Stainless-steel pipe for oil well and process for producing the same

Abstract

A steel composition contains: 0.05% or less of C; 0.5% or less of Si; 0.20% to 1.80% of Mn; 0.03% or less of P; 0.005% or less of S; 14.0% to 18.0% of Cr; 5.0% to 8.0% of Ni; 1.5% to 3.5% of Mo; 0.5% to 3.5% of Cu; 0.05% or less of Al; 0.20% or less of V; 0.01% to 0.15% of N; and 0.006% or less of O on a mass basis, and satisfies the following expressions: Cr+0.65Ni+0.6Mo+0.55Cu-20C>=18.5 and Cr+Mo+0.3Si-43.5C-0.4Mn-Ni-0.3Cu-9N<=11 (where Cr, Ni, Mo, Cu, C, Si, Mn, and N represent their respective contents (mass %)). After such a steel pipe material is formed into a steel pipe, the steel pipe is quenched by cooling after heating to a temperature of the AC3 transformation point or more and tempered at a temperature of the AC1 transformation point or less. The composition may further contain at least one element of Nb and Ti; at least one element selected from the group consisting of Zr, B, and W; or Ca, singly or in combination. Preferably, the steel pipe has a martensitic structure containing 5 to 25 percent by volume of a residual austenite phase, or further containing 5% percent by volume or less of a ferrite phase. Thus, the resulting stainless steel pipe for oil country tubular goods exhibits a superior corrosion resistance even in extremely severe, corrosive environments containing carbon dioxide gas (CO2), chloride ions (Cl<->), or the like.

Description

[0001] The present invention relates to steel pipes for oil country tubular goods used in crude oil wells and natural gas wells. In particular, the present invention relates to an improvement of corrosion resistance to extremely severe, corrosive environment in which carbon dioxide gas (CO.sub.2), chloride ions (Cl.sup.-), and the like are present.[0002] Deep oil wells, which have not conventionally been regarded at all, and corrosive sour gas wells, the development of which was abandoned for a time, have recently been developed increasingly on a world scale in order to cope with increase of crude oil price and anticipated oil resource depletion in the near future. These oil wells and gas wells generally lie at great depths in a severe, corrosive environment of a high-temperature atmosphere containing corrosive substances, such as CO.sub.2 and Cl.sup.-. Accordingly, steel pipes for oil country tubular goods used for digging such an oil or gas well have to be highly strong and corros...

AI Technical Summary

Benefits of technology

[0041] The element Si serves as a deoxidizer, and, preferably, its content is 0.05% or more in the present invention. However, a content of more than 0.50% reduces the CO.sub.2 corrosion resistance and further reduces the hot workability. Accordingly, the Si content is limited to 0.50% or less. Preferably, it is set in the range of 0.10% to 0.30%.

[0084] A quenching temperature of less than 800.degree. C. does not sufficiently achieve the effect of tempering to provide a desired strength. On the other hand, a quenching temperature of more than 1100.degree. C. coarsens the crystal grains to reduce the toughness of the steel. While a tempering temperature of less than 500.degree. C. does not pricipitate a sufficient amount of precipitations, a tempering temperature of more than 630.degree. C. remarkably reduces the strength of the steel.

Problems solved by technology

However, improved 13%-Cr martensitic stainless steel pipes manufactured by the techniques of Japanese Unexamined Patent Application Publication Nos. 8-120345, 9-268349, and 10-1755 and Japanese Patent Nos. 2814528 and 3251648 do not stably exhibit desired corrosion resistance in severe, corrosive environments at temperatures of more than 180.degree. C. containing CO.sub.2, Cl.sup.-, or the like.