20# round steel (GB/T 699) has a low-carbon composition: 0.17–0.24% carbon (C), 0.17–0.37% silicon (Si), 0.35–0.65% manganese (Mn), ≤0.035% phosphorus (P), ≤0.035% sulfur (S), and trace chromium (Cr ≤0.25%)/copper (Cu ≤0.25%).
Its chemical ductility stems primarily from low carbon content: 0.17–0.24% C minimizes carbide formation, as excess carbon would create hard, brittle Fe₃C particles that disrupt the iron matrix. Silicon acts as a mild deoxidizer, reducing oxygen-induced inclusions (e.g., SiO₂) that could create microcracks and reduce ductility. Manganese (0.35–0.65%) enhances ductility indirectly by reacting with sulfur to form ductile manganese sulfides (MnS) instead of brittle iron sulfides (FeS), which would cause hot shortness. Phosphorus is strictly limited to ≤0.035% because it segregates at grain boundaries, weakening intergranular bonds and reducing ductility-especially at low temperatures. Trace Cr/Cu have negligible effects on ductility but slightly improve atmospheric corrosion resistance by forming thin oxide films.