| 摘要 |
<P>PROBLEM TO BE SOLVED: To provide a method for production of a steel pipe having partially different strengths, capable of advantageously producing in view of reliability of a joined part, energy cost, surface property or product shape. <P>SOLUTION: The steel pipe contains, by mass, 0.05-0.20% C, 0.5-2.0% Si, 1.0-3.0% Mn, ≤0.1% P, ≤0.01% S, and the balance Fe with inevitable impurities, and has a bilayer composition comprising the ferrite and martensite phases or three-phase composition comprising the above bilayer and remaining austenite phase. An electric-resistance-welded steel pipe having the tensile strength of ≥980 MPa is used for an original pipe. The heat treatment comprising heating a plurality of different pipe length portions at 500-750°C and cooling to room temperature is applied to the original pipe and thereby, the mechanical properties such that a tensile strength is lower by ≥200 MPa than a heat untreatment part 4 and the product of tensile strength and elongation is 15,000 MPa×%, are imparted to a heat treatment part 3. <P>COPYRIGHT: (C)2013,JPO&INPIT |
