In-Situ Online Measurement of Rhombic Distortion in Billets
Dr Prabal Patra, Punit Rathore, Ashish Tiwari
Rhombic distortion in billets; shape defects in continuous casting; Quadrilateral detection; Hough transform; Gaussian penalty function; KL-divergence measure.
DD (diagonal-difference) is considered as measure of rhombic distortion, aka Rhomboidity, which is a shape related defect in square cross-section billets. Rhomboidity in billets starts with non-uniform shell solidification in the mold primarily due to inconsistent cooling causing irregular heat transfer. The higher diagonal difference greatly impacts the quality of billets to be rolled at various mills. Rhomboidity at or over 4% leads to billet twisting in the roughing stands of the rolling mill. Currently, billet rhomboidity is measured manually at end of casting operation. The presented work describes an optical, online & real-time image processing based method to determine the rhomboidity induced in each strand and alerts the operator to take corrective actions.
The online Rhomboidity Measurement System employs sophisticated image acquisition & processing techniques to determine face contours of the billet with sub-pixel accuracy. The key features of RMS are the construction of a gaussian penalty function for selection of suitable 4-lines combination that precisely fits the billet face and use of a highly efficient and accurate statistical indicator, based on KL-Divergence measure, to estimate the rhomboidity even in presence of partial occlusion of billet face by scales. The expected savings are to the tune of 0.27 Million USD.