Steel plate having low yield ratio, high strength and high uniform elongation and method for producing same

Abstract

Disclosed are a steel plate, which shows an excellent aging treatment resistance of API 5L X70 grade or lower and has a low yield ratio, a high strength and a high uniform elongation, and a method for producing the same. Specifically disclosed is a steel plate showing an excellent strain aging resistance and having a low yield ratio, a high strength and a high uniform elongation, which comprises, in terms of mass%, 0.06-0.12% of C, 0.01-1.0% of Si, 1.2-3.0% of Mn, 0.015% or less of P, 0.005% or less of S, 0.08% or less of Al, 0.005-0.07% of Nb, 0.005-0.025% of Ti, 0.010% or less of N, 0.005% or less of O, and Fe and unavoidable impurities as the remainder, and the metal tissue of which consists of two phases of bainite and island martensite wherein the area proportion of the island martensite is 3-20% and the circle-equivalent diameter thereof is 3.0 [mu]m or less, characterized in that the uniform elongation of the steel plate before and after a strain aging treatment for 30 minutes or shorter at a temperature of 250oC or lower is 7% or more and the yield ratio thereof is 85% or less.

Description

technical field [0001] The present invention relates to a low yield ratio, high strength and high uniform elongation steel plate mainly suitable for use in the field of line pipes, and a manufacturing method thereof , especially to a steel plate having low yield ratio, high strength, and high uniform elongation excellent in strain aging resistance, and a method for producing the same. In addition, the uniform elongation mentioned here is also called uniform elongation, and means the critical value of permanent elongation at which the parallel part of a test piece deforms substantially uniformly in a tensile test. Usually, it is obtained as the permanent elongation corresponding to the maximum tensile load. Background technique [0002] In recent years, in steel materials for welded structures, in addition to high strength and high toughness, low yield ratio and high uniform elongation are required from the viewpoint of earthquake resistance (earthquake-proof). For example,...

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Problems solved by technology

[0008] On the other hand, welded steel pipes such as UOE steel pipes and electric welded steel pipes used for line pipes have the following problems. After forming the steel plate into a tubular shape in a cold environment, after welding the abutting surface, usually from corrosion prevention From the viewpoint of steel pipes, polyethylene coating or powder epoxy coating is applied to the outer surface of steel pipes. heating, strain aging occurs, yield stress rises, and the yield ratio in the steel pipe will be larger than that in the steel plate

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