20Cr2Ni4A round steel (GB/T 3077) has a composition of 0.17–0.23% carbon (C), 0.17–0.37% silicon (Si), 0.30–0.60% manganese (Mn), 1.80–2.20% chromium (Cr), 3.60–4.00% nickel (Ni), ≤0.025% phosphorus (P), ≤0.025% sulfur (S).
Nickel (3.60–4.00%) is the core element enhancing cryogenic toughness (below -40°C) via two atomic-level mechanisms:
Ductile-brittle transition temperature (DBTT) suppression: Most ferritic steels become brittle below their DBTT, but nickel lowers 20Cr2Ni4A's DBTT to -70°C or lower. This is because Ni atoms disrupt the formation of brittle martensite at low temperatures-instead, the steel retains a ductile austenitic-ferritic mixed structure that can absorb energy without fracturing.
Grain boundary impurity binding: At cryogenic temperatures, impurities like phosphorus (P) tend to segregate at grain boundaries and weaken them. Nickel has a high affinity for P, forming stable Ni₃P compounds that prevent P from clustering at boundaries. This strengthens intergranular bonds, eliminating "cold brittleness" that plagues low-nickel steels (e.g., 20Cr with <1% Ni).
Chromium (1.80–2.20%) provides base strength by forming fine carbides, while ultra-low P/S limits (≤0.025%) further reduce brittle inclusions-together, these elements support nickel's cryogenic toughness effects.