High-strength cold rolled steel sheet having high hole expansion ratio, highstrength hot-dip galvanized steel sheet, and manufacturing methods therefor

1. A cold rolled steel sheet comprising:

by weight percent (wt %), 0.17 to 0.21% of carbon (C), 0.3 to 0.8% of silicon (Si), 2.7 to 3.3% of manganese (Mn), 0.3 to 0.7% of chromium (Cr), 0.01 to 0.3% of aluminum (Al), 0.01 to 0.03% of titanium (Ti), 0.001 to 0.003% of boron (B), 0.04% or less of phosphorus (P), 0.02% or less of sulfur (S), 0.01% or less of nitrogen (N), and a balance of iron (Fe) and other inevitable impurities, wherein the contents of carbon (C), silicon (Si), and aluminum (Al) satisfy Equation 1 below,

[C]+([Si]+[Al])/5≤0.35%  [Equation (1)]

wherein [C], [Si], [Al] refer to weight percents of C, Si, and Al, respectively; and

a microstructure consisting of: by area fraction, 3 to 7% of retained austenite, 5 to 15% of fresh martensite, and 5% or less (including 0%) of ferrite; by volume fraction, 1 to 3% of a cementite phase as a precipitate; and a balance of bainite or tempered martensite, wherein the cementite phase is distributed at least one of between laths of the bainite, in laths of the tempered martensite, and in a grain boundary of the tempered martensite.

2. The cold rolled steel sheet of claim 1 , wherein the cold rolled steel sheet further includes 0.1% or less of copper (Cu), 0.1% or less of nickel (Ni), and 0.1% or less of molybdenum (Mo).

3. The cold rolled steel sheet of claim 1 , wherein the cold rolled steel sheet further includes 0.03% or less of niobium (Nb) and 0.01% or less of vanadium (V).

4. The cold rolled steel sheet of claim 1 , wherein the cold rolled steel sheet has a tensile strength of 1180 MPa or more, a yield ratio of 0.65 to 0.85, a hole expansion ratio of 25% or more (HER), and an elongation of 5 to 13%.

5. A hot-dip galvanized steel sheet comprising: the cold rolled steel sheet of claim 1 and a hot-dip zinc plating layer on a surface thereof.