The Critical Function of Laminar Cooling Systems in Second-Hand Hot Rolling Mills​

Laminar cooling systems are essential post-rolling components in second-hand hot rolling mills, responsible for cooling hot-rolled strips from 800-950°C to room temperature in a controlled manner. Their performance directly influences the metallurgical structure and mechanical properties of the finished steel products.​

Mechanical Property Control​

In second-hand hot rolling mills, laminar cooling systems use high-pressure water jets (0.5-2 MPa) to achieve precise cooling rates (5-50°C/s). For example, cooling a low-alloy steel strip (Q355B) at 15°C/s promotes the formation of fine-grained ferrite-pearlite structures, increasing its yield strength by 30-40 MPa (from 355 MPa to 385-395 MPa) while maintaining good ductility (elongation ≥22%). For high-strength steel (e.g., Q690), a faster cooling rate (35°C/s) forms bainitic structures, further enhancing tensile strength to ≥700 MPa. Incorrect cooling rates in outdated systems can cause inconsistent properties—e.g., strength variations of ±50 MPa—leading to product rejection.​

Surface Quality Preservation​

Well-designed laminar cooling systems in second-hand hot rolling mills ensure uniform water distribution across the strip width (water flow deviation ≤5%). Uneven cooling causes thermal stress, leading to surface cracks (especially in high-carbon steels with ≥0.45% C content) or oxide scale peeling. By optimizing nozzle spacing (100-150mm) and angle (45° to strip surface), operators can reduce surface defect rates by 45-50%. Additionally, using demineralized water (conductivity ≤50 μS/cm) prevents scale formation from mineral deposits, improving the strip’s paint adhesion for automotive applications.​

Production Efficiency Improvement​

Modernized laminar cooling systems in refurbished second-hand hot rolling mills feature automatic cooling rate adjustment based on strip thickness (2mm-25mm) and speed (50-180m/min). This eliminates manual parameter changes, reducing cooling-related downtime by 20-25%. For example, when switching from 5mm to 15mm thick strips, the system automatically increases water flow by 60% and extends cooling length by 30%, maintaining consistent cooling efficiency. Older manual systems often require 10-15 minutes of setup time per change, significantly reducing throughput.​

Maintenance and Optimization Strategies​

To ensure the reliability of laminar cooling systems in second-hand hot rolling mills:​

• Inspect nozzles daily for clogging; clean or replace nozzles with flow rates 15% below the design value.​

• Check water temperature weekly (optimal range: 20-30°C); install a chiller if temperature exceeds 35°C to maintain cooling efficiency.​

• Descale the water tank monthly to remove iron oxide deposits, which can block water pumps.​

• Calibrate the cooling rate sensor quarterly using a thermal imaging camera to verify accuracy.​

For second-hand hot rolling mills processing thin strips (≤3mm), adding an air-knife system after the cooling section removes excess water, preventing water spots and reducing drying time by 60%.​

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