Surface cracks in continuous casting slabs are among the most critical defects affecting product quality and production efficiency in steelmaking. Longitudinal and transverse cracks are the two most common types, which can lead to delamination in rolled products, breakout incidents, or even entire heat rejection when severe. This article systematically analyzes the formation mechanisms of these cracks and proposes comprehensive control measures spanning mold operation to secondary cooling processes.
Causes and Prevention of Longitudinal Cracks
Formation Mechanism
Longitudinal cracks originate from uneven shell thickness in the meniscus region of the mold. When tensile stress exceeds the steel’s high-temperature strength, stress concentration occurs at weak points, leading to cracking. Key contributing factors include:
- Misaligned submerged entry nozzle (SEN) causing biased flow.
- Poor mold flux performance with abnormal liquid slag layer thickness (<10mm significantly increases risk).
- Excessive mold level fluctuation (>10mm results in 30% crack occurrence).
- High S/P content (S>0.02%, P>0.017%).
- Carbon content in peritectic range (0.12-0.17%).
Key Prevention Technologies
(1) Mold Optimization
- Ensure SEN alignment accuracy (<1mm deviation).
- Apply hot-top mold technology (embedding low-thermal-conductivity materials reduces heat flux by 50-70%).
- Optimize taper (0.9-1.3%/m adjusted per steel grade).
(2) Process Control
- Maintain mold level fluctuation within ±10mm.
- Control liquid slag layer at 10-15mm.
- Optimize SEN immersion depth (120-150mm).
Characteristics and Solutions for Transverse Cracks
Defect Features
Transverse cracks primarily hide in oscillation mark troughs on the inner radius, with depths up to 7mm upon pickling inspection. Microanalysis reveals propagation along ferrite networks with AlN precipitates, particularly sensitive in Nb/V microalloyed steels.
Root Causes
- Deep oscillation marks (>0.5mm) creating stress concentration.
- AlN precipitation at austenite grain boundaries (when Al>0.01%).
- Strand straightening in brittle temperature range (700-900°C).
- Excessive secondary cooling intensity.
Integrated Solutions
(1) Oscillation Improvement
- High-frequency/low-amplitude oscillation (200-400 cycles/min, 2-4mm) reduces mark depth by over 40%.
(2) Secondary Cooling Optimization
- Implement soft cooling (15-20% water reduction).
- Ensure straightening temperature >900°C.
- Adopt air-mist cooling for uniform heat transfer.
(3) Composition & Operation Control
- Limit Als content (<0.006%).
- Use low-viscosity mold flux (≤0.2Pa·s at 1300°C).
- Apply slow cooling or hot charging.
Case Study
A steel plant reduced Q345B longitudinal cracks from 8% to zero by:
- Replacing chrome plating with Ni-Fe alloy coating (30% better thermal uniformity).
- Adjusting flux basicity from 1.1 to 1.25 (stabilizing slag layer at 12±2mm).
- Reducing secondary cooling from 1.1 to 0.8L/kg water ratio.
Conclusion
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