Abstract: Aiming to increasing slab percentage of center segregation grade below 1.0 induced by relaxed displacement sensor calibration and poor roll gap accuracy, the accuracy of roll gap before the start of a sequence casts was measured through a multifunctional roll gap measurement instrument and the factors influencing online roll gap accuracy were analyzed. Considering the typical roll gap control system, incorrect offset values of displacement sensors upon cylinders on segments, especially on the solidification crater end zone, was the main reason of poor roll gap accuracy on the soft reduction region. Through strictly implement of offline calibration of displacement sensors, the online roll gap accuracy was highly improved with high precision between the left and right displacement sensors at the segments exit or entrance. Through the application of offline calibration of displacement sensor, the online roll gap deviation could be stably controlled within ±0.2 mm，resulting in the increasing percentage of center segregation grade below 1.0from 29.5 %to 92.0 % and eliminating triangular crack defect.

Key words: continuous casting slab, roll gap accuracy, calibration, internal quality