Cold-expansion process has been developed for ensuring dimensional precision of inside geometry on the end portions of line pipe. For expanding the inside diameter of line pipe, a plug insertion method by cold press in the axial direction from the end side was finally adopted. As the nature of end sizing operation, only very small amount of cold plastic strain is applicable by cold expansion, and in order to realize the high dimensional precision of end portion a special geometry of the plug had to be designed. If a single-taper plug is adopted the expanded portion tends to overshoot and the inside diameter of expanded portion is larger than the plug diameter. The new plug design allows smooth contact of the inside surface expanded by the inlet portion of the plug on the bearing portion of the plug and no overshooting phenomenon occurs. One of the simplest designs of this type of plug for practical use is a double-taper plug, and finite element method was carried out in order to determine the practical range of the plug geometry referring to the deep insight on the mechanism of expansion process.

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