Billet rhomboidity (difference in opposite diagonals) starts with non-uniform shell solidification in the mold which is due to inconsistent mold cooling causing irregular heat transfer.
The billet emerges from the bottom of the mold, essentially a square, but the uneven internal cooling mechanism forces a rhomboid shape just below the mold. Rhomboidity at or over a certain percentage leads to billet twisting in the roughing stands of the rolling mill.
This is a major problem at the steel melting shops since manual measurement of rhomboidity is difficult & unsafe due to the harsh environment & high temperatures. Further, manual measurements are delayed (by hours) & are subjective.
The rhomboidity measurement systems available in the market, though accurate & real time, are very costly.
Hence, an automatic, online, cost effective system for measurement of percentage rhomboidity of billet face emerging out of various strands of caster with the capability to operate under harsh environmental conditions was developed inhouse by Automation Division of Tata Steel.
A machine vision based Online Rhomboidity Measurement system was built and installed at the billet caster at Tata Steel. The camera is positioned at the side of the transfer bay so that it can inspect all the billets emerging out of various strands. The level of the lens is kept just at the center line of billet face.
The camera and other related electronics are protected inside a water cooled jacket and the entire assembly is behind a heat shield plate for protecting it from direct radiation from the billet transfer bay. The billet face images captured by camera are processed for detection of billet face corners, and hence the diagonals, to determine the Rhomboidity.