Patent ID: 7174761

Claim:
A process for manufacturing a seamless steel pipe, which comprises the steps of: preparing a mother tube; performing elongation rolling of the mother tube using a mandrel mill comprising a plurality of roll stands each having caliber rolls to form wall thickness variations in the circumferential direction of the mother tube; the position and amount of the wall thickness variations being predetermined in view of wall thickness variations to be formed in the circumferential direction of a seamless steel pipe during roll sizing, and performing roll sizing of the resulting mother tube from the elongation rolling to cancel the wall thickness variations formed during the elongation rolling, characterized in that (i) the wall thickness variation of the seamless pipe are portions where the wall thickness is decreased, (ii) the wall thickness variation is located in the circumferential direction of the mother tube at a position of around 45° with respect to the direction of reduction of the final roll stand of the elongation rolling, and (iii) the elongation rolling is carried our by reducing the roll gap of the mandrel mill smaller than a roll gap at which the shape of the grooves is a circle, and using a mandrel bar having a smaller outer diameter than the outer diameter of a mandrel bar with which a target wall thickness can be obtained at the exit of the mandrel mill when the roll gap is so adjusted that the shape of the groove is a circle.