STEEL PIPE HAVING HIGH FORMABILITY AND METHOD FOR PRODUCTION THEREOF

摘要

A steel pipe having high formability which has an r value in the longitudinal direction of 1.2 or more, preferably 1.6 or more over the whole region of perimeter direction containing a seamed portion; and a method of producing the steel pipe which comprises heating a steel pipe prepared by the electric welding of steel hoops to an Ac1 temperature or higher and subjecting the heated pipe to a stretch reducing of a diameter reduction percentage of 30 % or more in a temperature region of 600 ° or higher and up to an Ac1 temperature, and optionally heating again the pipe during or after completion of cooling after said reducing to a temperature of 600 to 900 ° and holding it at the temperature for 1 second or longer. The steel pipe is a highly formable steel pipe wherein a portion having been molten or transformed by seam welding has a high r value in the axial direction thereof which is of the same degree as that of a portion having not been affected by welding and is excellent particularly in bending formability.