Abstract
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.
Highlights
Steel is made in steel melting shop in the refractory lined vessels called LD Converters by blowing oxygen through the hot metal bath
Within the continuous casting sub-process of the steel manufacturing process, where the molten steel is passed through a water-cooled near-vertically-aligned lubricated mold of rectangular cross-section about a meter long to emerge in the form of a continuous strand consisting of a solidified shell encapsulating molten material, this strand being further cooled to complete solidification using water sprays even as the orientation is changed to horizontal using rollers [1, 2]
Billet Rhomboidity starts with non-uniform shell solidification in the mold which is due to inconsistent mold cooling causing irregular heat transfer, inducing thermal stress, making the emerging out billet face to be skewed to a have rhombus cross-sectional shape [2]
Summary
Steel is made in steel melting shop in the refractory lined vessels called LD Converters by blowing oxygen through the hot metal bath. To reduce rejection due to Rhomboidity defect and unsafe condition generated in handling billets, an online measurement system is required that will reduce time of inspection, batch holds and help immediate corrective actions to be taken. This can be very useful when trying to detect lines with short breaks in them due to noise, or when objects are partially occluded This is extremely important in our case as the emergent billet temperature is around 900 and, at this temperature, it reacts with ambient oxygen to form flakes of iron oxides which further get deposited on the surface as black patches. This distribution can be multimodal depending on number of lines present in the image. This is used for computing ‘Rhombic distortion index’ as the *+ & => ?> > between the probability mass function estimated from observed values from a given strand and the probability mass function of sample values where it was known that caster was producing normal shape billets
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