Seamless steel pipe for oil wells excellent in sulfide stress cracking resistance and method for producing the same

Abstract

A high-strength seamless steel pipe for oil wells excellent in sulfide stress cracking resistance which comprises, on the percent by mass basis, C: 0.1 to 0.20%, Si: 0.05 to 1.0%, Mn: 0.05 to 1.0%, Cr: 0.05 to 1.5%, Mo: 0.05 to 1.0%, Al: 0.10% or less, Ti: 0.002 to 0.05% and B: 0.0003 to 0.005%, with a value of equation “C+(Mn/6)+(Cr/5)+(Mo/3)” of 0.43 or more, with the balance being Fe and impurities, and in the impurities P: 0.025% or less, S: 0.010% or less and N: 0.007% or less. The seamless steel pipe may contain a specified amount of one or more element(s) of V and Nb, and/or a specified amount of one or more element(s) of Ca, Mg and REM. The seamless steel pipe can be produced at a low cost by adapting an in-line tube making and heat treatment process having a high production efficiency since a reheating treatment for refinement of grains is not required.

1 . A seamless steel pipe for oil wells which comprises, on the percent by mass basis, C: 0.1 to 0.20%, Si: 0.05 to 1.0%, Mn: 0.05 to 1.0%, Cr: 0.05 to 1.5%, Mo: 0.05 to 1.0%, Al: 0.10% or less, Ti: 0.002 to 0.05% and B: 0.0003 to 0.005%, with a value of A determined by the following equation (1) of 0.43 or more, with the balance being Fe and impurities, and in the impurities P: 0.025% or less, S: 0.010% or less and N: 0.007% or less:  
         A=C+ ( Mn/ 6)+( Cr/ 5)+( Mo/ 3)  (1),  
     wherein, in the equation (1), C, Mn, Cr and Mo each represent % by mass of the respective elements.  
   
   
       2 . A seamless steel pipe for oil wells which comprises, on the percent by mass basis, C: 0.1 to 0.20%, Si: 0.05 to 1.0%, Mn: 0.05 to 1.0%, Cr: 0.05 to 1.5%, Mo: 0.05 to 1.0%, Al: 0.10% or less, Ti: 0.002 to 0.05%, B: 0.0003 to 0.005%, and either one or both of V: 0.03 to 0.2% and Nb: 0.002 to 0.04%, with a value of A determined by the following equation (1) of 0.43 or more, with the balance being Fe and impurities, and in the impurities P: 0.025% or less, S: 0.010% or less and N: 0.007% or less:  
         A=C+ ( Mn/ 6)+( Cr/ 5)+( Mo/ 3)  (1),  
     wherein, in the equation (1), C, Mn, Cr and Mo each represent % by mass of the respective elements.  
   
   
       3 . A seamless steel pipe for oil wells which comprises, on the percent by mass basis, C: 0.1 to 0.20%, Si: 0.05 to 1.0%, Mn: 0.05 to 1.0%, Cr: 0.05 to 1.5%, Mo: 0.05 to 1.0%, Al: 0.10% or less, Ti: 0.002 to 0.05%, B: 0.0003 to 0.005%, and one or more element(s) selected from a group of Ca of 0.0003 to 0.005%, Mg of 0.0003 to 0.005% and REM of 0.0003 to 0.005%, with a value of A determined by the following equation (1) of 0.43 or more, with the balance being Fe and impurities, and in the impurities P: 0.025% or less, S: 0.010% or less and N: 0.007% or less:  
         A=C+ ( Mn/ 6)+( Cr/ 5)+( Mo/ 3)  (1),  
     wherein, in the equation (1), C, Mn, Cr, and Mo each represent % by mass of the respective elements.  
   
   
       4 . A seamless steel pipe for oil wells which comprises, on the percent by mass basis, C: 0.1 to 0.20%, Si: 0.05 to 1.0%, Mn: 0.05 to 1.0%, Cr: 0.05 to 1.5%, Mo: 0.05 to 1.0%, Al: 0.10% or less, Ti: 0.002 to 0.05%, B: 0.0003 to 0.005%, either one or both of V: 0.03 to 0.2% and Nb: 0.002 to 0.04%, and one or more element(s) selected from a group of Ca of 0.0003 to 0.005%, Mg of 0.0003 to 0.005% and REM of 0.0003 to 0.005%, with a value of A determined by the following equation (1) of 0.43 or more, with the balance being Fe and impurities, and in the impurities P: 0.025% or less, S: 0.010% or less and N: 0.007% or less:  
         A=C+ ( Mn/ 6)+( Cr/ 5)+( Mo/ 3)  (1),  
     wherein, in the equation (1), C, Mn, Cr and Mo each represent % by mass of the respective elements.  
   
