Three-Dimensional Evaluation of Internal Quality of the Continuous Casting Billet of a High Carbon Steel Using X-ray Computed Tomography

The internal quality of the continuous casting (CC) billet of a high carbon steel was three-dimensionally evaluated using X-ray computed tomography (X-CT). Internal defects were divided into porosities and large inclusions based on their sphericity. The porosities had a total number density of 1.62 #/mm3 and a total volume fraction of 449 ppm, while the number density and volume fraction of large inclusions were 0.383 #/mm3 and 68.6 ppm, respectively. The volume fraction of porosities increased gradually from the chilled layer toward the mixed zone of the billet, before increasing sharply in the equiaxed zone. Meanwhile, the number density of porosities increased sharply in both the mixed and equiaxed zones. There were three regions rich in large inclusions along the billet thickness: near the center of the equiaxed zone and the interfaces between the columnar and mixed zones. Two sources of large inclusions were proposed. The smaller inclusions (< 150 μm) with a significantly large quantity were estimated to mainly originate from endogenous inclusions generated by deoxidation or reoxidation, while the larger ones (> 150 μm) with a small quantity would mainly come from the entrainment of slag. Both the areal and bulk densities of the billet, 98.6 and 99.7 pct, respectively, were the smallest in the center of the equiaxed zone, which was consistent with the distribution of porosity defects and carbon segregation determined by traditional methods.