Slab defects vary widely and seriously impact steel quality and production efficiency. Slab defects primarily include surface cracks, internal cracks, and shape defects.
Surface Crack Defects
Longitudinal cracks are often caused by uneven thickness of the primary shell within the mold meniscus. Under the action of thermal and tensile stresses, cracks develop along interdendritic and austenite grain boundaries at weak points in the shell. For example, in the document “Control of Longitudinal Crack Defects on the Surface of SS400 Steel Continuously Cast Thin Slabs,” longitudinal cracks occur in SS400 steel continuously cast thin slabs due to solidification behavior and related factors within the mold, resulting in black streaking defects on the surface of the finished steel plate. Numerous factors influence these defects, including molten steel composition (for example, carbon content in the peritectic region increases the tendency for longitudinal cracking), molten steel temperature (high overheating can easily cause cracking), mold cooling (uneven or weak cooling can lead to longitudinal cracking), nozzle alignment, and mold slag properties.
Transverse cracks are categorized as corner transverse cracks and wide and subcutaneous transverse cracks. Corner transverse cracks are closely related to steel composition (such as microalloying), surface grain size of the ingot, cooling pattern, and process. In continuous casting ingots containing microalloys such as Nb, V, Al, and B, these microalloying elements easily combine with C and N to form carbides, nitrides, or carbonitrides, which precipitate extensively at austenite grain boundaries. This, combined with the traditional secondary cooling method of continuous casting, promotes the formation of film-like or network-like proeutectoid ferrite at the austenite grain boundaries, significantly weakening the surface plasticity and strength of the ingot, making it prone to transverse cracking at the corners of bending and straightening sections. Wide transverse cracks are often hidden beneath the ingot’s surface, on the surface, or along its edges, coexisting with oscillation marks. They often appear in the troughs of oscillation marks on the inner arc side of the ingot. Here, the solidified structure has coarse grains. In the third brittle temperature range of steel (600-900°C), precipitates form at the austenite grain boundaries and phosphorus and sulfur segregate, reducing the steel’s high-temperature strength and increasing crack susceptibility.
Star-shaped cracks, also known as network cracks, are closely related to the enrichment of copper at grain boundaries. Copper diffuses from the mold copper plate into the primary solidifying shell, penetrating along the austenite grain boundaries and reducing the high-temperature plasticity of the slab surface, thus forming star-shaped cracks. These cracks typically occur at the edges, but can also occur in the center of the slab, with a depth typically ranging from 1.5-3.0mm.
Edge Cracks: The edges of continuously cast slabs are subject to various stresses and inclusions, making them susceptible to longitudinal cracks, transverse cracks, and inclusion-induced cracks. During the rolling process, existing defects in the slab edges can rupture, extend, and expand, forming edge cracks. In severe cases, these defects can lead to edge breakage. Longitudinal edge cracks can be caused by defects such as longitudinal depressions, step edges, and corner folds on the wide edge of the slab. Transverse edge cracks are often caused by deep vibration marks and inappropriate secondary cooling systems. Inclusions (such as AlN precipitation) can cause irregular edge cracks.
Internal Crack Defects
Central cracks: These occur in the equiaxed crystal zone near the centerline of the strand thickness. At the end of the casting process, residual molten steel solidifies and shrinks. The developed columnar crystals hinder the flow of upstream molten steel to compensate for the shrinkage, which can easily lead to central cracks. If there is a large positive roll gap near the end of strand solidification, the strand shell will bulge under the ferrostatic pressure, further exacerbating the central crack defect.
Intermediate cracks: These typically occur in the columnar crystal zone from 20 mm below the skin to the center of the strand thickness. These cracks extend perpendicular to the inner and outer arc surfaces and along the width and along the casting direction. Improper secondary cooling processes can lead to uneven cooling and excessive reheating, resulting in significant thermal stresses. Furthermore, bulging of the strand shell, as well as the additional tensile loads generated during bending and straightening, can easily lead to central cracks.
Triangular cracks: These cracks occur in the triangular region of the slab, perpendicular to the narrow side of the strand. This defect is related to wide-surface bulging caused by poor alignment of the support rolls in the slab guide area. Factors such as excessive steel superheat, high S content and low Mn/S ratio in the molten steel composition, and a wide caster aperture all increase the likelihood of cracks in the triangular area.
Shape Defects
Bulging: The surface shell of the slab bulges into a convex shape due to the static pressure of the molten steel. This defect is categorized as large-surface bulging and side-arc bulging, with large-surface bulging being less common and side-arc bulging being more common. Bulging can cause solute-enriched molten steel to flow within the liquidus, leading to severe center segregation and cross-sectional cracking. In severe cases, it can prevent the slab from being pulled out, damage the equipment, and even cause steel leaks during cutting. High steel superheat, rapid casting speeds, poor cooling, wide segment aperture, improper mold taper, insufficient foot roll support, and segment equipment malfunctions are all contributing factors.
Concavity Defects: These include angle cavities and narrow-surface cavities. Offset angle depression may be accompanied by offset angle cracking or scarring defects, and in severe cases, can lead to offset angle cracking and steel leaks. Causes include: after the billet bulges within the sector, the pressure of the nip rolls on the bulged area on the face causes the narrow face and corner shell to sink inward; molten steel composition (for example, steels with a carbon content of 0.10%-0.15% undergo peritectic reaction during solidification, resulting in significant linear shrinkage, which is prone to depression); mold issues (copper plate wear, uneven water gaps, etc. leading to uneven cooling); mold slag effects (poor heat transfer); sector zero section problems (large deviation from the mold arc, bent zero section nip rolls, etc.); vibration (unsteady vibration); temperature and casting speed effects (excessive molten steel temperature or high casting speed leads to a thin and uneven billet shell); and poor nozzle alignment. Overall narrow face depression is generally caused by excessively hard foot roll adjustment. Minor overall narrow face depression, provided it meets relevant standards, has little impact on rolling.
