PRODUCTION OF SEAMLESS PIPE

PURPOSE:To allow an immediate constant-diameter rolling without subjecting a stock to a reduction drawing after piercing by setting the max. roll diameter in the outside diameter reduction part of the rolling rolls of a piercer larger than the roll diameter of the part corresponding to the outside diameter determining position of the stock. CONSTITUTION:The piercer 100 is obliquely disposed with the plural rolling rolls 11 in a pass line and is disposed with guide shoes 101. A plug 12 is disposed at the center. This piercer 100 has an I roll part 14 to IV roll part 17. The I roll part 14 is positioned on the inlet side of the rolling mill and has the face angle expanding toward the rolling outlet side. The II roll part 15 is continuous with the rolling outlet side of the I roll part 14 and has the face angle reducing toward the outlet and advancing while expanding the diameter of the stock 13. The III roll part 16 is continuous with the outlet side of the II roll part 15 and has the outside diameter drawing part to expand the diameter toward the outlet side. Further, the max. roll diameter DO in the outside diameter drawing part of the III roll part 16 is made larger than the roll diameter D1 of the part corresponding to the position where the reduction working of the stock 13 ends.