Steel sheet, steel pipe for line pipe, and production method therefor

Abstract

A steel sheet comprises 0.02-0.15 mass% of C, 0.02-0.50 mass% of Si, 0.6-2.0 mass% of Mn, more than 0 mass% but not more than 0.030 mass% of P, more than 0 mass% but not more than 0.003 mass% of S, 0.010-0.080 mass% of Al, 0.0003-0.0060 mass% of Ca, 0.001-0.01 mass% of N, and more than 0 mass% but not more than 0.0045 mass% of O, and satisfies formulas (1) and (2), the balance being iron and incidental impurities, wherein the area rate of a portion thereof having a defect echo height of 20% or more is 0.05% or less. (1): 3.0<=[Ca] / [S] and (2): ([Ca]-1.25*[S]) / [O]<=1.80, wherein [Ca], [S], and[O] represent the contained amounts (mass%) of Ca, S, and O, respectively.

Description

technical field [0001] The present invention relates to a steel pipe for steel plates and line pipes and a method of manufacturing the same. Background technique [0002] Mainly in line pipes for transportation of oil and gas, and storage containers, so-called sulfur resistance, such as hydrogen induced cracking resistance and stress corrosion cracking resistance, is required along with the development of low-quality resources containing hydrogen sulfide. It is known that hydrogen induced cracking (Hydrogen Induced Cracking, hereinafter referred to as "HIC") is the intrusion of hydrogen into the interior of the steel along with the corrosion reaction caused by the above-mentioned hydrogen sulfide, and the non-metallic inclusions such as MnS and Nb (C, N) Defects such as objects gather and gasify, resulting in cracks. When HIC occurs, there is a problem that the toughness of the structure decreases. In particular, since hydrogen penetrates from the surface portion of the st...

AI Technical Summary

Problems solved by technology

[0009] However, in Patent Document 1, the study was conducted to reduce the number of air cells in the slab, but the uncompressed air cells in the steel material of the final product were not considered.

Therefore, defects caused by uncompressed air bubbles remaining in the steel cannot be controlled, and HIC caused by uncompressed air bubbles cannot be suppressed.

[0010] In addition, in the method of Patent Document 2, although the study of reducing the content of Ar gas bubbles in the steel material was carried out, the size of the bubbles and the uncompressed bubbles of the steel material were not considered.

Therefore, even a small amount of coarse Ar bubbles cannot sufficiently suppress HIC.

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