Why do longitudinal cracks occur in continuously cast round billets?

Longitudinal cracks in continuously cast round billets occur when the thermal stress and phase transformation stress generated during the solidification process of molten steel exceed the load-bearing limit of the billet shell. Furthermore, the billet’s own crystal structure and composition weaken its crack resistance, ultimately leading to the formation and propagation of longitudinal cracks in the weak areas. The main causes are related to the non-uniformity of the molten steel solidification process, the characteristics of the steel grade, and the poor plasticity in the high-temperature brittle range.

Non-uniformity of the molten steel solidification process

The solidification of molten steel is a gradual process from a chilled layer to a columnar crystal layer and finally to a central equiaxed crystal layer. The structural non-uniformity in this process is the root cause of stress. Molten steel cools rapidly in the crystallizer, first forming a thin and dense chilled layer (fine equiaxed crystals) on the outer surface, while the internal molten steel slowly solidifies to form a columnar crystal layer. Significant temperature gradients exist between the quench layer and the columnar crystal layer, and between the columnar crystal layer and the central equiaxed crystal layer. The quench layer, which cools first, is “constrained” by the cooling and contraction of the subsequent columnar crystal layer, generating radial and circumferential tensile stresses, with the tensile stress increasing closer to the outer edge of the billet. Uneven cooling in the crystallizer, variations in molten steel flow, or fluctuations in the liquid level can lead to uneven thickness of the initial billet shell and a shift in the solidification center. In this case, the thinner areas of the billet shell become weak points in terms of stress resistance, where tensile stress is highly concentrated. When the stress exceeds the tensile strength of the billet shell, initial longitudinal cracks will form.

Steel Grade Characteristics

The solidification process of molten steel is affected by different chemical compositions and crystal formation during solidification. The inherent characteristics of some steel grades can significantly reduce crack resistance, becoming a contributing factor to longitudinal cracking. In crack-sensitive steel grades, columnar crystal growth is vigorous and the grains are coarse, with the columnar crystals arranged radially, forming “weak grain boundaries.” Grain boundaries are the weakest areas of atomic bonding; coarse columnar crystals further reduce grain boundary bonding. When subjected to tensile stress, cracks easily initiate along grain boundaries (intergranular fracture), and the directional growth of columnar crystals guides crack propagation longitudinally. Medium-carbon manganese steel (containing 0.3%-0.4% carbon) and other steel grades undergo a δ-γ phase transformation (body-centered cubic lattice → face-centered cubic lattice) during solidification and cooling. This phase transformation is accompanied by approximately 1% volume expansion, leading to additional phase transformation stress between grains. Meanwhile, this type of steel exhibits a significant tendency for rhombic transformation, and the heat flux density of the crystallizer has a substantial impact on the solidification process. Inadequate cooling intensity in the crystallizer can exacerbate the superposition of phase transformation stress and thermal stress, further increasing the risk of cracking. Residual elements (Sn, Cu, As, etc.) in the steel accumulate at austenite grain boundaries, reducing surface energy and atomic bonding forces at the grain boundaries, and hindering grain boundary migration and dynamic recrystallization. This leads to a sharp decrease in the hot plasticity of the billet in the high-temperature range (925-1000℃). At this point, even small thermal stresses can cause the billet shell to crack along the grain boundaries, and Sn and Cu can form low-melting-point alloys (such as Cu-Sn alloys), which penetrate along the grain boundaries, further weakening the grain boundary strength.

Poor Plasticity in the High-Temperature Brittle Range

The billet exhibits a brittle range within a specific temperature range, where the plasticity and toughness of the billet shell decrease significantly, becoming a critical stage where cracks are easily triggered:

925-1000℃ High-Temperature Brittle Range: This temperature range is the core triggering zone for longitudinal cracks. For steel grades with high residual element content (e.g., Cu equivalent≥0.3%), elements such as Sn and Cu segregate at the austenite grain boundaries, leading to deterioration of the billet’s thermoplasticity and a significant reduction in area. Simultaneously, if the casting speed is too slow (e.g., ≤ 1 m/min), the temperature of the initial billet shell in the mold will fall into this brittle zone, and the shell cannot withstand the thermal stress, directly forming initial cracks.

The phase transformation brittle zone around 591℃: Steel grades such as 37Mn5 undergo phase transformation and significant volume expansion when cooled to around 591℃, generating substantial phase transformation stress. If the billet is not cooled slowly at this point (e.g., only stacked cooling without insulation pits/covers), the rapid temperature drop prevents the phase transformation stress from being released, further exacerbating crack propagation due to the superposition of previous thermal stresses.

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