Low alloy steel

Abstract

A low alloy steel subjected to post weld heat treatment, containing, by mass percent, of C: 0.01 to 0.15%, Si: 3% or less, Mn: 3% or less, and Al: 0.08% or less, one or more kinds of elements selected from Ti, V and Nb: the range satisfying Formula (1), and the balance being Fe and impurities, wherein in the impurities, N: 0.01% or less, P: 0.05% or less, S: 0.03% or less, and O: 0.03% or less:0.1×[C(%)]≦[Ti(%)]+[V(%)]+0.5×[Nb(%)]≦0.2  (1)where, the symbol of element in the formula represents the content (mass %) of each element.In the alloy steel, a HAZ subjected to PWHT, especially short-time PWHT, has excellent hydrogen embrittlement resistance in wet hydrogen sulfide environments or the like.

Description

TECHNICAL FIELD[0001]The present invention relates to a low alloy steel. More particularly, it relates to a low alloy steel in which a weld heat affected zone that has been subjected to postweld heat treatment has excellent resistance to embrittlement attributable to hydrogen, such as stress corrosion cracking in wet hydrogen sulfide environments.BACKGROUND ART[0002]In the development of submarine oilfields, a steel pipe called a riser, flowline, or trunkline is used for transmission of crude oil or natural gas between an oil well or gas well located at the bottom of the sea and a platform on the sea or between the platform and a refinery station on the land. On the other hand, with the worldwide exhaustion of fossil fuels, oil fields containing much hydrogen sulfide having corrosiveness have been developed actively. A steel pipe for transmitting crude oil or natural gas exploited from oil fields containing such a corrosive gas is sometimes broken by embrittlement attributable to hy...

AI Technical Summary

Benefits of technology

[0030]According to the present invention, there can be provided a low alloy steel in which a HAZ subjected to PWHT, especially short-time PWHT, has excellent hydrogen embrittlement resistance in wet hydrogen sulfide environments or the like.

Problems solved by technology

Unfortunately, for example, as described in non-Patent Document 1, it is widely known that the SSC susceptibility is increased by the increase in hardness.

As a result, however much the HIC resistance and SSC resistance of the steel itself are enhanced, practically sufficient performance of a welded structure cannot be achieved in many cases.