Method for producing seamless tube with groved inner surface

A method for producing a seamless tube with a grooved inner surface, comprising the steps of:

• drawing a metal tube to gradually reduce a diameter of the metal tube by means of a holding die disposed externally of the metal tube and a holding plug disposed internally of the tube and engaged with the holding die;
• rotating a plurality of first rolls in contact with an outer surface of the metal tube with a reduced diameter such that a rotation axis of each of the first rolls is orthogonal to an axial direction of the metal tube, while disposing, at a position inside the metal tube corresponding to the first rolls, a first grooved plug formed with a grooved outer surface and coupled in relatively rotatable relation to the holding die via a first coupling shaft, and pressing the metal tube against the first grooved plug by means of the first rolls to form a first groove in a portion of an inner surface of the metal tube in a circumferential direction of the tube and thereby form a plurality of first grooved zones;
• performing a sizing process with respect to the metal tube formed with the first grooved zones by using a sizing device;
• and
• rotating a plurality of second rolls in contact with the outer surface of the metal tube such that a rotation axis of each of the second rolls is orthogonal to the axial direction of the metal tube and deviates from the rotation axis of each of the first rolls, while disposing, at a position inside the metal tube corresponding to the second rolls, a second grooved plug formed with a grooved outer surface and coupled in relatively rotatable relation to the first grooved plug via a second coupling shaft, and pressing the metal tube against the second grooved plug by means of the second rolls to form a second groove in a portion of the inner surface of the metal tube in the circumferential direction of the tube and thereby form a plurality of second grooved zones.