Seamless steel tubes and pipes for use in oil well

Abstract

Disclosed are seamless steel tubes for oil well use, comprising C: 0.14-0.35%, Si: 0.05-1.0%, Mn: 0.05-2.0%, Cr: 0.05-1.5%, Mo: 0.05-2.0%, Ti: 0-0.05%, V: 0-0.1%, and Al: not less than 0.010%, wherein the concentration product by Al and N content, corrected by Ti and V, is within the range of 0.00001 to 0.00050, and the residuals are Fe and impurities including P: 0.025% or less, and S: 0.010% or less. Ti, V, Nb, or B is preferably contained to enhance the quench hardenability as well as the resistance to sulfide stress corrosion cracking, and further Ca, Mg and / or REM is preferably contained to improve the shape of non-metallic inclusions, enhancing the resistance to sulfide stress corrosion cracking. Thus, said tubes by the invention can be produced by efficient means realizing energy savings, and widely used as ones having excellent stability in mechanical strength.

Description

BACKGROUND OF THE INVENTION [0001] 1. Technical Field [0002] The present invention relates to seamless steel tubes and pipes (hereinafter, simply referred to as tube(s)) for use in oil well, more particularly, to tubes having less variation of tensile strength, i.e., excellent stability in mechanical strength. [0003] 2. Description of the Related Art [0004] Seamless steel tubes having higher reliability compared to welded tubes, are frequently used in hostile oil well environment and / or high temperature environment, so that the tubes having higher mechanical strength in association with stable strength, higher toughness and higher resistance to sour gas environment degradation are required. [0005] For instance, in Japanese Patent Application Publication No. 2001-73086, seamless steel tubes containing V, Nb, Ti, Cr and Mo that are arranged to satisfy the relationship expressed by the preset equation to thereby suppress the formation of M23C6 type carbide, which are in general formed ...

AI Technical Summary

Benefits of technology

[0009] The present invention is started in view of the above circumstances, and the task thereof is to provide seamless steel tubes for use in oil well having excellent stability in mechanical strength, which can be produced by efficient means leading up to realization of energy saving.

[0013] (c) In order to secure the mechanical strength of steel tube upon enhancing the quench hardenability and to prevent the toughness from decreasing, it is effective to control each content of C, Si, Mn and Cr, while it is effective to control Mo content in order to enhance the quench hardenability and the resistance to sulfide stress corrosion cracking.

[0014] (d) When Ti is contained, N is immobilized as nitride to thereby leave B in the dissolved state, so that the quench hardenability can be enhanced. Also, when V is contained, fine carbide precipitates during tempering treatment to thereby enhance the mechanical strength. Nb contributes to enhance the resistance to sulfide stress corrosion cracking by forming carbonitride, and so does B by enhancing the quench hardenability which leading up to increasing martensite. Further, by containing one or more of Ca, Mg and REM, the shape of non-metallic inclusions can be improved to enhance the resistance to sulfide stress corrosion cracking.

[0021] In the present invention, “excellent stability in mechanical strength” is meant to that there is less variation of strength like tensile strength of steel, which will be recited at the later stage.

Problems solved by technology

However, this much measure could not realize the stable mechanical strength.

In this in-line heat treatment, there is an issue that the quench hardenability fluctuates according to the variation of grain size of prior austenite, thus making it difficult to obtain the stable mechanical strength.

Images