratio steel with highly uniform elongation characteristics comprises: heating the steel having the above composition to Ac₃ or higher, then hot-rolling the steel at a finish-rolling temperature of Ar₃ point or higher; and air-cooling or accelerated-cooling the hot-rolled steel at a cooling stop temperature of 500°C, then reheating the steel at Ac₁ -Ac₃. The welded steel pipe uses the above steel as the basic material part and is provided with weld metal having a specific composition.;COPYRIGHT: (C)2012,JPO&INPIT