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

Abstract

A thick-walled steel pipe for linepipes 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 and in which the base metal and the weld heat-affected zone have excellent toughness. The welded steel pipe for linepipes, which has superior compressive strength and superior toughness, contains, in terms of mass%, 0.03-0.08% C, up to 0.10% Si, 1.00-2.00% Mn, up to 0.010% P, up to 0.0030% S, up to 0.06% Al, 0.005-0.020% Nb, 0.005-0.025% Ti, and 0.0010-0.0060% N, Ti (%) / N (%) being 2-4, the value of Ceq being 0.30 or more, and the remainder comprising Fe and incidental impurities. The steel pipe is characterized in that the metallographic structure of the base metal has specific values of the contents of bainite, island martensite (MA), and cementite, etc. and that the weld heat-affected zone has a specific metallographic structure. Also provided is a process for producing the welded steel pipe.

Description

technical field [0001] The present invention relates to a linepipe for transporting crude oil, natural gas, etc., and in particular to a thick-walled deep-sea linepipe that requires high collapse resistant performance. A line pipe steel pipe excellent in high compressive strength and high toughness among (line pipe for deep-sea) and a manufacturing method thereof. 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 [0002] With the increase in demand for energy in recent years, the devel...

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Problems solved by technology

[0007] However, in the method of Patent Document 2, separate management of the heating temperature and heating time of the outer surface (outer surface) and the inner surface (inner surface) of the steel pipe is carried out in actual production, especially in a mass production process (mass production process). Quality control is extremely difficult, but the method of Patent Document 3 has the following problem: Since the stop temperature of accelerated cooling in steel plate production needs to be set at a relatively low temperature of 300° C. or lower, the strain (distortion) of the steel plate becomes large, and the The roundness decreases when the steel pipe is formed in the UOE process, and in order to remove Ar 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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