1. Base Elements (Essential for Strength & Formability)
Carbon (C): ≤0.12%
Kept low to ensure good weldability and ductility while providing sufficient strength.
Silicon (Si): 0.15–0.50%
Enhances strength and deoxidation during steelmaking; contributes to rust layer stability.
Manganese (Mn): 0.90–1.50%
Improves hardenability and toughness; balances sulfur's negative effects.
Phosphorus (P): ≤0.030%
Controlled to avoid brittleness but slightly elevated compared to carbon steels for corrosion resistance.
Sulfur (S): ≤0.025%
Minimized to prevent hot cracking during welding.
2. Weathering Alloying Elements (Critical for Corrosion Resistance)
Copper (Cu): 0.25–0.55%
The most critical element for atmospheric corrosion resistance; promotes dense, adherent rust layer formation.
Chromium (Cr): 0.40–0.80%
Enhances oxidation resistance and stabilizes the protective patina.
Nickel (Ni): ≤0.65%
Improves toughness and corrosion resistance, especially in acidic or marine environments.
(Optional additions in some grades: Vanadium (V) or Molybdenum (Mo) for micro-alloy strengthening.)
3. Impurities & Residuals (Strictly Controlled)
Aluminum (Al): Added for deoxidation (typically ≤0.015%).
Nitrogen (N): Limited to ≤0.015% to avoid aging brittleness.
4. Carbon Equivalent (CEV) & Weldability
CEV (IIW formula): ~0.40–0.45
Higher than Q235NH/Q355NH due to alloy content; may require preheating (100–150°C) for thick-section welding.
