Seamless steel pipe

Abstract

There is provided a seamless steel pipe that has high strength and excellent low-temperature toughness and has hydrogen embrittlement resistance properties. The seamless steel pipe has a chemical composition described in the specification, and the chemical composition satisfies [5C+Mo+Cr≥1.00], and satisfies [GN−1.96×(Mn+70P+100N)≥7.50] and [GN−1.37×(Mn+85P−30Ca)≥8.90] in conjunction with a prior-y grain size number GN. The seamless steel pipe has a tensile strength of 1200 MPa or more, and a critical hydrogen concentration of 2.5 ppm or more.

1 . A seamless steel pipe having a chemical composition consisting of, in mass %:
 C: 0.05 to 0.20%,   Si: 0.05 to 0.50%,   Mn: 0.40 to 1.50%,   P: 0.025% or less,   S: 0.020% or less,   Cu: 0.10 to 0.50%,   Ni: 0.10 to 0.50%,   Cr: 0.10 to 1.20%,   Mo: 0.10 to 0.50%,   Ti: 0.005 to 0.050%,   Nb: 0.005 to 0.100%,   Ca: 0.0005 to 0.0025%,   Al: 0.080% or less,   N: 0.0100% or less,   V: 0 to 0.100%,   B: 0 to 0.0050%,   Mg: 0 to 0.0050%, and   REM: 0 to 0.0050%,   with the balance: Fe and impurities, wherein   contents of elements satisfy Formula (i) shown below on a precondition that the contents fall within respective ranges described above,   the chemical composition further satisfies Formula (ii) and Formula (iii) shown below in conjunction with a prior-austenite grain size number,   a tensile strength of the seamless steel pipe is 1200 MPa or more, and   a critical hydrogen concentration of the seamless steel pipe is 2.5 ppm or more:   
       
         
           
             
               
                 
                   
                     
                       
                         5 
                         ⁢ 
                         C 
                       
                       + 
                       Mo 
                       + 
                       Cr 
                     
                     ≥ 
                     1. 
                   
                 
                 
                   
                     ( 
                     i 
                     ) 
                   
                 
               
             
           
         
         
           
             
               
                 
                   
                     
                       GN 
                       - 
                       
                         1.96 
                         × 
                         
                           ( 
                           
                             Mn 
                             + 
                             
                               70 
                               ⁢ 
                               P 
                             
                             + 
                             
                               100 
                               ⁢ 
                               N 
                             
                           
                           ) 
                         
                       
                     
                     ≥ 
                     
                       
                         7 
                         . 
                         5 
                       
                       ⁢ 
                       0 
                     
                   
                 
                 
                   
                     ( 
                     ii 
                     ) 
                   
                 
               
             
           
         
         
           
             
               
                 
                   
                     
                       GN 
                       - 
                       
                         1.37 
                         × 
                         
                           ( 
                           
                             Mn 
                             + 
                             
                               85 
                               ⁢ 
                               P 
                             
                             - 
                             
                               30 
                               ⁢ 
                               Ca 
                             
                           
                           ) 
                         
                       
                     
                     ≥ 
                     
                       
                         8 
                         . 
                         9 
                       
                       ⁢ 
                       0 
                     
                   
                 
                 
                   
                     ( 
                     iii 
                     ) 
                   
                 
               
             
           
         
         where symbols of the elements in the formula mean contents (mass %) of the elements in the steel, and when an element is not contained, zero will be set to the corresponding symbol, and where GN means the prior-austenite grain size number. 
       
     
     
         2 . The seamless steel pipe according to  claim 1 , wherein the chemical composition contains one or more elements selected from, in mass %:
 V: 0.001 to 0.100%,   B: 0.0001 to 0.0050%,   Mg: 0.0001 to 0.0100%, and   REM: 0.0001 to 0.0100%.