STEEL PIPES EXCELLENT IN DEFORMATION CHARACTERISTICS AND PROCESS FOR MANUFACTURING THE SAME

摘要

The invention provides steel pipes excellent in deformability, particularly steel pipes for pipe expansion for oil well use excellent in pipe expansion characteristics and low-yield-ratio line pipes, and a process for manufacturing the pipes without water cooling necessitating large-scale heat treatment equipment. A steel pipe which contains by mass C: 0.04 to 0.10%, Mn: 1.00 to 2.50%, Si: 0.80% or below, P: 0.03% or below, S: 0.01% or below, Al: 0.10% or below, and N: 0.01% or below and further contains one or more of Ni: 1.00% or below, Mo: 0.60% or below, Cr: 1.00% or below, and Cu: 1.00% or below in such amounts as to satisfy the relationship: Mn + Cr + Ni + 2Mo + Cu > 2.00 with the balance consisting of iron and unavoidable impurities and whose microstructure is a dual-phase structure composed of 2 to 10% (in area fraction) of a martensite-austenite hybrid and soft phase; and a process for manufacturing the pipes which comprises heating a mother steel pipe to a temperature ranging from Ac1 + 10°C to Ac1 + 60°C and then subjecting the resulting pipe to air cooling.