Triangular cracks in slabs are a typical internal defect in continuous casting production. They often occur in the “triangular region” (50-150mm from the narrow edge and 30-80mm from the surface) at the junction of the wide and narrow faces of the slab. Because they are difficult to weld after rolling, they often lead to slab downgrading or scrapping. The main causes are a combination of four factors: compositional segregation, uneven cooling, stress concentration, and insufficient equipment precision. Systematic improvements are needed in various aspects, including the formation mechanism, process control, and equipment calibration.
The core cause of triangular cracks is “weakened grain boundary strength + excessive local stress,” specifically due to the following four key factors:
Steel composition segregation and high-temperature brittleness
(a) Sulfur (S) and phosphorus (P) are the most significant crack-inducing elements in steel:
During solidification, sulfur easily forms low-melting-point compounds (such as FeS, melting point 988℃) at grain boundaries, or combines with oxygen to form Fe(S,O) (even lower melting point), forming a liquid phase film that directly weakens grain boundary bonding. When S content > 0.020%, the incidence of triangular cracks increases significantly; when S > 0.030%, cracks are difficult to control (verified by production data from Panzhihua Iron and Steel Group and Pingxiang Iron and Steel Group).
Phosphorus has a strong tendency for grain boundary segregation, which exacerbates interdendritic embrittlement, especially in peritectic steel with a carbon content of 0.10%-0.15%. This, combined with the volume shrinkage caused by the peritectic phase transformation (δ→γ), further reduces the crack resistance at the solidification front.
(2) Insufficient Manganese-Sulfur Ratio Exacerbates Embrittlement Risk
Manganese (Mn) can combine with sulfur to form high-melting-point MnS (melting point 1620℃), dispersed in the matrix rather than at grain boundaries. However, if Mn/S < 25, manganese cannot completely fix sulfur, leaving residual free sulfur forming a low-melting-point phase. Production data shows that when Mn/S < 25, the probability of triangular region cracking increases by more than 30%, while when Mn/S ≥ 30, the cracking rate can be reduced to below 5%.
(3) Microalloying Element Precipitation Induces Embrittlement
In microalloyed steels containing Nb, V, and Ti, carbonitrides such as Nb(CN) and AlN easily precipitate at high temperatures. These precipitates pin austenite grain boundaries, inhibiting dynamic recrystallization, causing a sharp drop in ductility in the triangular region at the straightening temperature (600-900℃, the third brittle region), making it prone to cracking along grain boundaries.
Uneven Secondary Cooling and Temperature Gradient Imbalance
The triangular zone is the most complex area for billet cooling (due to the superposition of cooling from both wide and narrow faces). Uneven cooling directly generates thermal stress, specifically manifested as:
Corner Overcooling and Temperature Fluctuations
Clogged nozzles on the narrow face of the secondary cooling zone, spray angle deviations, or insufficient water flow in the edge loop can lead to localized overcooling in the triangular zone (a sudden temperature drop of 100-150℃). Subsequently, upon entering the radiant cooling zone, the temperature rapidly recovers (the recovery rate can reach over 200℃/m). Under the repeated effects of thermal expansion and contraction, fatigue stress is generated at the grain boundaries, inducing cracks.
Differences in Cooling Across Width
Wide slabs (width > 1500mm) are prone to exhibiting a “cold center, hot edges” or “high-temperature zone at 1/4 of the width.” During the later stages of solidification, the expansion of molten steel in the high-temperature zone can pull apart the newly solidified shell in the adjacent low-temperature zone (triangular zone), leading to cracks (thermal imaging at Ningbo Steel confirms that the risk of cracking increases sharply when the temperature difference across the width exceeds 50℃).
Improper Cooling Matching in the Copper mould tube
A mismatch in cooling intensity between the wide and narrow sides of the crystallizer (e.g., excessive water flow on the narrow side) can cause the shell on the narrow side to become concave. Disordered columnar crystal growth in the triangular region leads to “multi-directional columnar crystal convergence,” resulting in weak intergranular bonding and becoming a crack initiation point.
Equipment Adjustment Deviations
â‘´ Roller Gap Deviation: Excessive opening of the casting machine’s fan-shaped section (especially sections 6-8 at the end of solidification), roller gap deviation > 0.3mm, or clamping roll deflection > 0.4mm can cause the billet to bulge under the static pressure of molten steel. Because the billet shell in the triangular region is relatively thin (only 20-30mm), it is easily stretched, resulting in deformation exceeding the critical strain (0.2%-0.5%), leading to cracking.
⑵ Poor Arc Alignment: When the arc alignment deviation of the arc-shaped continuous casting machine is > 0.3mm, the triangular region will bear additional bending stress during billet straightening. If the straightening temperature happens to fall into the brittle zone III (600-900℃), the triangular region, lacking ductility, is prone to cracking along the columnar crystals.
(3) Slag Accumulation on Side Guide Rollers Slag accumulation on side guide rollers, or excessively tight installation (extending into the crystallizer > 5mm), will apply additional mechanical pressure to the triangular region. Especially when the casting speed fluctuates, the pressure repeatedly acts on the weak solidification front, directly inducing cracks.
Process Fluctuations: Casting Speed
Fluctuations and High Superheat Frequent increases and decreases in casting speed (such as during the initial stage of the first heat in the tundish) will lead to instability at the solidification end position. The molten steel will wash away the solidification front, causing the billet shell to thin. When the superheat is > 30℃, excessive growth of columnar crystals (the length of the inner arc columnar crystals increases by 20%) increases the grain boundary area in the triangular region, increasing the risk of embrittlement.
FAQ
What is the key role of the manganese-sulfur ratio (Mn/S ratio) in preventing internal cracks in slabs?
A: Sulfur (S) in steel forms low-melting-point FeS, which remains in the liquid state for a long time at grain boundaries, easily inducing cracks. By increasing the Mn/S ratio (usually >25 or higher), sulfur can be converted into MnS inclusions with higher melting points, thereby improving the high-temperature plasticity of the steel. In modern high-efficiency continuous casting, strictly controlling the Mn/S ratio is fundamental to reducing internal cracks in segregation-sensitive grades (such as medium carbon steel and low alloy steel).
Why is temperature reheating in the secondary cooling zone a major cause of cracking?
A: Uneven spraying or abrupt changes in cooling intensity in the secondary cooling zone can lead to a drastic temperature reheating on the slab surface. Surface thermal expansion generates tensile stress in the not-yet-fully-solidified central region. According to industry standards, slab surface temperature fluctuations should be controlled within 100℃/m to prevent excessive internal stress due to thermal stress accumulation.
