Low alloy steel, seamless steel oil country tubular goods, and method for producing seamless steel pipe

Abstract

A low alloy steel comprising, by mass %, C: 0.10 to 0.20%, Si: 0.05 to 1.0%, Mn: 0.05 to 1.5%, Cr: 1.0 to 2.0%, Mo: 0.05 to 2.0%, Al: 0.10 % or less and Ti: 0.002 to 0.05%, and with a Ceq value obtained by the following formula (1) of 0.65 or more, with the balance being Fe and impurities, wherein in the impurities, P is 0.025% or less, S is 0.010% or less, N is 0.007% or less, and B is less than 0.0003%, and the number per unit area of M23C6 type precipitates (M: a metal element) whose grain diameter is 1 μm or more is 0.1 / mm2 or less. This invention provides a low alloy steel possessing both hardenability and toughness and improves the resistance to sulfide stress corrosion cracking.Ceq=C+(Mn / 6)+(Cr+Mo+V) / 5   formula (1)where C, Mn, Cr, Mo and V in the formula (1) denote the mass % of respective elements.

Description

[0001]The disclosure of Japan Patent Application No. 2007-092144 filed Mar. 30, 2007 including specification, drawings and claims is incorporated herein by reference in its entirety.TECHNICAL FIELD[0002]The present invention relates to a low alloy steel, and relates in particular to a low alloy steel suitable for use in a highly corrosive deep oil wells containing hydrogen sulfide at high pressure, a seamless steel oil country tubular goods, and a method for producing a seamless steel pipe.BACKGROUND ART[0003]Steel utilized in harsh, high temperature environments such as oil wells must possess better performance in terms of strength, toughness and sour resistance. In deeper wells, the steel must possess even higher strength and even better stress corrosion cracking resistance.[0004]In steel products, the hardness becomes higher as the material strength increases, which in turn raises the dislocation density so the hydrogen content in the steel product increases making it become brit...

AI Technical Summary

Benefits of technology

[0034]The low-alloy steel of the present invention improves resistance to sulfide stress corrosion cracking and provides hardenability and toughness. The low-alloy steel of the present invention is effective when used hydrogen sulfide environments at 2 atm or more, and especially under the 5 to 10 atm environment most vulnerable to sulfide stress corrosion cracking.

Problems solved by technology

In steel products, the hardness becomes higher as the material strength increases, which in turn raises the dislocation density so the hydrogen content in the steel product increases making it become brittle with stress.

Strengthening the steel product therefore usually causes poor resistance to sulfide stress corrosion cracking.

In particular, when a steel member is produced at a desired yield strength in a steel product whose “yield strength / tensile strength” ratio (hereinafter called “yield ratio”) is low, then the tensile strength and hardness tend to become higher, so the sulfide stress corrosion cracking resistance drastically deteriorates.