High strength steel plate having low yield ratio excellent in terms of strain ageing resistance, method of manufacturing the same and high strength welded steel pipe made of the same

A high strength steel plate having 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 an area fraction of island martensite is 3% or more and 15% or less, wherein an 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 a thickness direction of ΔHV30 or less in terms of Vickers hardness, a hardness variation in a width direction of ΔHV30 or less in terms of Vickers hardness, a maximum hardness in 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.