Patent ID: 9017494
Filing Date: 2015-04-28
Classification: C21D,C22C

Abstract:
1. 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 consisting of, 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 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, 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 the Ar 3 transformation point to 1100 degrees centigrade, then following the final rolling step, the finally rolled steel pipe is assistantly heated in a temperature range from the Ar 3 transformation point to 1000 degrees centigrade in-line before the surface temperature becomes lower than the Ar 3 transformation point, and then the assistantly heated pipe is quenched from a temperature of the Ar 3 transformation point or higher and then tempered at a temperature lower than the Ac 1 transformation point, while keeping the pipe at a temperature of the Ar 3 transformation point or higher from tube-making to quenching: wherein, in the equation (1), C, Mn, Cr and Mo each represent % by mass of the respective elements.