Steel plate and line pipe for line pipe

Abstract

A steel plate for a line pipe, the steel plate having a plate thickness of 25 mm or more comprising in terms of mass%: 0.040 to 0.080% of C, 0.05 to 0.40% of Si, 1.60 to 2.00% of Mn, 0.020% or less of P, 0.0025% or less of S, 0.05 to 0.20% of Mo, 0.0011 to 0.0050% of Ca, 0.060% or less of Al, 0.010 to 0.030% of Nb, 0.008 to 0.020% of Ti, 0.0015 to 0.0060% of N, and 0.0040% or less of O, a remainder consisting of Fe and unavoidable impurities, wherein a content ratio of Ca to S [Ca / S] is from 0.90 to 2.70, and a content ratio of Ti to N [Ti / N] is 2.20 or higher" wherein Ceq, which is defined by the following Formula (1), is from 0.380 to 0.480: Ceq = C+Mn / 6+(Ni+Cu) / 15+(Cr+Mo+V) / 5; and wherein: at a position of 1 / 4 of the plate thickness, a ferrite fraction (F1) is from 20 to 60% and a remainder is a structure of bainite; at a position of 1 / 2 of the plate thickness, a ferrite fraction (F2) is from 5 to 60% and a remainder is a structure of bainite or a structure of bainite and martensite; a ratio (F1 / F2) of the ferrite fraction (F1) to the ferrite fraction (F2) is from 1.00 to 5.00; at a position of 1 / 4 of the plate thickness an average grain diameter of ferrite is from 2.0 to 15.0 µm, and at a position of 1 / 2 of the plate thickness the average grain diameter of ferrite is from 5.0 to 20.0 µm; and the hardness at a position of 1 / 2 of the plate thickness is 400 Hv or less, and the length of MnS at a position of 1 / 2 of the plate thickness is 1.00 mm or less.

Description

technical field [0001] The present invention relates to steel plates for line pipes and line pipes. Background technique [0002] At present, the production areas of crude oil and natural gas are expanding to the polar regions, and the environment for laying pipelines for transporting crude oil or natural gas is becoming more and more severe. For example, there has been an increase in the transport of crude oil or natural gas containing hydrogen sulfide, depending on the linepipe. Therefore, the line pipe and the steel plate for line pipe which are the material of the line pipe may be required to have acid resistance. In addition, acid resistance refers to hydrogen-induced cracking resistance (HIC resistance) (Hydrogen-Induced Cracking Resistance) and sulfide stress cracking resistance (SSC resistance) (Sulfide Stress Cracking Resistance) in a corrosive environment containing hydrogen sulfide. [0003] However, it is known that the acid resistance of steel is deteriorated ...

AI Technical Summary

Problems solved by technology

[0009] In addition, when a thick steel plate (for example, a steel plate with a thickness of 25 mm or more) is produced, the reduction of the recrystallized region and the non-recrystallized region becomes insufficient. In the toughness evaluation by Weight Tear Test; DWTT), it may be difficult to ensure the characteristics (hereinafter also referred to as "DWTT characteristics")

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