| 摘要 |
A steel plate having a low yield ratio and high strength of an API 5L X70 grade or less, a method for manufacturing the steel plate, and a high strength welded steel pipe made of the steel plate are provided. A high strength steel plate has a low yield ratio, the steel plate having a chemical composition containing, by mass%, C: 0.03% or more and 0.08% or less, Si: 0.01% or more and 1.0% or less, Mn: 1.2% or more and 3.0% or less, P: 0.015% or less, S: 0.005% or less, Al: 0.08% or less, Nb: 0.005% or more and 0.07% or less, Ti: 0.005% or more and 0.025% or less, N: 0.010% or less, O: 0.005% or less and the balance being Fe and inevitable impurities, a metallographic structure being a dual-phase structure consisting of a bainite phase and island martensite, wherein the area fraction of the island martensite is 3% or more and 15% or less, wherein the equivalent circle diameter of the island martensite is 3.0 µm or less, and wherein the remainder of the metallographic structure is a bainite phase, a hardness variation in the thickness direction of ”HV30 or less in terms of Vickers hardness, a hardness variation in the width direction of ”HV30 or less in terms of Vickers hardness, a maximum hardness in the surface portions of the steel plate of HV230 or less in terms of Vickers hardness and a yield ratio of 85% or less and an elongation of 22% or more in a full-thickness tensile test using a test piece having a shape in accordance with GOST standards. |
