STEEL SULFIDE-STRESS-CRACKING TEST METHOD AND SEAMLESS STEEL PIPE HAVING EXCELLENT SULFIDE-STRESS-CRACKING RESISTANCE

When the resistance to sulfide stress corrosion cracking is evaluated by applying a constant load stress à (MPa) to a round bar tensile test specimen immersed in a test solution and checking whether failure occurs before a particular time passes, the round bar tensile test specimen including a parallel section, a shoulder section, and a grip section is used. In the round bar tensile test specimen, the shoulder section is formed by a curve having two or more radii of curvature, a radius of curvature R1 (mm) of a portion adjacent to the parallel section is 15 mm or more and satisfies (0.22à - 119) ‰¤ R1 ‰¤ 100, a length X1 (mm) of a portion of the curve having the radius of curvature R1 in a longitudinal direction of the test specimen satisfies X1 ‰¥ ˆš{(r/8) × (R1 - r 2 /4)} (r: radius (mm) of the round bar tensile test specimen in the parallel section), and the radius of curvature R1 is larger than other radii of curvature. Thus, even when the steel is a high-strength steel having a yield strength of 758 MPa or higher, the occurrence of the failure in the shoulder section is suppressed, and the resistance to sulfide stress corrosion cracking can be properly evaluated.