Ultrahigh-strength thin steel sheet

An ultrahigh-strength thin steel sheet having excellent corrosion resistance and hydrogen embrittlement resistance, consisting essentially of:

• more than 0.25 but not more than 0.60 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;
• 0.005 to 0.1 mass % of Nb;
• 0.0002 to 0.01 mass % of B;
• 0.005 mass % or less of Mo;
• and
• a balance of iron and inevitable impurities;
• wherein:
• a metallographic texture of the steel sheet after tensile processing at a working rate of 3% contains 1% or more of residual austenite in terms of an area ratio with respect to the metallographic texture;
• in the metallographic texture, 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.

An ultrahigh-strength thin steel sheet having excellent corrosion resistance and hydrogen embrittlement resistance, consisting 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;
• 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.

An ultrahigh-strength thin steel sheet having excellent corrosion resistance and hydrogen embrittlement resistance, consisting of:

• more than 0.25 but not more than 0.60 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;
• 0.005 to 0.1 mass % of Nb;
• 0.0002 to 0.01 mass % of B;
• and
• a balance of iron and inevitable impurities;
• wherein:
• a metallographic texture of the steel sheet after tensile processing at a working rate of 3% contains 1% or more of residual austenite in terms of an area ratio with respect to the metallographic texture;
• in the metallographic texture, 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.