   
       5 . The seamless steel pipe for oil wells according to  claim 1 , wherein the tensile strength is not more than 931 MPa.  
   
   
       6 . The seamless steel pipe for oil wells according to  claim 2 , wherein the tensile strength is not more than 931 MPa.  
   
   
       7 . The seamless steel pipe for oil wells according to  claim 3 , wherein the tensile strength is not more than 931 MPa.  
   
   
       8 . The seamless steel pipe for oil wells according to  claim 4 , wherein the tensile strength is not more than 931 MPa.  
   
   
       9 . A method for producing a seamless steel pipe for oil wells, which comprises the steps of making a pipe by hot-piercing a steel billet having a chemical composition according to  claim 1 , with a value of A determined by the following equation (1) of 0.43 or more followed by elongating and rolling, and then finally rolling at a final rolling temperature adjusted to 800 to 1100 degrees centigrade, assistantly heating the resulting steel pipe in a temperature range from the Ar 3  transformation point to 1000 degrees centigrade in-line, and then quenching it from a temperature of the Ar 3  transformation point or higher followed by tempering at a temperature lower than the Ac 1  transformation point:  
         A=C+ ( Mn/ 6)+( Cr/ 5)+( Mo/ 3)  (1),  
     wherein, in the equation (1), C, Mn, Cr and Mo each represent % by mass of the respective elements.  
   
   
       10 . The method for producing a seamless steel pipe for oil wells according to  claim 9 , wherein the temperature of assistant heating in-line is the Ac 3  transformation point to 1000 degrees centigrade.  
   
   
       11 . A method for producing a seamless steel pipe for oil wells, which comprises the steps of making a pipe by hot-piercing a steel billet having a chemical composition according to  claim 2 , with a value of A determined by the following equation (1) of 0.43 or more followed by elongating and rolling, and then finally rolling at a final rolling temperature adjusted to 800 to 1100 degrees centigrade, assistantly heating the resulting steel pipe in a temperature range from the Ar 3  transformation point to 1000 degrees centigrade in-line, and then quenching it from a temperature of the Ar 3  transformation point or higher followed by tempering at a temperature lower than the Ac 1  transformation point:  
         A=C+ ( Mn/ 6)+( Cr/ 5)+( Mo/ 3)  (1),  
     wherein, in the equation (1), C, Mn, Cr and Mo each represent % by mass of the respective elements.  
   
   
       12 . A method for producing a seamless steel pipe for oil wells, which comprises the steps of making a pipe by hot-piercing a steel billet having a chemical composition according to  claim 3 , with a value of A determined by the following equation (1) of 0.43 or more followed by elongating and rolling, and then finally rolling at a final rolling temperature adjusted to 800 to 1100 degrees centigrade, assistantly heating the resulting steel pipe in a temperature range from the Ar 3  transformation point to 1000 degrees centigrade in-line, and then quenching it from a temperature of the Ar 3  transformation point or higher followed by tempering at a temperature lower than the Ac 1  transformation point:  
         A=C+ ( Mn/ 6)+( Cr/ 5)+( Mo/ 3)  (1),  
     wherein, in the equation (1), C, Mn, Cr and Mo each represent % by mass of the respective elements.  
   
   
       13 . A method for producing a seamless steel pipe for oil wells, which comprises the steps of making a pipe by hot-piercing a steel billet having a chemical composition according to  claim 4 , with a value of A determined by the following equation (1) of 0.43 or more followed by elongating and rolling, and then finally rolling at a final rolling temperature adjusted to 800 to 1100 degrees centigrade, assistantly heating the resulting steel pipe in a temperature range from the Ar 3  transformation point to 1000 degrees centigrade in-line, and then quenching it from a temperature of the Ar 3  transformation point or higher followed by tempering at a temperature lower than the Ac 1  transformation point:  
         A=C+ ( Mn/ 6)+( Cr/ 5)+( Mo/ 3)  (1),  
     wherein, in the equation (1), C, Mn, Cr and Mo each represent % by mass of the respective elements.