Cold rolled steel sheet and method of manufacturing thereof

A method of manufacturing a cold rolled steel sheet, comprising:

• heating a slab at 1100 to 1300° C., the slab including, by weight %, carbon (C):
• 0.23 to 0.31%, silicon (Si):
• 0.03 to 0.30%, manganese (Mn):
• 0.60 to 1.30%, chromium (Cr):
• 0.05 to 0.50%, molybdenum (Mo):
• 0.05 to 0.50%, boron (B):
• 0.0005 to 0.005%, phosphorus (P):
• 0.04% or less (excluding 0%), sulfur(S):
• 0.003% or less (excluding 0%), nitrogen (N):
• 0.01% or less (excluding 0%), aluminum (Al):
• 0.015 to 0.10%, titanium (Ti):
• 0.01 to 0.05%, and a balance of Fe and other inevitable impurities, and satisfying the following relational expressions 1 to 4;
• finishing hot-rolling the heated slab at Ar3 to Ar3+120° C. to obtain a hot-rolled steel sheet;
• coiling the hot-rolled steel sheet at Ms to 600° C.;
• cold-rolling the coiled hot-rolled steel sheet at a cold reduction ratio of 35 to 70% to obtain a cold rolled steel sheet;
• continuously annealing the cold rolled steel sheet at Ac3+10° C. to Ac3+80° C.;
• primarily cooling the continuously annealed cold rolled steel sheet to a primary cooling end temperature (T1) of 670 to 750° C. at an average cooling rate of 1 to 6° C./s;
• secondarily cooling the primarily cooled cold rolled steel sheet to a secondary cooling end temperature (T2) of 50 to 200° C. at an average cooling rate of 40 to 90° C./s;
• reheating the secondarily-cooled cold rolled steel sheet to an overaging treatment temperature (H) of 100 to 280° C., and then performing an overaging treatment for 5 to 12 minutes;
• and
• subjecting the overaging cold rolled steel sheet to temper rolling with reduction force of 500 to 1,000 tons,
• wherein the primary cooling end temperature (T1)−secondary cooling end temperature (T2) is controlled to be 750° C. or lower, and
• the overaging treatment temperature (H)−secondary cooling end temperature (T2) is controlled to be 50° C. or higher,
• 95
• ≤
• X
• =
• 48.8
• +
• 49
• ⁢
• log
• ⁢
• C
• +
• 35.1
• Mn
• +
• 25.9
• Si
• +
• 14.5
• Ni
• +
• 9.6
• Cu
• +
• 76.5
• Cr
• +
• 105.9
• Mo
• +
• 1325
• ⁢
• Nb
• +
• 10000
• ⁢
• B
• ≤
• 150
• [
• Relational
• ⁢
• Expression
• ⁢
• 1
• ]
• 19
• ≤
• Y
• =
• 13
• +
• 10
• ⁢
• C
• +
• 15
• ⁢
• Si
• +
• 5
• ⁢
• (
• Mn
• +
• Cr
• +
• Mo
• )
• ≤
• 30
• [
• Relational
• ⁢
• Expression
• ⁢
• 2
• ]
• Z
• =
• C
• +
• Mn
• /
• 20
• +
• Si
• /
• 30
• +
• 2
• ⁢
• P
• +
• 4
• ⁢
• S
• ≤
• 0.4
• [
• Relational
• ⁢
• Expression
• ⁢
• 3
• ]
• 0.16
• ≤
• Y
• /
• X
• ≤
• 0.23
• [
• Relational
• ⁢
• Expression
• ⁢
• 4
• ]