Method for adjusting rolling positions of rolling rolls constituting three-roll mandrel mill and method for manufacturing seamless pipes or tubes

A method for adjusting the rolling positions of rolling rolls disposed on at least a first rolling stand and a second rolling stand constituting a three-roll mandrel mill during rolling a pipe or tube material, the method comprising:

• using a pressing-down device for moving the rolling rolls in the rolling direction and a control unit for controlling the pressing-down device;
• immediately after a front edge part of the pipe or tube material has begun to be rolled by the rolling rolls, making the control unit control the pressing-down device to move the rolling rolls in the rolling direction so as to approach the center of the pipe or tube material at a speed of 16 millimeters per second or higher;
• and
• thereafter making the control unit control the pressing-down device to keep the rolling positions of the moved rolling rolls;
• wherein the first rolling stand indicates a rolling stand that is situated on the most upstream side in the conveyance direction of the pipe or tube material among rolling stands constituting the three-roll mandrel mill, the second rolling stand indicates a rolling stand that is directly adjacent to the first rolling stand among rolling stands constituting the three-roll mandrel mill, and the front edge part of the pipe or tube material indicates an edge part on the downstream side in the conveyance direction of the pipe or tube material in the three-roll mandrel mill.