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

The present invention provides a high-strength, high-strength, high-strength steel plate excellent in resistance to deformation aging properties of API 5L X70 grade or less, a method for producing the steel plate, and a high strength welded steel pipe using the steel plate. The steel sheet having a low resistance and a high strength according to the present invention has a predetermined composition and the metallic structure is composed of bainite and stalactite martensite and further contains polygonal ferrite in the surface layer within 5 mm from the front and back surfaces. Wherein the area fraction of the stalactite martensite is 3 to 15% and the circle-equivalent diameter is 3.0 m or less, the area fraction of the polygonal ferrite in the surface layer portion is 10% or more and less than 80%, the remainder is bainite, In a full thickness tensile test using a test piece having a shape determined by the GOST standard, the deviation of the hardness in the plate thickness direction is not more than 30HV, the deviation of the hardness in the plate width direction is not more than 30HV and the maximum hardness of the surface layer of the steel plate is not more than 230HV Is 85% or less and the elongation is 22% or more.