| 摘要 |
A heavy wall and high strength seamless steel pipe having high sour resistance is provided. In particular, a quenching and tempering treatment is conducted to adjust the yield strength to be higher than 450 MPa and adjust the wickers hardness HV5 that can be measured at an outermost side or an innermost side of the pipe under a 5 kgf load (test load: 49 N) to be 250 HV5 or less. In order to achieve this, surface layers are ground from surfaces by a depth of 0.3 mm or more in a wall thickness direction after the quenching treatment, or the quenching treatment is designed to include holding a heating temperature equal to or higher than the Ac3 transformation point for 120 seconds or more in an air atmosphere and then performing water-cooling in a nucleate boiling region or performing water-cooling in a film boiling region and then in a nucleate boiling region. As a result of this quenching treatment, the hardness of the surface layers decreases to 250 HV5 or less and a steel pipe that has an M-shape hardness distribution in which the maximum hardness appears at some point in the profile other than the center of the wall or a U-shape hardness distribution in which the hardness of the surface layers is the highest but not more than 250 HV5 or a flat shape hardness distribution can be obtained. As a result, the sour resistance is significantly improved. |
