Low alloy high strength seamless steel pipe for oil country tubular goods

Abstract

A low alloy high strength seamless steel pipe for oil country tubular goods is provided including a composition containing, in terms of mass %, C: 0.25 to 0.31%, Si: 0.01 to 0.35%, Mn: 0.45 to 0.70%, P: 0.010% or less, S: 0.001% or less, 0: 0.0015% or less, Al: 0.015 to 0.080%, Cu: 0.02 to 0.09%, Cr: 0.8 to 1.5%, Mo: 1.1 to 1.6%, V: 0.01 to 0.06%, Nb: 0.005 to 0.015%, B: 0.0015 to 0.0030%, Ti: 0.005 to 0.020%, and N: 0.005% or less, and having a ratio of the Ti content to the N content of 3.0 to 4.0, with the balance being Fe and inevitable impurities, the steel pipe having a ratio of a stress at a strain of 0.7% to a stress at a strain of 0.4% in a stress-strain curve of 1.02 or less and a yield strength of 861 MPa or more.

1 . A low alloy high strength seamless steel pipe for oil country tubular goods comprising a composition containing, in terms of mass %,
 C: 0.25 to 0.31%,   Si: 0.01 to 0.35%,   Mn: 0.45 to 0.70%,   P: 0.010% or less,   S: 0.001% or less,   O: 0.0015% or less,   Al: 0.015 to 0.080%,   Cu: 0.02 to 0.09%,   Cr: 0.8 to 1.5%,   Mo: 1.1 to 1.6%,   V: 0.01 to 0.06%,   Nb: 0.005 to 0.015%,   B: 0.0015 to 0.0030%,   Ti: 0.005 to 0.020%, and   N: 0.005% or less,   and having a value of a ratio of the Ti content to the N content (Ti/N) of 3.0 to 4.0,   with the balance being Fe and inevitable impurities,   the steel pipe having a value (σ 0.7 /σ 0.4 ), as a ratio of a stress at a strain of 0.7% to a stress at a strain of 0.4% in a stress-strain curve, of 1.02 or less and a yield strength of 861 MPa or more.   
     
     
         2 . The low alloy high strength seamless steel pipe for oil country tubular goods according to  claim 1 , which further contains, in addition to the composition, one or more selected from, in terms of mass %,
 W: 0.1 to 0.2%, and   Zr: 0.005 to 0.03%.   
     
     
         3 . The low alloy high strength seamless steel pipe for oil country tubular goods according to  claim 1 , which further contains, in addition to the composition, in terms of mass %,
 Ca: 0.0005 to 0.0030%,   and has the number of oxide-based non-metallic inclusions in steel comprising of Ca and Al and having a major diameter of 5 μm or more, whose composition ratio satisfies, in terms of mass %, the following equation (1), of 20 or less per 100 mm 2 :
   (CaO)/(Al 2 O 3 )≥4.0   (1)
 
   
     
     
         4 . The low alloy high strength seamless steel pipe for oil country tubular goods according to  claim 2 , which further contains, in addition to the composition, in terms of mass %,
 Ca: 0.0005 to 0.0030%,   and has the number of oxide-based non-metallic inclusions in steel comprising of Ca and Al and having a major diameter of 5 μm or more, whose composition ratio satisfies, in terms of mass %, the following equation (1), of 20 or less per 100 mm 2 :
   (CaO)/(Al 2 O 3 )≥4.0   (1)