Q235 round steel (GB/T 700) has a low-alloy composition: 0.12–0.20% carbon (C), 0.17–0.37% silicon (Si), 0.30–0.70% manganese (Mn), ≤0.045% phosphorus (P), ≤0.045% sulfur (S), and trace elements (Cr ≤0.30%, Ni ≤0.30%, Cu ≤0.30%).
Carbon (0.12–0.20%) provides minimal strengthening but keeps the steel chemically ductile, as higher carbon would increase carbide formation. Silicon acts as a deoxidizer, lowering oxygen content to reduce oxide inclusions that disrupt chemical uniformity. Manganese (0.30–0.70%) improves hardenability but has a limited effect on chemical resistance. The most impactful elements here are impurities: phosphorus (≤0.045%) can segregate at grain boundaries, causing cold brittleness by weakening intergranular bonds, while sulfur (≤0.045%) forms iron sulfides that create brittle zones. Trace chromium, nickel, and copper slightly improve atmospheric corrosion resistance but are present in too low concentrations to significantly alter the steel's core chemical properties.