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. UOE steel pipe characterized by containing, 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, and N: 0.001 to 0.01%, and one or both of REM: 0.002 to 0.02%, and Mg: 0.0001 to 0.006%, and a balance of Fe and unavoidable impurities and in that a ratio of 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 than 1.0 from the center of plate thickness to the outside surface, said steel pipe produced by successively C-ing, U-ing, and O-ing steel pipe, seam welding the ends of the steel pipe, expanding the pipe, and applying heat treatment after producing the UOE steel pipe so that 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×Int 1 (1)
T 2 −407≧−31.3×Int 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. UOE steel pipe as set forth in claim 1 , characterized by further containing, by wt %, one or more of Ni: 1% or less, Mo: 0.6% or less, Cr: 1% or less, Cu: 1% or less, V: 0.3% or less, and B: 0.0003 to 0.003%.
3. UOE steel pipe as set forth in claim 1 , said UOE steel pipe characterized in that a ratio b/a between a collapse strength a (MPa) of steel pipe produced by said UOE process and a collapse strength b (MPa) of steel pipe produced by the UOE process and after heating to a range from an outside surface of said steel pipe to a center of thickness of 80-550° C. is at least 1.10.
4. UOE steel pipe as set forth in claim 1 , said UOE steel pipe characterized by being coated during said heat treatment or cooling after heat treatment.
5. UOE steel pipe as set forth in claim 1 , characterized in that said ratio of compression and tension of yield strength in the circumferential direction near the inside surface is at least 1.10.