Cold rolled steel sheet and manufacturing method therefor

A cold-rolled steel sheet, comprising by weight:

• C:
• 0.10 to 0.16%, Si:
• 0.05 to 0.495%, Al:
• 0.01 to 0.18%, Mn:
• 2.4 to 3.5%, Cr:
• 0.05 to 0.8%, Mo:
• 0.05 to 0.8%, B:
• 0.0001 to 0.003%, Nb:
• 0.005 to 0.07%, Ti:
• 0.005 to 0.07%, with a remainder of Fe and other unavoidable impurities,
• wherein the cold-rolled steel sheet has a tensile strength (TS):
• 1185 to 1400 MPa, and a yield ratio (YS/TS):
• 0.70 or more,
• wherein the cold-rolled steel sheet has a microstructure, the microstructure including, by area:
• fresh martensite:
• 1 to 11%, one or two of tempered martensite and bainite:
• 80 to 97%, retained austenite:
• 1 to 9%, and ferrite:
• 7% or less (including 0%), and includes
• at least one of MC, M(C,N), and their composite precipitates (M=Nb, Ti, Si, Cr, Mo, Fe) with an average size of 20 nm or less in a fraction of 50/μm2 or more, and
• wherein the cold-rolled steel sheet satisfies following Relational Expressions 1 to 3:
• [
• Relational
• ⁢
• Expression
• ⁢
• 1
• ]
• 1120
• ≤
• X
• =
• 2301
• ×
• C
• +
• 287
• ×
• Mn
• +
• 1533
• ×
• Nb
• +
• 288
• ×
• Cr
• -
• 71
• ×
• Si
• -
• 84.1
• ×
• A
• ⁢
• l
• ≤
• 1380
• 0.25
• ≤
• Y
• =
• C
• +
• Si
• /
• 30
• +
• Mn
• /
• 20
• ≤
• 0.36
• [
• Relational
• ⁢
• Expression
• ⁢
• 2
• ]
• 3420
• ≤
• X
• /
• Y
• ≤
• 4360
• [
• Relational
• ⁢
• Expression
• ⁢
• 3
• ]
• where a content of each alloy element in the above Relational Expressions 1 to 3 is % by weight.