Effect of depressions on the three-dimensional propagation of longitudinal surface crack in continuous casting strands of hypo-peritectic steel

Surface depressions are common defects in the continuous casting process of hypo-peritectic steel, categorised into transverse and longitudinal depressions. While the formation processes of these depressions are well understood, their influence on the propagation of longitudinal surface cracks remains unclear. This study developed a new method for large-area surface depth measurement of the strand based on optical focusing. Utilising this method, we investigated the impact of surface depressions on the three-dimensional propagation of longitudinal surface crack in terms of width, depth and casting direction through macrostructure characteristics and finite element static analysis. The results indicated that transverse and longitudinal depressions affect crack propagation differently. Specifically, the depth of longitudinal depressions increases crack width, while transverse depressions can inhibit it to some extent. Macrostructure analysis of the cross-section revealed a continuous segregation line at the bottom of the depression, closely related to heat transfer, thermal stress, and the flow of solute-enriched liquid steel. Statistical analysis indicated that the depth of longitudinal depressions significantly affects the length of the segregation line, which in turn influences crack depth. However, no surface cracks were found at the bottom of transverse depressions adjacent to longitudinal depressions. This may be due to the banded grains at the bottom of transverse depressions resisting tensile thermal stress parallel to the strand's width direction. Along the casting direction, crack propagation primarily occurs along the bottom of longitudinal depressions due to continuous thermal disturbances at the meniscus. Meanwhile, the transverse depression can restrict the propagation of the existing crack while promoting the formation of the new crack at the other site. Finally, the study proposed mechanisms by which depressions influence the propagation of the longitudinal surface crack. This research further elucidates the impact patterns of surface depressions on macrostructure and stress distribution and how these factors subsequently influence crack propagation in three dimensions. It underscores the importance of controlling surface depressions to mitigate the longitudinal surface crack.