1. Main Structural Elements (Guarantee Basic Strength and Weldability)
Carbon (C): Maximum content ≤ 0.18%. It balances the steel's strength and weldability; a lower carbon content avoids hardening cracks during welding, which is critical for structural applications like bridges or sign frames.
Manganese (Mn): Maximum content ≤ 1.60%. It enhances tensile strength and low-temperature impact toughness, preventing brittle fracture in cold environments (aligning with the "K2" grade's -20°C toughness requirement).
Silicon (Si): Maximum content ≤ 0.55%. It boosts oxidation resistance and helps form a dense initial oxide film, laying the foundation for the steel's weather-resistant performance.
2. Weather-Resistant Alloying Elements (Core for Anti-Corrosion)
Copper (Cu): Minimum content ≥ 0.20% (typically 0.20–0.50%). It is the key element for weather resistance, promoting the formation of a tight, adherent patina that blocks further corrosion from air and moisture.
Chromium (Cr): Minimum content ≥ 0.30% (typically 0.30–1.20%). It cooperates with Cu to densify the patina, improving resistance to harsh environments like coastal areas (where salt spray accelerates corrosion).
Nickel (Ni): Maximum content ≤ 0.50% (optional, but ≥0.05% if added). It enhances low-temperature impact toughness and further improves corrosion resistance, making the steel more adaptable to cold regions.
3. Impurity Elements (Strictly Limited to Avoid Defects)
Phosphorus (P): Maximum content ≤ 0.035%. Excessive P makes the steel brittle at low temperatures, so strict limits reduce the risk of sudden fracture.
Sulfur (S): Maximum content ≤ 0.035%. It minimizes welding defects like porosity or hot cracks, ensuring the steel's structural integrity after welding.
Nitrogen (N): Maximum content ≤ 0.015%. It prevents "age hardening" (a process that reduces toughness over time), keeping the steel's mechanical properties stable long-term.
