US Patent 7780800 Method of manufacturing a bent pipe

A bent pipe corresponding to at least API X100 grade and having a base metal with high strength and toughness and a weld metal with high toughness is provided. A steel plate prepared by cooling after hot rolling at a cooling rate at the central portion in the plate thickness direction of at most 5° C. per second at 700-500° C. is formed into a bend mother pipe, which is then heated to 900-1100° C. and subjected to bending, then it is cooled to a temperature of at most 300° C. at a cooling rate in the central portion of the thickness direction of at least 5° C. per second at 700-500° C., after which it is tempered at 300-500° C.