UOE steel pipe excellent in collapse strength and method of production thereof
The present invention relates to UOE steel pipe excellent in collapse characteristics formed by the UOE production method and a method for forming this UOE steel pipe, said UOE steel pipe characterized in that a ratio between compression and tension of yield strength in the circumferential direction is at least 1.05 near the inside surface and is at least 0.9 to not more to 1.0 from the center of plate thickness to the outside surface.
1. A method of production of UOE steel pipe excellent in collapse strength, comprising successively C-ing, U-ing, and O-ing steel plate, seam welding the ends of the steel pipe, and expanding the pipe, said method of production of UOE steel pipe characterized by applying heat treatment after producing the UOE steel pipe, 0consisting essentially of, by wt %, C: 0.03 to 0.15%, Si: 0.8% or less, Mn: 0.3 to 2.5%, P: 0.03% or less, S: 0.01% or less, Nb: 0.01 to 0.3%, Ti: 0.005 to 0.03%, Al: 0.1% or less, N: 0.001 to 0.01%, Mo: 0.6% or less, one or more of Ca: 0.01% or less, REM: 0.02% or less, and Mg: 0.006% or less, and a balance of Fe and unavoidable impurities, so that a heating temperature T 1 of the outside surface of said steel pipe and a heating time t 1 of the outside surface satisfy the following formula (1) and a heating temperature T 2 of an inside surface of said steel pipe and a heating time t 2 of the inside surface satisfy the following formula (2):
T 1 −503≧−37.6×ln t 1 (1)
T 2 −407<−31.3×ln t 2 (2)
where,
T 1 : heating temperature of outside surface of steel pipe (° C.), t 1 : heating time of outside surface of steel pipe (s),
T 2 : heating temperature of inside surface of steel pipe (° C.), t 2 : heating time of inside surface of steel pipe (s).
2. A method of production of UOE steel pipe excellent in collapse strength as set forth in claim 1 , said method of production of UOE steel pipe characterized in that said heat treatment is performed by induction heating.
3. A method of production of UOE steel pipe excellent in collapse strength as set forth in claim 2 , said method of production of a UOE steel pipe characterized by coating the pipe during said heat treatment or during the cooling after the heat treatment.