| 摘要 |
PROBLEM TO BE SOLVED: To provide a high-strength seamless steel tube for oil wells with superior sulfide stress cracking resistance (SSC resistance).SOLUTION: The seamless steel tube has composition which contains, by mass, 0.15-0.50% of C, 0.1-1.0% of Si, 0.3-1.0% of Mn, 0.015% or less of P, 0.005% or less of S, 0.01-0.1% of Al, 0.01% or less of N, 0.1-1.7% of Cr, 0.40-1.1% of Mo, 0.01-0.08% of V, 0.01-0.08% of Nb, 0.03% or less of Ti, and 0.0005-0.003% of B, and has structure which includes a tempered martensite phase as the main phase and which contains prior-austenite grains having a grain size number of 8.5 or larger. The steel tube has such hardness distribution that the hardness is 295HV10 or less at each of an inner-side point located in the region at a distance of 2.54-3.81 mm from the inner surface of the tube, an outer-side point located at the same distance apart from the outer surface of the tube, and a point located at the center of the sheet thickness, the points being located at each of four positions apart from each other at intervals of 90° along in the circumferential direction. |
