Investigation of sub-surface cracks in continuous cast billets

Sub-surface cracks in the continuous cast product of steel were studied to understand the causes of such defects and to suggest remedial measures. Even after hot rolling the cracks were not welded. The diameter of the as cast billets were 225 mm and most of the cracks were located around l cm away from the edges. The cracks were visible with the naked eye at transverse sections. The cracks were not continuous but scattered. The objective of this work was to identify the root cause of the formation of the sub-surface defects on the concast product and to ascertain whether it was due to any material related problem or due to any other cause such as processing. The existing cracks were opened through mechanical testing. The fracture surfaces were observed by SEM. Metallography and SEM observations revealed that the cracks were generated due to preferential segregation of sulphur, Cu and subsequent formation of various types of sulphide inclusions. The presence of a higher amount of Cu, particularly in the cracked region is of great concern. Cu is very harmful in steel if it exceeds the tolerable limit, particularly with the presence of high amount of S. The sulphide inclusions are found at the existing cracked regions. Inclusions at the crack is indicative of a correlation between inclusions and cracks. The inclusions act as a second phase and may influence the solidification of the matrix by restriction in directional solidification, differential contraction and even acting as a source of stress concentration—hence the appearance of cracks. The location-specific segregation of the inclusions is however surprising. Had there been dispersion of inclusions, probably the cracks would not have resulted.