| 摘要 |
<p><P>PROBLEM TO BE SOLVED: To provide a high strength steel pipe having excellent weldability and tube-expandability for oil-well use. <P>SOLUTION: The steel pipe having a composition consisting of C: 0.04% or less, Si: 0.5-2.0%, Mn: 2.0-4.0%, P: 0.07% or less, S: 0.01% or less, and Al: 0.05% or less is subjected to a first heat treatment in which the steel pipe is heated at the heating temperature ranging 800-1,050°C, and cooled at a cooling rate equal to or higher than that of air cooling, and then a second heat treatment in which the steel pipe is heated at the heating temperature ranging 650-750°C, kept for 300 seconds or longer, and then air-cooled to a room temperature. This provides the steel pipe being capable of precipitating a right amount of stable residual austenite phase, having the excellent tube-expandability to endure an expanding process under such a severe condition as 40% or higher expanding rate, having a tensile strength of 550 MPa or higher, and also having an excellent weldability. <P>COPYRIGHT: (C)2012,JPO&INPIT</p> |
