Patent ID: 7146836

Claim:
A piercing method for manufacturing a seamless pipe, comprising: holding a feed angle β and a cross angle γ of both-ends-supported cone-type main rolls provided to be opposite laterally or vertically across a pass line in a range satisfying the following expressions (1) to (3); setting the relationship between the outer diameter “d 0 ” of a solid billet and the outer diameter “d” and thickness “t” of a hollow piece after piercing so as to satisfy the following expression (4); and setting the relationship between inlet diameter D 1 and outlet diameter D 2 of the main rolls with the above-mentioned “d 0 ”, “d” and γ so as to satisfy the following expression (5) 8°≦β≦20° (1) 5°≦γ≦35° (2) 15°≦β+γ≦50° (3) 1.5≦−ψ r /ψ θ ≦4.5 (4) ( d/d 0 )/(0.75+0.025γ)≦ D 2 /D 1 (5) In the expression (4), ψ r 1n(2t/d 0 ) and ψ θ =1n{2(d−t)/d 0 }.