Central quality assessment of continuously cast high carbon billets using ultrasonic c-scan imaging

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Quality improvement in continuous cast steel products with respect to internal and subsurface level is of increasing importance for inclination towards clean steel
production. In the process of continuous casting, when the steel billet solidifies during primary and secondary cooling, alloying elements have a tendency to segregate at the centre of billet around central porosity in the equiaxed zone. This happens because of lower solubility of alloying elements in solid steel rather than in liquid state. Central porosity in the equiaxed zone is obtained mainly due to formation of liquid crater during
solidification. In high carbon steel, carbon segregation may result in cementite formation at localized regions thereby increasing hardness ultimately causing breakage
during wire drawing operation. Macro-etching, sulfur printing and ultrasonic imaging,these are the various techniques of finding the central porosity in cast billets. Macroetching and sulfur printing provide only surface information whereas ultrasonic imaging gives the through thickness information of the billet. It is many times very
important to know the plane-wise/depth wise defect distribution in a material. In this work an image processing algorithm has been developed in Matlab to determine depth wise defects in the test material. This algorithm is capable of detecting and 3D- positioning of the central porosity region and carbon segregation in the billet samples produced by varying process parameters. Further development of this technique would
be useful in detection and 3D-positioning of various defects such as pores, segregations and inclusions in steel products, paving the path for controlling the process parameters to achieve clean steel.