Welded steel pipe for linepipe with superior compressive strength, and process for producing same

Abstract

A thick-walled steel pipe for linepipes that has high compressive strength is provided in which the decrease in yield stress caused by the Bauschinger effect is inhibited by optimizing the metallographic structure of a steel plate without requiring special forming conditions for steel pipe formation or requiring any heat treatment after pipe production. The steel pipe for linepipes has superior compressive strength and excellent sour resistance and contains, in terms of mass%, 0.03-0.10% C, up to 0.30% Si, 1.00-2.00% Mn, up to 0.015% P, up to 0.003% S, up to 0.080% Al, 0.005-0.035% Nb, and 0.005-0.020% Ti, C (%)-0.065Nb (%) being 0.025 or more, the value of Ceq being 0.3 or more, and the remainder comprising Fe and incidental impurities. The steel pipe is characterized in that the metallographic structure has a bainite content of 60% or more, a processed-ferrite content of 5% or less, an island martensite (MA) content of 3% or less, an average MA grain diameter of 2 [Mu]m or smaller, and an MA aspect ratio of 5 or less. Also provided is a process for producing the steel pipe.

Description

technical field [0001] The present invention relates to a line pipe for transporting crude oil, natural gas, etc., and in particular to a thick-walled deep-sea line pipe that requires high collapse resistant performance. For deep-sea), a welded steel pipe for pipes excellent in high compressive strength or further a welded steel pipe for pipes excellent in high compressive strength and acid resistance, and methods for producing them. It should be noted that unless otherwise specified, the compressive strength in the present invention refers to compressive yield strength (compressive yield strength) or 0.5% compressive strength (compressive proof strength). In addition, unless otherwise specified, the tensile yield strength (tensile yield strength) refers to the tensile yield strength (tensile yield strength) or 0.5% tensile strength, and the tensile strength (tensile strength) refers to the tensile test as the general definition the maximum stress. Background technique [...

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Problems solved by technology

[0007] However, in the method of Patent Document 2, the heating temperature and heating time of the outer surface (outer surface) and the inner surface (inner surface) of the steel pipe are separately managed, and quality control is carried out in actual manufacturing, especially in mass production process (mass production process). Management is extremely difficult, but the method of Patent Document 3 has the following problem: Since the stop temperature of accelerated cooling in steel sheet production needs to be set at a relatively low temperature of 300° C. or lower, the strain (distortion) of the steel sheet becomes large. When the steel pipe is formed in the process, the roundness decreases, and in order to 3 Accelerated cooling starts above the temperature, and rolling at a higher temperature is required, and the fracture toughness deteriorates

[0012] However, in the technique described in Patent Document 6, it is necessary to heat up to the center of the steel plate during heating, which will lead to a decrease in DWTT performance, so it is difficult to apply it to thick-walled linepipes for deep sea

[0013] In addition, the Bauschinger effect is affected by various microstructure factors such as crystal grain size and amount of solid solution carbon. Therefore, as in the technology described in Patent Document 7, only by reducing However, steel pipes with high compressive strength cannot be obtained due to the second phase of solid material. In addition, under the disclosed reheating conditions, the solid solution C It is difficult to obtain the balance of excellent tensile strength, compressive strength and DWTT performance

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