High yield ratio-type high-strength steel sheet and method for manufacturing same

A method of manufacturing a high yield ratio-type high strength steel sheet, comprising:

• preparing a slab containing, in weight %, 0.04 to 0.09% of C, 0.2% or less (excluding 0%) of Si, 2.0 to 3.5% of Mn, 0.3 to 1.2% of Cr, 0.03 to 0.08% of Ti, 0.01 to 0.05% of Nb, 0.0010 to 0.0050% of B, 0.01 to 0.10% of Sol.Al, 0.001 to 0.10% of P, 0.010% or less (including 0%) of S, 0.010% or less (excluding 0%) of N, a remainder of Fe and unavoidable impurities, the contents of the Si, the Cr and the C satisfying the following relationship 1,
• (
• Si
• +
• Cr
• )
• /
• 2
• ⁢
• C
• ≥
• 5
• ,
• [
• Relationship
• ⁢
• 1
• ]
• where each component indicates a weight content;
• hot rolling the slab such that a temperature at a finish rolling outlet side is Ar3 to Ar3+50° C.° C., so as to obtain a hot rolled steel sheet;
• coiling the hot rolled steel sheet at a temperature in a range of 600 to 750° C.;
• cold rolling the hot rolled steel sheet at a reduction ratio of 40 to 70% so as to obtain a cold rolled steel sheet;
• continuously annealing the cold rolled steel sheet at a temperature in a range of Ac3−60 to Ac3° C.;
• performing a primary cooling to a temperature of 600 to 700° C. at a cooling rate of 1 to 20° C./sec after the continuously annealing;
• after the primary cooling, performing a secondary cooling to a temperature of Ms−100 to Ms ° C. at a cooling rate of 5 to 20° C./second;
• and
• after the secondary cooling, performing an overaging treatment,
• wherein contents of the Mn and the B and an annealing temperature (a,° C.) satisfy the following relationship 3,
• 19000
• ⁢
• [
• B
• ]
• +
• 4.5
• [
• Mn
• ]
• +
• 0
• .
• 0
• ⁢
• 6
• ⁢
• a
• ≤
• 1
• ⁢
• 0
• ⁢
• 4
• [
• Relationship
• ⁢
• 3
• ]
• where [B] and [Mn] indicate weight percent of a corresponding element.