Ultrahigh strength cold-rolled steel sheet and manufacturing method thereof

An ultrahigh strength cold-rolled steel sheet, comprising, in percentage by weight:

• C:
• 0.25 to 0.4%;
• Si:
• 0.5% or less, excluding 0;
• Mn:
• 3.53 to 4.0%;
• P:
• 0.03% or less, excluding 0;
• S:
• 0.015% or less, excluding 0;
• Al:
• 0.1% or less, excluding 0;
• Cr:
• 1% or less, excluding 0;
• Ti:
• 48/14*[N] or more and 0.1% or less;
• Nb:
• 0.1% or less, excluding 0;
• B:
• 0.005% or less, excluding 0;
• N:
• 0.0042% to 0.01%;
• and
• a balance of Fe and other unavoidable impurities,
• wherein a wave height (ΔH) is 3 mm or less in an edge portion after cutting a steel sheet in the width direction thereof to provide a sample 1000 mm long in the longitudinal direction,
• wherein the cold-rolled steel sheet is an as-rolled cold-rolled steel sheet,
• wherein the as-rolled cold-rolled steel sheet has a microstructure consisting of 90% or more (including 100%) of martensite, and one or two of 10% or less (including 0%) of ferrite and bainite, and
• wherein the as-rolled cold-rolled steel sheet has:
• a tensile strength of 1700 MPa or more, and
• a yield strength of 1200 MPa or more.