Method of manufacturing seamless pipes

A method of manufacturing seamless pipes by carrying out piercing-rolling using a piercer provided with a pusher disposed on the entry side along the pass line, a plug disposed on the exit side along the pass line and a pair of inclined rolls disposed so as to face each other across the plug which method is characterized in that it has the following features (a) to (d):

• Feature (a):
• That the piercing-rolling is carried out under conditions such that the plug tip draft (TDFT) is not higher than 0.04 and/or the root of the product of the plug tip draft (TDFT) and the number of billet revolutions (N), namely (TDFT×N)0.5, is not greater than 0.4;
• Feature (b):
• That the positions of the inclined rolls are selected so that the gorge draft (GDFT) indicating the ratio of the roll gap (Rg) which is minimal in the gorge section between the inclined rolls to the billet outside diameter (Bd), namely Rg/Bd, may satisfy the relations defined by the formula (1) given below;
• Feature (c):
• That the piercing-rolling is carried out using a plug having a shape satisfying the relations defined by the formula (2) given below;
• Feature (d):
• That the pusher pushes the billet at least in the unsteady region of the piercing-rolling:
• −0.01053×EL+0.8768≦GDFT≦−0.01765×EL+0.9717 (1)
• −0.95×(TDFT×N)0.5+1.4≦L2/d2≦−1.4×(TDFT×N)0.5+3.15 (2)
• In the above formulas,
• TDFT=1−(d1/Bd)
• where d1:
• minimum roll-to-roll distance (mm) at the plug tip position and
• Bd:
• billet outside diameter (mm),
• N=(Ld×EL)/(0.5×π×Bd×tan β)
• where Ld:
• projected contact length (mm) from the billet gripped point to the plug tip,
• EL:
• piercing ratio or elongation, namely [hollow raw pipe length]/[billet length],
• β:
• roll feed angle,
• L2:
• rolling section length (mm) of the plug, and
• d2:
• outside diameter (mm) of the plug at the boundary position between the rolling section and reeling section thereof.