High strength martensitic stainless steel excellent in carbon dioxide gas corrosion resistance and sulfide stress-corrosion cracking resistance

Abstract

The present invention provides a martensitic stainless steel in which specified elements in a steel composition are limited. The martensitic stainless steel can have high strength of 0.2% proof stress of 860 MPa or more and excellent carbon dioxide gas corrosion resistance and sulfide stress-corrosion cracking resistance by limiting the steel composition of specified elements and defining Mo content in the steel by relationships with IM values as well as by forming microstructure of the steel with main tempered martensite, carbide precipitated during tempering, and intermetallic compounds such as a Laves phase, a σ phase and the like. As a result the martensitic stainless steels of the present invention can be applied to practical steels, which can be widely used in oil well tubes and the like under environment including carbon dioxide gas, hydrogen sulfide, chlorine ions or two or more of them, in wide fields.

Description

BACKGROUND OF THE INVENTION [0001] 1. Field of the Invention [0002] The present invention relates to a steel material suitable for its use in severe corrosion environment containing corrosive materials such as carbon dioxide gas, hydrogen sulfide, chlorine ions and the like. Specifically, the present invention relates to a steel material for a seamless steel tube and a seam welded steel tube such as an electric resistance welding steel tube, a laser welding steel tube, a spiral welding tube or the like, which is used in applications for petroleum or natural gas production facilities, facilities for eliminating carbon dioxide gas, or for geo-thermal power generation, or for a tank for liquid containing carbon dioxide gas, especially to a steel material for oil well tubes for oil wells or gas wells. [0003] 2. Description of the Related Art [0004] From the viewpoint of exhaustion of petroleum resources, which is expected in the near future, development of an oil well under severe envir...

AI Technical Summary

Benefits of technology

[0018] (b) A high strength martensitic stainless steel excellent in carbon dioxide gas corrosion resistance and sulfide stress-corrosion cracking resistance and having 0.2% proof stress of 860 MPa or more, which comprises the compositions defined in any one of (a); being subjected to tempering in which (20+log t)(T+273) satisfies 13500-17700 when, after quenching the steel at a quenching temperature of 880° C.-1000° C., a range of a tempering temperature is set to 450° C.-620° C., a tempering temperature is set to T (° C.) and tempering time is set to t (hour); and satisfying the above mentioned expression (1) wherein the microstructure of said steel mainly comprises tempered martensite, carbide precipitated during tempering, and intermetallic compounds such as a Laves phase, a σ phase and the like finely precipitated during tempering.

Problems solved by technology

However, as the environment of the well becomes severe, steel which contains increased amount of alloying elements, has been used.

However, although the SUS 420 steel has excellent corrosion resistance to carbon dioxide gas, it has poor corrosion resistance to hydrogen sulfide.

Thus, the SUS 420 steel is liable to generate sulfide stress-corrosion cracking (SSCC) under the environment containing carbon dioxide gas and hydrogen sulfide simultaneously.

However, such a low carbon-content steel described in these publications may not have the enough strength required for use in a deep well, that is proof stress of 860 MPa or more.

However, since Co is an expensive element, it is desirable not to use.

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