Formation of longitudinal, midface cracks in continuously-cast slabs

Abstract

Based on a series of related investigations, a mechanism for the formation of longitudinal, midface cracks in strand-cast slabs has been elucidated. Metallographic studies, X-ray analyses, and high-temperature tensile testing of as-cast slab samples, together with computer predictions of heat flow, have been combined to show that the cracks open first close to the solidification front where the ductility is extremely low. Tensile strains in the solid shell, resulting from rapid surface cooling or inadequate end-plate taper, concentrate in local regions of high temperature near the meniscus or at the level of nozzlestream impingement on the broad face. While the interior of the shell is opening, the surface which is cooler and more ductile, flows plastically to form a local surface depression. The subsurface cracks may subsequently break through to the surface in the mold or upper sprays if the tensile strains are sufficiently high. The influence of the impinging metal stream, mold-powder practice, metal-level control, steel composition, end-plate taper, submold support and spray practice on the formation of longitudinal, midface cracks is explained in terms of this mechanism.