1.How is the all-hydrogen bell-type furnace operated in practice?
This is the most common annealing method for cold-rolled coils. Core atmosphere: 100% pure hydrogen (H₂)
Formulation: H₂ content 95%–100%, the remainder N₂.
Purity requirements: Pure hydrogen or high-purity hydrogen must be used (dew point ≤ -60℃, oxygen content < 2ppm).
Why not use a nitrogen-hydrogen mixture?
Hydrogen's thermal conductivity is 7 times that of nitrogen. Bell-type furnaces rely on convection heating; the purer the hydrogen, the faster the heating and the more uniform the furnace temperature. Introducing a large amount of nitrogen will cause a lag in the heating of the core of the steel coil, resulting in uneven hardness within the coil.
Practical logic:
Large cleaning stage: After vacuuming, introduce a small amount of N₂ to break the vacuum, then immediately introduce a large flow of H₂ to drive out the N₂.
Heating stage: Maintain pressure with pure H₂ (furnace pressure 10~30mbar), actively introducing nitrogen is not allowed.
Cooling stage: To save costs, N₂ can be introduced for auxiliary cooling after the fire is shut down, but air must not be mixed in above 650°C.
2.What is the standard composition ratio for a continuous annealing furnace?
H₂: 3%–5% (10%–15% used in some high-strength steels)
N₂: 95%–97%
O₂ is strictly prohibited (controlled below 10 ppm)
3.Why is there so little hydrogen?
Explosion Prevention: Continuous furnaces have multiple openings, keeping 3%~5% H₂ within a safe range (below 4% non-flammable? Actually, it must be below 1%? – Correction: The lower explosive limit of H₂ in air is 4%, but in a N₂ environment, below 5% is safe). The furnace inlet and outlet are sealed with fire curtains; using low-hydrogen content is safer.
Surface Control: 3%~5% H₂ is sufficient to reduce the extremely thin oxide film on the strip surface, giving it a silvery-gray appearance.
Dew Point Determines Surface Condition: This is the core technique.
For a bright surface (ordinary cold-rolled sheet): the dew point must be ≤ -40℃, and the furnace must be extremely dry.
For a plating-compatible surface (hot-dip galvanized substrate): the dew point must be controlled between 0℃ and +10℃! Intentionally allowing the strip surface to undergo slight oxidation forms SiO₂/MnO oxides, inhibiting excessive reduction of pure iron and improving zinc layer adhesion.
4.What is the practical logic?
Furnace running thin material (0.3mm): Fast pace, dew point is prone to being too high, requiring increased hydrogen flow rate.
Furnace running high-strength steel (DP980): Prone to surface oxidation, requiring appropriate increase of H₂% and reduction of dew point.
5.How to make a quick assessment on-site?
Blue/Darkening of the Edges of the Steel Coil After Removal
→ High Atmosphere Dew Point (Water Not Drained Completely) or Furnace Leak (Oxygen Ingress).
→ Solution: Increase Hydrogen Flow Rate, Check Nitrogen Purity.
Interlayer Adhesion in Steel Coil
→ Excessive N₂ in the Pure Hydrogen Furnace, leading to Uneven Heat Transfer and Localized Overheating and Melting.
→ Solution: Revert to Pure Hydrogen, Reduce Heating Rate.
Surface Carbon Black/Hyperpipeline
→ Residual Oxygen Introduced into the N₂ Pipeline, Combustion with Rolling Oil, and Carbon Deposits.
→ Solution: Check Furnace Airtightness, Increase Hydrogen Content and Clean.