Control method for reducing tail shrinkage hole of square billet

The invention discloses an automatic control method for reducing a shrinkage hole of a tail billet by a square billet continuous casting machine. The method comprises the following steps: position sensors are arranged on pressure roller oil cylinders of withdrawal and straightening units; a solid-phase ratio of a cast blank under each withdrawal and straightening unit is obtained through the temperature field distribution calculation of a model computer to select and determine the compression acting range of the tail billet; a multi-frame substep compression mode is adopted in the compression acting range; after the compression starting condition is satisfied, each withdrawal and straightening unit in the compression range is switched from a pressure control mode to a position control mode one by one, and the displacement quantity of each pressure roller oil cylinder is adjusted through the position closed loop adjustment according to the set compression quantity, so that a pressure roller of each withdrawal and straightening unit reaches the set pressure position; the local deceleration process in drawing of the tail billet is performed: when the tundish molten steel weight is reduced to the preset quantity, the billet drawing speed of the withdrawal and straightening units is decelerated; after the tail of the cast blank leaves from a crystallizer, the billet drawing speed is recovered to the standard drawing speed; and after the compression ending condition is satisfied, the compression process of the tail billet is finished. The method reduces the mass loss of the cast blank.