The Hidden Battle in Steel Leveling: Taming Residual Stress Beyond Surface Flatness

Q: If steel plates pass flatness inspection at the leveling plant but warp after cutting, is residual stress always the culprit?​​

​A:​​ Not necessarily! Thermal stress from uneven expansion during hot cutting (localized temperatures >1,200°C) is often the primary cause. Rapid verification method: Rotate the same plate 90° before cutting. If deformation patterns change, thermal stress dominates; symmetric warping (e.g., left-bending paired with right-bending) indicates residual stress.

​Q: How should responsibility be allocated when post-cutting deformation occurs despite acceptable delivered flatness?​​

​A:​​ Accountability requires tripartite collaboration:

1. ​Steel mills (Primary)​: Non-uniform cooling during thermo-mechanical controlled processing (TMCP) creates widthwise stress gradients (edge stress 60 MPa > center). Levelers lack roll-bending capabilities to fully correct this.
2. 2.​Leveling plants (Secondary)​: High-strength steels (e.g., DP780) demand precise thermal control—a 20% line-speed increase causes ±8% yield strength fluctuation, necessitating dynamic reduction compensation.
3. ​End users (Critical)​: Blast cleaning before cutting removes oxides that exacerbate thermal deformation. Pre-cut stress-relief annealing further mitigates risks.

​Q: What is the core mechanism for residual stress elimination during leveling?​​

​A:​​ Stratified plastic deformation achieves stress redistribution:

• Surface layer (>80% plastic deformation)​: Roller pressure forcibly restructures grain orientation, disrupting original stress patterns.
• ​Core layer (elastic state)​: Partially inherits stress but acts as a stabilized "stress reservoir," preventing localized concentration.

​Q: Does increasing reduction linearly reduce residual stress?​​

​A:​​ No—it follows a U-curve relationship:

• Under-reduction (plastic deformation <80%)​: Leaves original stress intact.
• Over-reduction: Introduces new bending stress exceeding eliminated stress. Case study: Increasing reduction from 1.2mm to 1.8mm for Q355 steel raisedresidual stress by 15%.

​Q: How does equipment condition affect stress distribution?​​

​A:​​ Mechanical defects directly induce stress imbalance:

• Roll misalignment → Unilateral stress spikes.
• Worn rollers → Wave-like stress distribution.
• Insufficient frame rigidity → 0.12mm frame deflection under 3,000-ton loads transfers as internal stress (15% of plate thickness).

​Q: How do plants quantify residual stress control given measurement limitations?​​

​A:​​ Industry relies on empirical methods due to detection challenges:

• X-ray diffraction: Limited to 0.1mm surface depth.
• Ultrasonic testing: Requires extensive calibration.
• Practical test: Cutting a 30mm strip along the rolling direction. Warping >3mm/m triggers immediate parameter adjustments.

​Q: What integrated solutions balance precision and adaptability?​​

​A:​​ Synergizing hardware, data, and process innovation:

• ​Precision hardware: Laser-guided 9-roller systems enable 0.15mm/m dynamic compensation; DLC diamond-coated rollers triple wear resistance.
• Data-driven intuition: Master operators compile "bend force-rebound maps" from 100,000+ datasets for instinctive tuning.
• ​Thermal-process synergy: Integrating induction heating into high-strength leveling lines reduced cutting distortion from 12% to 1.5% in one implementation.

Baohui Steel Limited

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