Ultrahigh-strength steel sheet with excellent bendability and stretch flangeability, and manufacturing method therefor

An ultrahigh strength steel sheet comprising, by weight:

• carbon (C):
• 0.12 to 0.4%, silicon (Si):
• 0.5% or less (excluding 0%), manganese (Mn):
• 2.5 to 4.0%, phosphorous (P):
• 0.03% or less (excluding 0%), sulfur (S):
• 0.012% or less (excluding 0%), aluminum (Al):
• 0.1% or less (excluding 0%), chromium (Cr):
• 1% or less (excluding 0%), titanium (Ti):
• 48/14×[N]˜0.1%, niobium (Nb):
• 0.1% or less (excluding 0%), boron (B):
• 0.005% or less (excluding 0%), nitrogen (N):
• 0.01% or less (excluding 0%), with balance of Fe and other impurities,
• wherein the steel sheet comprises, as a microstructure, by area, martensite:
• 90% or more, and a sum of ferrite and bainite:
• 10% or less,
• wherein a value of M defined by Relational expression 1 below satisfies a range of 100 to 500.
• M=Psize×Pnumber×[C]0.5×[Mn]2×[S] [Relational expression 1]
• In the above Relational expression 1, Psize is an average diameter of inclusions having a diameter of 1 μm or more, and Pnumber is an average number of inclusions having a diameter of 1 μm or more. [C] and [Mn] are average weight % contents of elements in parentheses in the steel sheet, respectively, and [S] is an average ppm content of element in parentheses in the steel sheet.