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 M 23 C 6 type precipitates (M: a metal element) whose grain diameter is 1 μm or more is 0.1/mm 2 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.

1 . 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 M 23 C 6  type precipitates (M: a metal element) whose grain diameter is 1 μm or more is 0.1/mm 2  or less.
   Ceq=C+(Mn/6)+(Cr+Mo+V)/5   formula (1)   where C, Mn, Cr, Mo and V in the formula (1) denote the content of respective elements (mass %).   
   
   
       2 . The low alloy steel according to  claim 1 , comprising either one or both of 0.03 to 0.2% V and 0.002 to 0.04% Nb. 
   
   
       3 . The low alloy steel according to  claim 1 , comprising at least one element selected from 0.0003 to 0.005% Ca, 0.0003 to 0.005% Mg and 0.0003 to 0.005% REM. 
   
   
       4 . A seamless steel oil country tubular goods using the low alloy steel according to  claim 1 . 
   
   
       5 . A method for producing a seamless steel pipe comprising the following steps:
 (a) hot piercing a steel billet possessing the chemical composition claimed  claim 1  and a Ceq value obtained by the following formula (1) of 0.65 or more;   (b) elongation rolling; producing a pipe at a final rolling temperature of 800 to 1100° C.;   (c) supplementary heating the resultant steel pipe in-line in a temperature range from the Ar 3  transition point to 1000° C.;   (d) quenching the pipe from a temperature of the Ar 3  transition point or higher; and then   (e) tempering the pipe at a temperature of the Ac 1  transition point or lower.
   Ceq=C+(Mn/6)+(Cr+Mo+V)/5   formula (1) 
   where C, Mn, Cr, Mo and V in the formula (1) denote the content of the respective elements (mass %).   
   
   
       6 . The low alloy steel according to  claim 2 , comprising at least one element selected from 0.0003 to 0.005% Ca, 0.0003 to 0.005% Mg and 0.0003 to 0.005% REM. 
   
   
       7 . A seamless steel oil country tubular goods using the low alloy steel according to  claim 2 . 
   
   
       8 . A seamless steel oil country tubular goods using the low alloy steel according to  claim 3 . 
   
   
       9 . A method for producing a seamless steel pipe comprising the following steps:
 (a) hot piercing a steel billet possessing the chemical composition claimed in  claim 2  and a Ceq value obtained by the following formula (1) of 0.65 or more;   (b) elongation rolling; producing a pipe at a final rolling temperature of 800 to 1100° C.;   (c) supplementary heating the resultant steel pipe in-line in a temperature range from the Ar 3  transition point to 1000° C.;   (d) quenching the pipe from a temperature of the Ar 3  transition point or higher; and then   (e) tempering the pipe at a temperature of the Ac 1  transition point or lower.
   Ceq=C+(Mn/6)+(Cr+Mo+V)/5   formula (1) 
   where C, Mn, Cr, Mo and V in the formula (1) denote the content of the respective elements (mass %).   
   
   
       10 . A method for producing a seamless steel pipe comprising the following steps:
 (a) hot piercing a steel billet possessing the chemical composition claimed in  claim 3  and a Ceq value obtained by the following formula (1) of 0.65 or more;   (b) elongation rolling; producing a pipe at a final rolling temperature of 800 to 1100° C.;   (c) supplementary heating the resultant steel pipe in-line in a temperature range from the Ar 3  transition point to 1000° C.;   (d) quenching the pipe from a temperature of the Ar 3  transition point or higher; and then   (e) tempering the pipe at a temperature of the Ac 1  transition point or lower.
   Ceq=C+(Mn/6)+(Cr+Mo+V)/5   formula (1) 
   where C, Mn, Cr, Mo and V in the formula (1) denote the content of the respective elements (mass %).