Working principle of a leveling roll: 

Initial bending (entry section)
The sheet first enters the entry roll set, where rollers arranged alternately on the top and bottom apply bending forces far exceeding the yield strength, causing the surface layer to stretch and the core layer to compress, eliminating large waves or warping.

Repeated bending (middle section)
Traveling through a dense roll set, the sheet undergoes alternately forward and reverse bending (S-shaped path), so that the stretched/compressed areas from the previous stage are swapped in the next bending, disrupting and redistributing residual stress, progressively correcting flatness.

Elastic recovery control (exit section)
The exit roll gap is slightly larger than in the middle section, applying a force slightly above the yield strength, allowing the sheet to leave under controlled elastic recovery, preventing reverse warping or insufficient correction.