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 steel plate with a resistance less than API 5L X70 grade, a manufacturing method thereof, and a high strength welded steel pipe using the steel sheet. The steel sheet according to the present invention has a predetermined composition and the metallic structure is a two-phase structure of bainite and stalactite martensite. The area fraction of the stalactite martensite is 3 to 15% The equivalent diameter is 3.0 mu m or less, the balance is bainite, the deviation of the hardness in the plate thickness direction is DELTA HV 30 or less, the deviation of the hardness in the plate width direction is DELTA HV 30 or less and the maximum hardness of the surface plate portion of the steel plate is HV 230 or less , A yield ratio in a full thickness tensile test using a test specimen of a shape specified by the GOST standard is 85% or less and a stretch ratio is 22% or more.