HIGH YIELD RATIO-TYPE HIGH-STRENGTH STEEL SHEET AND METHOD FOR MANUFACTURING SAME

1 . A high yield ratio-type high strength steel sheet comprising:

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 Si, Cr and C satisfying the following relationship 1,

(Si+Cr)/2C≥5  [Relationship 1]

where each component indicates a weight content; and

as a microstructure, in an area fraction, 50% or more of bainite, 20 to 40% of tempered martensite, and residual ferrite, in which a hardness ratio of a bainite phase and a tempered martensite phase at a ¼t point of a thickness of a steel sheet (where t is the thickness of the steel sheet (mm)), represented by the following relationship 2, is 1.3 or less,

( HB/HTM )≤1.3  [Relationship 2]

where HB is a bainite hardness, and HTM is a tempered martensite hardness.

2 . The high yield ratio-type high strength steel sheet of claim 1 , wherein the tempered martensite has an average particle diameter of 2 μm or less, the bainite has an average particle diameter of 3 μm or less, and an area fraction of a bainite structure having a particle size of more than 3 μm is 5% or less.

3 . The high yield ratio-type high strength steel sheet of claim 2 , wherein a distribution density of a nano precipitate having a diameter of 10 nm or less among the bainite and tempered martensite structures is 150 pieces/μm 2 or more.

4 . The high yield ratio-type high strength steel sheet of claim 1 , wherein the steel sheet has a tensile strength of 980 MPa or higher, a yield strength of 780 MPa or higher, a yield ratio of 0.8 or higher, and a HER value of 50% or higher.

5 . The high yield ratio-type high strength steel sheet of claim 1 , wherein the steel sheet is provided with a zinc-based plating layer disposed on at least one surface thereof.

6 . 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)/2C≥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 Ar 3 to Ar 3 +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 Ac 3 -60 to Ac 3 ° 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]+0.06a≤104  [Relationship 3]

where [B] and [Mn] indicate weight percent of a corresponding element.

7 . The method of manufacturing a high yield ratio-type high strength steel sheet of claim 6 , wherein an aging time in the overaging treatment is controlled such that in an entire structure, an area fraction of bainite is 50% or more, and an area fraction of tempered martensite is 20 to 40%.

8 . The method of manufacturing a high yield ratio-type high strength steel sheet of claim 6 , further comprising a zinc-based plating operation of forming a zinc-based plating layer on at least one surface of a steel sheet after the overaging treatment.

9 . The method of manufacturing a high yield ratio-type high strength steel sheet of claim 6 , further comprising an operation of skin-pass rolling at an elongation of 0.1 to 1.0% after the overaging treatment or after the zinc-based plating.