| 摘要 |
<p>Provided is a low yield ratio, high strength and high toughness steel plate having excellent strain ageing resistance equivalent to API 5L X70 Grade or lower and a method for manufacturing the same. In particular, the steel plate has a composition containing 0.03% to 0.06% C, 0.01% to 1.0% Si, 1.2% to 3.0% Mn, 0.015% or less P, 0.005% or less S, 0.08% or less Al, 0.005% to 0.07% Nb, 0.005% to 0.025% Ti, 0.010% or less N, and 0.005% or less O on a mass basis, the remainder being Fe and unavoidable impurities. The steel plate has a metallographic microstructure that is a three-phase microstructure consisting of bainite, M-A constituent, and quasi€•polygonal ferrite, the area fraction of the bainite being 5% to 70%, the area fraction of the M€•A constituent being 3% to 20%, the remainder being the quasi€• polygonal ferrite, the equivalent circle diameter of the M-A constituent being 3.0 µm or less. The steel plate has a yield ratio of 85% or less and a Charpy impact test absorbed energy of 200 J or more at €•30°C before or after being subjected to strain ageing treatment at a temperature of 250°C or lower for 30 minutes or less.</p> |
