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 low yield ratio high-strength steel plate that has superior strain aging resistance at a grade of API 5L X70 or lower, a production method for said steel plate, and a high-strength welded steel pipe that employs the steel plate. The low yield ratio high-strength steel plate of the present invention is characterized in that: said steel plate has a predetermined component composition; the metal structure of the steel plate comprises bainite and island-shaped martensite, and polygonal ferrite in a surface layer that is within 5 mm of the front and back surfaces; the island-shaped martensite has an area fraction of 3-15% and an equivalent circular diameter of 3.0 µm or less, the polygonal ferrite in the surface layer has an area fraction of 10% or more but less than 80%, and the remaining portion is bainite; the hardness variation in the plate thickness direction is ?HV30 or less; the hardness variation in the plate width direction is ?HV30 or less; the maximum hardness of the surface layer of the steel plate is HV230 or less; and in that the yield ratio is 85% or less and elongation is 22% or more according to a full thickness tensile test using a test fragment having a shape defined by the GOST standard.