Method for producing ultrathin-wall seamless metal tube by cold rolling method

A method for producing an ultrathin-wall seamless metal tube by a cold rolling method in which employed is a cold pilger mill of mechatronics drive type that includes:

• an electric control system to control a reciprocating movement of a roll stand and a shell drive of a tube material;
• and a mechanism to give a feed and a turn angle to the tube material immediately before the start of a forward stroke and immediately before the start of a backward stroke, the method comprising:
• utilizing a roll having a groove whose diameter gradually increases, remains constant, or gradually decreases from an engaging entry side to a finishing exit side of a pair of rolls, and a tapered mandrel whose diameter similarly gradually increases from the engaging entry side to the finishing exit side;
• and
• giving a turn angle and a feed to the tube material immediately before the start of a forward stroke, and also giving a shell drive to the tube material immediately before the start of a backward stroke so as to bring a load balance between forward and backward strokes while suppressing the upper limit of rolling load during the course from the engaging entry side to the finishing exit side, thereby elongating the tube material by reducing a wall thickness while expanding a mid-wall diameter which is an average diameter of an outer diameter and an inner diameter of the tube material.