Evaluation of the Effect of Electro-Magnetic Stirring on Soundness of Continuously Cast Billets Using Ultrasonic Technique

Effect of electro-magnetic stirring on soundness of continuously cast billets and slabs can be assessed by many methods like visual inspection of macro-etch & sulphur print evaluation. Ultrasonic assessment provides through thickness information of the test samples, whereas, macro-etching and sulphur print methods provide information in one plane only. An attempt has been made to evaluate the soundness (inhomogeneities / flaws as well as effect of columnar / equiaxed grains) of continuously cast low carbon and high carbon continuously cast billets by ultrasonic attenuation as well as high gain pulse-echo technique in transverse cut slices. One low carbon grade (Grade – LC) and one high carbon grades (Grade – HC) were selected for this purpose. Assessment of over all flaws / inhomogeneities was done by counting number of high gain internally reflected signals (flaw-echoes) using 6 MHz twin crystal probe. Using 10 MHz probe and attenuation technique (attenuation indicated by back-wall echo drop) was used to evaluate the overall macro-structure (including columnar / equiaxed grains as well as inhomogeneities / flaws). The result revealed no significant improvement in the equiaxed zone in case of low carbon grade billets (i.e. Grade – LC), whereas, there was significant improvement in the equiaxed zone of high carbon grade billets (Grade – HC). The ultrasonic flaws in the inclusion concentration bands in all the billets were found to decrease after electro-magnetic stirring. In low carbon billet, however, the flaws were found to increase in the central region after electro-magnetic stirring.