Low-yield-ratio high-strength steel sheet excellent in strain aging resistance, method for producing same, and high-strength welded steel pipe using the same

Abstract

The present invention provides a low-yield-ratio high-strength steel plate with excellent strain aging resistance below API 5L X70 grade, a manufacturing method thereof, and a high-strength welded steel pipe using the steel plate. The low-yield-ratio high-strength steel sheet of the present invention is characterized in that its component composition is a predetermined component composition, its metal structure is a surface layer part that includes bainite and island martensite, and is within 5 mm from the surface and the back surface. A structure that also includes polygonal ferrite, the area percentage of the island-shaped martensite is 3 to 15% and the equivalent circle diameter is 3.0 μm or less, and the area percentage of polygonal ferrite in the surface layer is 10%. More than and less than 80%, the balance is the above-mentioned bainite, the deviation of the hardness in the thickness direction is ΔHV30 or less, the hardness deviation in the width direction of the plate is ΔHV30 or less, and the maximum hardness of the surface layer of the steel plate is HV230 or less. When using GOST The yield ratio in the through-thickness tensile test of the test piece having the shape specified in the standard is 85% or less, and the elongation is 22% or more.

Description

technical field [0001] The present invention relates to a steel plate with low yield ratio, high strength and high toughness mainly suitable for use in the field of line pipe, a manufacturing method thereof and a high-strength welded steel pipe, in particular to a steel plate with low yield ratio and high strength which is excellent in strain aging resistance. A high-toughness steel plate, a method for producing the same, and a high-strength welded steel pipe using the steel plate as a raw material with excellent buckling resistance and elongation properties. Background technique [0002] In recent years, steel materials for welded structures have been required to have low yield ratio and high uniform elongation from the viewpoint of earthquake resistance in addition to high strength and high toughness. It is generally known that bainite (hereinafter, sometimes referred to as β), martensite (hereinafter, sometimes referred to as β), and martensite (hereinafter, sometimes ref...

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Problems solved by technology

Therefore, there is a problem that the yield stress of the steel pipe is larger than the yield ratio of the steel plate due to strain aging caused by the processing strain during the pipe making and the heating during the coating treatment, and the yield stress increases.

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