Method for producing seamless pipe and method for determining length of billet for use in producing seamless pipe

A method for producing a seamless pipe comprising steps of a billet-heating process, a billet-cutting process, a piercing-rolling process and elongation-rolling process, wherein:

• the method using the billets satisfying the following formulas (1) to (3):
• lmin≦l≦lmax (1)
• LminL≦Lmax (2)
• F≧Fmin (3)
• wherein L is the length (m) of the parent billet and is represented by the following formula (4), and F is the billet-heating-furnace filling factor (%) and is represented by the following formula (5):
• L=l×nB+k×(nB−1) (4)
• F=100×L/Lmax (5)
• wherein in the above-described formulas, the meanings of the individual symbols are as follows:
• Lmin:
• The minimum length (m) of the parent billet allowed on the basis of the conditions of the billet-heating furnace
• Lmax:
• The maximum length (m) of the parent billet allowed on the basis of the conditions of the billet-heating furnace
• l:
• The length (m) of the child billet
• lmin:
• The minimum value (m) of the length of the child billet allowed on the basis of the predetermined tolerances and the rolling conditions
• lmax:
• The maximum value (m) of the length of the child billet allowed on the basis of the predetermined tolerances and the rolling conditions
• k:
• Billet cutting margin (m)
• nB:
• The number (number of pieces) of the child billets obtainable from one piece of parent billet
• Fmin:
• The allowable minimum value (%) of the heating-furnace filling factor
• wherein, the method adopts the following respective values in the following respective cases (A) to (C) in each of which the relation between the number-of-obtainable-billet indexes I0 and Il calculated respectively from the following formulas (6) and (7) is specified:
• (A) A case where I0=Il:
• L obtained by substituting nB=I0 and l=lmax into the above-described formula (4) is adopted as the length L of the parent billet, lmaxis adopted as the length l of the child billet, and I0 is adopted as the number nB of the obtainable child billets
• (B) A case where I0<Ii, and F satisfies the above-described formula (3) wherein F is obtained by substituting, into the above-described formula (5), L obtained by substituting nB=I0 and l=lmax into the above-described formula (4):
• L obtained by substituting nB =I0 and l=lmax into the above-described formula (4) is adopted as the length L of the parent billet, lmax is adopted as the length l of the child billet, and l0 is adopted as the number nB of the obtainable child billets
• (C) A case where Io<Ii, and F does not satisfy the above-described formula (3) wherein F is obtained by substituting, into the above-described formula (5), L obtained by substituting nB=I0 and l=lmax into the above-described formula (4):
• Lmax is adopted as the length L of the parent billet,(Lmax−k×nB −l))/nB is adopted as the length l of the child billet, and I0+l is adopted as the number nB of the obtainable child billets
• I0=Int((Lmax+k)/(lmax+k)) (6)
• Il=Int((Lmax+k)/(lmin+k)) (7)
• wherein Int(X) means the maximum integer that does not exceed the specified numerical value X.