Patent ID: 7887648

Claim:
An ultrahigh-strength thin steel sheet having excellent corrosion resistance and hydrogen embrittlement resistance, consisting essentially of: 0.10 to 0.25 mass % of C; 1.0 to 3.0 mass % of Si; 1.0 to 3.5 mass % of Mn; 0.15 mass % or less of P; 0.02 mass % or less of S; 1.5 mass % or less of Al; 0.003 to 2.0 mass % of Cr; 0.003 to 0.5 mass % of Cu; 0.003 to 1.0 mass % of Ni; 0.003 to 1.0 mass % of Ti; 0.1 mass % or less of Nb; 0.0005 to 0.01 mass % of B; 0.005 mass % or less of Mo; and a balance of iron and inevitable impurities; wherein: the steel sheet contains 1% or more of residual austenite in terms of an area ratio with respect to a total texture of the steel sheet; and grains of the residual austenite have an average axis ratio (major axis/minor axis) of 5 or more; the grains of the residual austenite have an average minor axis length of 1 μm or less; and the grains of the residual austenite have a nearest-neighbor distance between grains of 1 μm or less.