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In addition to the super piercer. "the newsuper piercer" which allows expansion piercing has been deveioped by the authors. The development of the skewing technology for disc roll axes and its application to the cone-type piercing technology results in remarkable increase in the expansion~atio. And then, studied in detail were the influences of the expansion ratio, feed and cross angle on the rotary forging effects, and redundant shear deformations in the expansion piercing process. Noteworthy is that the larger the expansion ratio is, the more remarkable is the decrease in the number of rotary forging times. Accordingly, the larger the expansion ratio is, the more remarkable is the increase in the elongation vaiue at the tip of the plug. Namely, whenthe expansion ratio becomes la!ge~, any possible cause of the initiation of inside bore defects can be eliminated.In order to confirm the abovementioned effects, the influences of the expansion ratio, heating temperature, feed and cross angle on the inside bore defects in materials with poor hot workability were studied in this paper.