Patent ID: 7780800

Claim:
A method of manufacturing a bent pipe comprising preparing a steel plate by air cooling at a cooling rate in the central portion of the thickness direction of the plate of less than 5° C. per second in a temperature range of 700-500° C. after hot rolling, preparing a bend mother pipe in the form of a welded steel pipe from the steel plate, performing hot bending of the bend mother pipe, and then cooling at a cooling rate in the central portion of the thickness direction of at least 5° C. per second in a temperature range of 700-500° C. to manufacture a bent pipe having a base metal with a tensile strength which is higher than the tensile strength of the base metal of the bend mother pipe, wherein the bent pipe has a base metal having a steel composition comprising, in mass %, C:0.03%-0.12%, Si: 0.05%-0.50%, Mn: 1.4%-2.2%, S: at most 0.01%, Mo: 0.05%-1.0%, Al: 0.005%-0.06%, N: at most 0.008%, at least one of Cu: 0.05%-1.0%, Ni: 0.05%-2.0%, and Cr: 0.05%-1.0%, at least one of Nb: 0.005%-0.1%, V: 0.005%-0.1%, and Ti: 0.005%-0.03%, and a remainder of Fe and impurities, with the carbon equivalent Ceq given by the following equation being at least 0.45%: Ceq =C+Mn/6+(Cr+Mo+V)/5+(Cu+Ni)/15.