High strength seamless steel pipe excellent in hydrogen-induced cracking resistance and its production method

Abstract

The present invention relates to a high strength seamless steel pipe excellent in hydrogen-induced cracking resistance, characterized by consisting of, by mass %, C: 0.03-0.11%, Si: 0.05-0.5%, Mn: 0.8-1.6%, P: 0.025% or less, S: 0.003% or less, Ti: 0.002-0.017%, Al: 0.001-0.10%, Cr: 0.05-0.5%, Mo: 0.02-0.3%, V: 0.02-0.20%, Ca: 0.0005-0.005%, N: 0.008% or less and O (Oxygen): 0.004% or less, and the balance Fe and impurities, and also characterized in that the microstructure of the steel is bainite and/or martensite, ferrite is precipitated at grain boundaries and yield stress is 483 MPa or more. Further, to ensure high strength of the steel, the seamless steel pipe preferably contains, by mass %, at least one of Cu: 0.05-0.5% and Ni: 0.05-0.5%. To produce the above-mentioned steel pipe, it is desirable to limit a starting temperature of quenching after rolling, a cooling rate and a tempering temperature. By this configuration a seamless steel pipe having an yield stress of 483 MPa or more and excellent HIC resistance, which is suitable for a pipeline, can be provided.

1 - 2 . (canceled) 
   
   
       3 . A production method of a high strength seamless steel pipe excellent in hydrogen-induced cracking resistance, characterized in that after rolling a billet having a composition according to claim  1  to a seamless steel pipe by hot rolling, said seamless steel pipe is immediately soaked and then cooled at a starting temperature of quenching of (Ar 3  point+50° C.) to 1100° C. and at a cooling rate of 5° C./sec or more, and then said seamless steel pipe is tempered at 550° C. to Ac 1  points, whereby a seamless steel pipe in which the microstructure of steel is bainite and/or martensite, ferrite is precipitated at grain boundaries and yield stress is 483 MPa or more is produced. 
   
   
       4 . A production method of a high strength seamless steel pipe excellent in hydrogen-induced cracking resistance, characterized in that after rolling a billet having a composition according to claim  2  to a seamless steel pipe by hot rolling, said seamless steel pipe is immediately soaked and then cooled at a starting temperature of quenching of (Ar 3  point+50° C.) to 1100° C. and at a cooling rate of 5 □/sec or more, and then said seamless steel pipe is tempered at 550° C. to Ac 1  points, whereby a seamless steel pipe in which the microstructure of steel is Bainite and/or martensite, ferrite is precipitated at grain boundaries and yield stress is 483 MPa or more is produced.