Causes and Prevention of Steel Spool Shrinkage Deformation

Causes and Prevention of Steel Spool Shrinkage Deformation 

    Without changing the inner diameter,Compatible Expansion and Contraction Spool the combined approach of removable in-furnace internal support+end-section thermal insulation/heat equalization temperature control + surface roughness zoning(thin h?BN spray coating in the mid-section),the permanent shrinkage of the middle section can be stably reduced by approximately 50 – 70%,while significantly improving roundness retention. 

1.Background and Problem 

    Typical case:The steel spool length 2350mm holds a concentrically wound aluminum coil (1,600 mm wide, rolling weight~15 t). After heating/annealing cycles,Commonly occurs OD at mid-section becomes smaller than the ends and roundness degrades, shortening service life and affecting quality 

2.Why Does the Middle Shrink More? (Key Mechanisms) 

  · Hotter and longer at mid-section:The ends dissipate heat rapidly, while the middle section remains at high temperature for longer, softening the material and making it prone to compression-induced collapse. 

  ·Thermal expansion mismatch + friction →radial pressure: aluminum expands more than steel; limited sliding presses the spool inward, peaking near mid-span. 

  · Structurally more soft:The free mid-span section exhibits the lowest stiffness, undergoing early ovalization and transitioning from elastic deformation to irreversible shrinkage." 

3. Quick Self-Check 

  ·Mid-section hardness lower than at the ends. 

  ·Under no-load heating conditions (without coil), shrinkage is negligible or significantly mitigated. 

  ·More severe under N? atmosphere (thin oxide film, high friction, restricted slip). 

  ·Mid-section OD decreases cycle by cycle; ends change little.

4. Overall strategy (Without Altering Inner Diameter) 

5. Practical options and parameters 

5.1 Removable in-furnace internal support(For in-furnace use only，remove upon removal from furnace.) 

  ·Coverage length:Coverage ≥1600 mm + 50–100 mm per side (1700–1800 mm total). 

  · Tensioning and contact pressure:Radial expansion 0.2 – 0.5 mm; target contact pressure 5–10 MPa; runout ≤0.05 mm. 

  · Material/Surface:Alloy steel body:wear-resistant surface + high-temp solid lubricants (h-BN/MoS?). No Coils. 

  ·Usage Process:Insert/expand before heat → keep through heat/soak/cool → depressurize <150 °C and remove. 

  ·Expected reduction: ~40–70% (Enhanced stability with temp/zoning control overlay). 

5.2 End insulation + uniform temp. control (mandatory). 

  ·Targets: axialΔT (Coil section - end portion)≤30–40°C; through-thickness ΔT ≤40 –60 °C. 

  ·Test Method: Procedure: Employ 25–50 mm ceramic fiber boards with metallic reflective layers at terminal sections; implement heating/cooling at 2–4 ℃/min; conduct a 10–20 min heating period prior to reaching the target temperature. 

  ·Monitoring: end/mid/end thermocouples; correct if ΔT exceeds limits. 

  ·Expected reduction range: approximately 15–30%. 

5.3 Surface zoning + thin h?BN at mid-span(Reduce the peak value of mid-section extrusion pressure.)

  ·Middle section (width: 1200–1600 mm): Ra 12–15 μm, Rpk≈2 μm; thinly coated with 5–15 μm h-BN (high purity, temperature resistance >900 ℃). 

  ·Two Ends (200–300 mm each): Ra 20–25 μm, Rpk 3–4 μm, providing grip to prevent overall slippage. 

  ·Slip handling：Prioritize widening high Rpk band or slightly increasing Ra at ends; maintain low friction in middle 

  ·Expected reduction: ~15–25% (more pronounced in N? atmosphere). 

5.4 Optional Enhancement: Internal/External Furnace Idlers/Saddles 

  ·Two heat-resistant idlers/saddles can be arranged in the non-coil area outside the coil edge to share the load, reducing mid-span bending moment and ovalization. 

  ·Expected reduction ~10–20%. 

5.5 Long-term Upgrade:(Inner Diameter Unchanged) 

  ·Increasing wall thickness from 30 to 35 mm enhances stability and reduces high-temp creep rate; combine with 5.1–5.3. 

  ·It is necessary to evaluate the impacts of increased weight and heating time on cycle time and energy consumption. 

5.6 On-Machine Phase: Optimization of tensioning pressure for expandable/contractible reels 

  · Under the premise of ensuring adequate torque transmission capabilities,Use "minimum necessary pressure" (safety factor 1.3–1.5) to reduce inner bore stress. 

  ·Suggest "torque-pressure-slip" calibration to generate on-site performance. curves. 

6.Fast Implementation Roadmap 

7.SOP highlights 

7.1 Removable In-furnace Internal Support 

  ·Inspect → Insert and align → Expand to set position (≈6–8 MPa) → Retain during heating/holding/cooling → Depressurize below 150 °C post-cooling → Extract and inspect. 

  ·Check every 100–200 h; runout ≤0.05 mm. 

7.2 End insulation&soak 

      Baffles (25–50 mm ceramic fiber board + metallic reflective surface) securely fixed; 2– 4 °C/min heat/cool; 10–20 min soak; ΔT alarm for correction. 

7.3 Surface zoning&h?BN 

      Mid: Ra 12–15 μm, Rpk≈2 μm + thin h-BN spray, low-temp cure; Ends: Ra 20–25 μm, Rpk 3–4 μm 

8.Acceptance&Monitoring 

Support 

Email: guangwei@gwspool.com 

Company: GW Precision Technology Co., Ltd. 

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