Patent ID: 7892368
Filing Date: 2011-02-22
Classification: B21C,C21D,C22C,Y10T

Abstract:
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), where, T T