| 摘要 |
A seamless steel tube for OCTG which possesses both of high strength of yield strength YS of 95 ksi grade (665 to 758 MPa) and excellent low-temperature toughness, and a manufacturing method of the seamless steel tube are provided. To be more specific, to a stainless steel seamless tube having the composition which contains, by mass%, 0.020% or less C, 10 to 14% Cr, 3% or less Ni, 0.05% or less N, 0.03 to 0.2% Nb, and optionally further, 1.0% or less Si, 0.1 to 2.0% Mn, 0.020% or less P, 0.010% or less S, 0.10% or less Al, and Fe and unavoidable impurities as a balance, quenching in which the seamless tube is heated at a quenching temperature of A c3 transformation temperature or above and, thereafter, the seamless tube is cooled to a temperature range of 100°C or less at a cooling rate of air cooling or more, and tempering which follows the quenching and in which the seamless tube is heated at a tempering temperature of 550°C or above and is cooled are applied. Due to such treatment, a martensitic stainless steel seamless tube for country tubular goods which has the tempered martensitic structure where a precipitated Nb quantity is 0.020% or more, and possesses both of high strength at yield strength of 95 ksi grade and the excellent low-temperature toughness of fracture transition temperature vTrs of -40°C or below, and allows the hot straightening. In addition to the above-mentioned composition, the seamless steel tube may further contain one or two kinds of elements selected from Cu, Mo and/or one or two kinds selected from V, Ti, B in addition to the above-mentioned composition. |
