HIGH STRENGTH STEEL PLATE HAVING LOW YIELD RATIO EXCELLENT IN TERMS OF STRAIN AGEING RESISTANCE, METHOD FOR MANUFACTURING THE SAME AND HIGH STRENGTH WELDED STEEL PIPE MADE OF THE SAME

摘要

A high strength steel plate having a low yield ratio,and a strain ageing resistance of an API 5L X70 grade orless is provided. The microstructure was a bainite phaseand island martensite, and polygonal ferrite in surfaceportions within 5 mm from the upper and lower surfaces.Vickers hardness variation in the thickness direction is= .DELTA.HV30. Vickers hardness variation in the widthdirection is = .DELTA.HV30. Maximum Vickers hardness in thesurface portions of the steel plate is = HV230. Yieldratio is 85% or less and elongation is 22% or more in afull-thickness tensile test using a test piece having ashape in accordance with GOST standards.