30CrMnSiA round steel (GB/T 3077) has a composition of 0.27–0.34% carbon (C), 0.90–1.20% silicon (Si), 1.10–1.40% manganese (Mn), 1.10–1.40% chromium (Cr), ≤0.025% phosphorus (P), ≤0.025% sulfur (S).
Silicon (0.90–1.20%) is the key element enhancing high-temperature chemical stability, acting through two mechanisms:
Oxide layer reinforcement: At elevated temperatures (400–600°C), silicon diffuses to the steel surface and reacts with oxygen to form a thin, dense silicon dioxide (SiO₂) layer beneath the chromium oxide (Cr₂O₃) layer. SiO₂ is more stable than Cr₂O₃ at high temperatures, resisting breakdown and preventing oxygen from penetrating to the steel matrix. This dual oxide layer (Cr₂O₃ + SiO₂) provides better oxidation resistance than Cr₂O₃ alone.
Carbide stabilization: Silicon dissolves in the steel matrix and interacts with chromium carbides (Cr₂₃C₆), slowing their coarsening at high temperatures. Coarsened carbides lose their ability to pin dislocations and strengthen the steel-silicon's presence keeps carbides fine and effective, maintaining the steel's chemical structure and strength at elevated temperatures.
Chromium (1.10–1.40%) complements silicon by forming the outer Cr₂O₃ layer, while manganese (1.10–1.40%) enhances hardenability-both support silicon's high-temperature effects without compromising stability.