LOW YIELD RATIO HIGH-STRENGTH STEEL PLATE HAVING SUPERIOR STRAIN AGING RESISTANCE, PRODUCTION METHOD THEREFOR, AND HIGH-STRENGTH WELDED STEEL PIPE USING SAME

摘要

The present invention provides a low yield ratio high-strength steel plate that is API 5L X70 grade 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 is a two-phase structure comprising bainite and island-shaped martensite; the island-shaped martensite has an area fraction of 3-15% and an equivalent circular diameter of 3.0 µm or less, and the remaining portion is bainite; the hardness variation in the plate thickness direction is DeltaHV30 or less; the hardness variation in the plate width direction is DeltaHV30 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.