High-strength steel having excellent fracture initiation resistance and fracture propagation arrestability at low temperature and method of manufacturing the same

1. A high-strength steel comprising:

by wt %, 0.02 to 0.09% of C, 0.005 to 0.019% of Si, 0.5 to 1.68% of Mn, 0.001 to 0.035% of Sol.Al, 0.03% or less of Nb, excluding 0%, 0.01% or less of V, excluding 0%, 0.001 to 0.02% or Ti, 0.01 to 1.0% of Cu, 0.01 to 2.0% of Ni, 0.01 to 0.03% of Cr, 0.001 to 0.5% of Mo, 0.0002 to 0.005% of Ca, 0.001 to 0.006% of N, 0.02% or less of P, excluding 0%, 0.003% or less of S, excluding 0%, 0.002% or less of 0, excluding 0%, and a balance of Fe and inevitable impurities,

wherein the high-strength steel satisfies Relational Expression 1 below,

5*C+Si+10*sol.Al≤0.6  Relational Expression 1

where each element symbol indicates a content of each element by wt %,

wherein a microstructure comprises a sum of polygonal ferrite and acicular ferrite of 50 area % or higher, and comprises a martensite-austenite multiphase, an MA phase, of 3.5 area % or lower, and

wherein the steel comprises inclusions, and a number of inclusions having a size of 10 μm or greater is 11 count/cm 2 or lower.

2. The high-strength steel of claim 1 , wherein an average size of the MA phase measured in equivalent circular diameter is 2.5 μm or less.

3. The high-strength steel of claim 1 , wherein polygonal ferrite and acicular ferrite are not process-hardened by a hot-rolling process.

4. The high-strength steel of claim 1 , wherein the steel has yield strength of 355 MPa or higher, an impact energy value of 300 J or higher at −60° C., and a Crack Tip Opening Displacement Test (CTOD) value of 0.3 mm or higher at −40° C.

5. The high-strength steel of claim 1 , wherein the steel has tensile strength of 450 MPa or higher.

6. A method of manufacturing a high-strength steel, the method comprising:

preparing a slab comprising, by wt %, 0.02 to 0.09% of C, 0.005 to 0.019% of Si, 0.5 to 1.68% of Mn, 0.001 to 0.035% of Sol.Al, 0.03% or less of Nb, excluding 0%, 0.01% or less of V, excluding 0%, 0.001 to 0.02% or Ti, 0.01 to 1.0% of Cu, 0.01 to 2.0% of Ni, 0.01 to 0.03% of Cr, 0.001 to 0.5% of Mo, 0.0002 to 0.005% of Ca, 0.001 to 0.006% of N, 0.02% or less of P, excluding 0%, 0.003% or less of S, excluding 0%, 0.002% or less of 0, excluding 0%, and a balance of Fe and inevitable impurities, and satisfying Relational Expression 1 below,

5*C+Si+10*sol.Al≤0.6  Relational Expression 1

where each element symbol indicates a content of each element by wt %; heating the slab to 1000 to 1200° C.;

finish-hot-rolling the heated slab at 680° C. or higher and obtaining a hot-rolled steel sheet;

cooling the hot-rolled steel sheet, and

a tempering process in which the cooled hot-rolled steel sheet is heated to 450 to 700° C., and the heated hot-rolled steel sheet is maintained for 1.3*t+10 minutes to 1.3*t+200 minutes and is cooled, where t is a value of a thickness of the hot-rolled steel sheet measured in mm unit,

wherein the cooling comprises cooling the hot-rolled steel sheet to a cooling terminating temperature of 300 to 650° C. at a cooling rate of 2 to 10° C./s, and

wherein the preparing the slab comprises:

adding Ca or Ca alloys to molten steel in a final stage of a secondary refining process; and

performing bubbling and refluxing processes using an Ar gas for at least three minutes after adding Ca or Ca alloys.