In the previous article, we discussed controlling titanium inclusions for high-quality steel applications. Today, we focus on a critical internal defect that lies beneath the visible surface of continuous casting slabs: subsurface cracks. These cracks, located 2-15mm below the slab surface, are invisible to visual inspection but become exposed during subsequent rolling, leading to surface peeling, lamination, and rejection of finished products. For automotive exposed panels, tinplate, and thin-gauge electrical steel, subsurface cracks are a "zero tolerance" defect. How can you prevent subsurface cracks from forming during continuous casting and ensure the internal integrity of your slabs? Wuxi WeiDa Cored Wire Co.,Ltd provides comprehensive solutions based on mold metallurgy optimization, secondary cooling control, and steel composition management.
The Formation Mechanism of Subsurface Cracks
Subsurface cracks form in the continuous casting mold during the initial stages of solidification. As the solidifying shell moves down through the mold, it experiences complex thermal and mechanical stresses. The following mechanisms are primarily responsible. Uneven shell growth due to non-uniform heat transfer creates thin spots in the shell that are weak points for crack initiation. Thermal stress from rapid cooling causes the shell to contract; if the contraction is constrained, tensile stresses develop that can tear the shell. Ferrostatic pressure from the liquid core pushes outward against the shell; if the shell is too thin or has weak spots, it bulges and cracks. Mold friction between the shell and mold wall generates tensile stresses in the shell, particularly during oscillation cycle changes. Bending and straightening of the strand imposes additional mechanical stresses on the subsurface region.
Critical Factors That Increase Subsurface Crack Risk
Several process and material factors contribute to subsurface crack formation. High casting speed increases shell growth rate requirements and stress levels. Excessive mold cooling intensity creates steep thermal gradients. Poor mold flux performance leads to uneven lubrication and heat transfer. High sulfur or phosphorus content in the steel promotes hot shortness and reduces high-temperature ductility. Improper secondary cooling causes surface temperature fluctuations that induce thermal stress. Brittle phase formation such as grain boundary carbides, nitrides, or sulfides weakens the shell.
Our Solution: Integrated Crack Prevention Strategy
First, optimize mold heat transfer and shell growth uniformity. The mold is where the shell first forms; its condition determines crack susceptibility. Our recommendations include using mold flux with controlled crystallization properties to promote uniform heat transfer, maintaining proper mold taper to match shell shrinkage, controlling mold cooling water flow rate and temperature to avoid excessive cooling, ensuring mold level stability within ±3mm to prevent meniscus fluctuations, and using mold electromagnetic braking to stabilize flow and prevent uneven shell growth.
Second, optimize oscillation parameters. The oscillation marks on the slab surface are directly related to subsurface crack formation. Deep oscillation marks create stress concentration sites where cracks initiate. We recommend using high-frequency, small-amplitude oscillation (180-300 cpm, 3-6mm amplitude) with negative strip time of 0.08-0.12 seconds. These parameters minimize oscillation mark depth while maintaining adequate lubrication.
Third, control secondary cooling intensity and uniformity. After the slab exits the mold, secondary cooling continues the solidification process. Our dynamic secondary cooling control system recommendations include adjusting water flow rates in each cooling zone based on casting speed and steel grade, avoiding surface temperature recovery (reheating) that causes thermal stress, maintaining uniform cooling across slab width to avoid edge-to-center temperature differences, and keeping surface temperature above the ductility trough (typically >900°C for most steel grades) to avoid brittle fracture.
Fourth, optimize steel composition for high-temperature ductility. The steel's intrinsic hot ductility determines its resistance to cracking. Our cored wire products help improve ductility:
•Calcium treatment modifies harmful sulfides (MnS) into less harmful globular CaS, reducing hot shortness.
•Rare earth treatment is even more effective at modifying sulfides and also ties up tramp elements like copper and tin that cause hot shortness.
•Boron microalloying (0.001-0.003%) improves grain boundary strength and hot ductility.
•Titanium microalloying forms TiN particles that refine the grain structure and improve ductility.
•Avoid excess nitrogen which forms grain boundary nitrides; use low-nitrogen charge materials.
Fifth, control harmful residual elements. Copper, tin, antimony, and arsenic are particularly damaging to hot ductility. These elements segregate to grain boundaries and form low-melting-point liquid films that cause intergranular cracking. Our dilution strategies using high-purity charge materials (DRI/HBI) and rare earth modification help mitigate the effects of these elements.
Sixth, optimize bending and straightening practices. Mechanical stress from strand bending and straightening is a major cause of subsurface cracks in the lower portions of the caster. We recommend: keeping strand temperature during straightening above the ductility trough (typically >900°C), using multi-point straightening to distribute stress over several points rather than a single point, and maintaining proper roll alignment to avoid localized stress concentrations.
Seventh, implement online monitoring and rapid response. Detecting problems early prevents widespread defects. We recommend equipping your caster with mold thermal monitoring systems (thermocouples in the mold copper plates) to detect uneven heat transfer, oscillation condition monitoring to detect abnormal friction, surface temperature pyrometers in the secondary cooling zone to verify cooling uniformity, and eddy current surface inspection for early detection of surface cracks that correlate with subsurface defects.
Troubleshooting Subsurface Cracks: Systematic Approach
When subsurface cracks occur, systematic troubleshooting is essential. Our technical team follows a structured methodology. Step one: locate and characterize the cracks using acid etching or ultrasonic testing to determine depth, orientation, and distribution. Step two: analyze process data from the suspect period including casting speed, mold level, cooling water parameters, and secondary cooling profiles. Step three: examine mold flux performance including consumption rate, slag rim condition, and properties. Step four: review steel composition including carbon equivalent, sulfur, phosphorus, and residual elements. Step five: implement corrective actions based on identified root causes, then verify effectiveness.
From Occasional Failures to Reliable Production
Subsurface cracks are among the most difficult defects to detect and most costly to correct. They are invisible until downstream processing, at which point the value added makes rejection expensive. With Wuxi WeiDa's integrated crack prevention strategy, you can move from reactive troubleshooting to proactive prevention.
If you are experiencing subsurface cracks that cause surface defects on your rolled products, or if you wish to validate your current practices to ensure you are not at risk, please visit https://www.weidamaterials.com/ to discuss our subsurface crack prevention solutions.