Hic-resistant thick steel plate and uoe steel pipe

Abstract

A thick steel plate and a UOE steel pipe having a high strength of at least X60 grade and improved resistance to HIC have a chemical composition consisting essentially of, in mass percent, C: 0.02-0.07%, Si: 0.05-0.50%, Mn: 1.1-1.6%, P: at most 0.015%, S: at most 0.002%, Nb: 0.005-0.060%, Ti: 0.005-0.030%, Al: 0.005-0.06%, Ca: 0.0005-0.0060%, N: 0.0015-0.007%, at least one of Cu, Ni, Cr, and Mo in a total amount of greater than 0.1% and less than 1.5%, and a remainder of Fe and impurities, wherein the degrees of segregation of Nb and Ti are both at most 2.0, and the ratio of (the degree of Nb segregation) / (the degree of Mn segregation) and the ratio of (the degree of Ti segregation) / (the degree of Mn segregation) are both at least 1.0 and at most 1.5.

Description

TECHNICAL FIELD[0001]This invention relates to a thick steel plate and a UOE steel pipe. More specifically, it relates to a high strength thick steel plate of at least X60 grade which is improved in resistance to hydrogen induced cracking (HIC resistance) and which is suitable for use in applications such as line pipe, offshore structures, and pressure vessels. It also relates to a UOE steel pipe made from this thick steel plate. In this description, a thick steel plate means a steel plate with a thickness of at least 6.0 mm.BACKGROUND ART[0002]Line pipe used for transporting crude oil and natural gas requires not only ordinary properties such as strength, toughness, and weldability, but it is also important for it to have resistance to hydrogen induced cracking (abbreviated below as HIC) because it is sometimes used in corrosive environments containing hydrogen sulfide.[0003]In the past, oil leaks, breakage, and explosions have actually taken place in connection with line pipes and...

AI Technical Summary

Benefits of technology

[0017]The object of the present invention is to provide a thick steel plate having excellent HIC resistance (referred to below as a HIC-resistant thick steel plate) and a UOE steel pipe which can prevent HIC, which could not be prevented by previously known countermeasures against HIC, in a thick steel plate having a strength level of at least X60 grade and particularly in a thick steel plate with a thickness of at least 10 mm in which marked segregation easily occurs.

[0023]As stated above, a thick steel plate means a steel plate having a plate thickness of at least 6.0 mm. A preferred plate thickness is at least 10 mm in which segregation occurs particularly easily. There is no particular upper limit on the plate thickness, but the present invention can achieve a HIC-resistant thick steel plate having a thickness up to 40 mm.

[0026]A HIC-resistant thick steel plate and UOE steel pipe according to the present invention can achieve a high strength of at least X60 grade (YP on the order of 70 ksi). In addition, elements such as Mn, C, P, and S which have been proposed in the past as causes of HIC are controlled, and HIC caused by carbonitrides of Nb or Ti can be prevented. Therefore, the occurrence of HIC in a high strength thick steel plate can be prevented with certainty.

[0027]A thick steel plate according to the present invention having improved HIC resistance can be used in structures such as offshore structures or pressure vessels, or it can be used for the manufacture of UOE steel pipes. A UOE steel pipe according to the present invention having improved HIC resistance is particularly suitable for line pipe, but it can also be used for offshore structures. Because the occurrence of HIC can be prevented with certainty, the reliability of products is markedly increased.

Problems solved by technology

In the past, oil leaks, breakage, and explosions have actually taken place in connection with line pipes and oil country tubular goods due to HIC.

Cracks develop due to the internal pressure of the gas.

(3) As the strength specifications which are recently being demanded of steel have become more stringent, individual countermeasures against the above-described center segregation_or formation of MnS are often inadequate.

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