Abstract: The low carbon-high manganese HFW pipe is tested and evaluated by means of chemical analysis, mechanical property test, optical microscopy metallographic observation and scanning electron microscope characterization, etc.And the weld seam impact specimens with poor toughness are analyzed.As a result, it is revealed that the main cause for the low impact toughness of the weld seam is the formation of martensite as caused by the segregation of Mn in the welding metal streamline close and parallel to the weld seam, and distributed on the welding metal streamline and along the weld seam direction.Thanks to the follow-up annealing treatment of the pipe, the martensitic brittle phase in the weld seam structure is eliminated, and in turn impact toughness of the weld seam is obviously improved.