HIGH-STRENGTH STEEL WITH EXCELLENT TOUGHNESS OF HEAT-AFFECTED ZONE AND MANUFACTURING METHOD THEREOF

1 . A high-strength steel having excellent toughness in a heat-affected zone, comprising:

in weight %, C: 0.16 to 0.20%, Mn: 1.0 to 1.5%, Si: 0.3% or less (excluding O), Al: 0.005 to 0.5%, P: 0.02% or less, S: 0.01% or less, Ti: 0.005 to 0.02%, Nb: 0.01 to 0.1%, and N: 0.006 to 0.01%; and

at least one selected from the group consisting of Ca: 0.006% or less, V: 0.03% or less, Ni: 2.0% or less, Cu: 1.0% or less, Cr: 1.0% or less, and Mo: 1.0% or less, and a balance of Fe and unavoidable impurities,

wherein a microstructure of the high-strength steel is composed of a ferrite-pearlite composite structure, and

after welding and a stress relief heat treatment, in the microstructure, 1.27×10 6 or more of precipitates of 100 nm or less are present per 1 mm 2 , and 900 or more of precipitates are distributed in a single crystal grain.

2 . The high-strength steel having excellent toughness in a heat-affected zone of claim 1 , wherein the precipitates are Nb-based carbides.

3 . The high-strength steel having excellent toughness in a heat-affected zone of claim 1 , wherein the steel has a surface crack sensitivity index (Cs) of 0.3 or less, defined by the following relational expression 1,

Cs=(71.4×[C] 2 )−(30.3×[C])+3.32,  [Relational Expression 1]

where [C] is a weight % content of a corresponding component.

4 . The high-strength steel having excellent toughness in a heat-affected zone of claim 1 , wherein the steel has free-N defined by the following relational expression 2, exceeding 0,

Free-N=[N]−{([Ti]/47.887)×14.01}−{([B]/10.81)×14.01},  [Relational Expression 2]

where [N], [Ti] and [B] are weight % contents of corresponding components.

5 . The high-strength steel having excellent toughness in a heat-affected zone of claim 1 , wherein the steel has a tensile strength of 500 MPa or higher and a Charpy impact absorption energy of 150 J or higher even after stress relief heat treatment.

6 . The high-strength steel having excellent toughness in a heat-affected zone of claim 1 , wherein in the heat-affected zone of the steel, a martensite austenite constituent (MA) is 3% or less in area fraction.

7 . The high-strength steel having excellent toughness in a heat-affected zone of claim 1 , wherein in the steel, the heat-affected zone has Charpy impact absorption energy of 100 J or higher at 0° C. after large heat input welding.

8 . A method of manufacturing a high-strength steel having excellent toughness in a heat-affected zone, comprising:

preparing a steel slab,

the steel slab including,

in weight %, C: 0.16 to 0.20%, Mn: 1.0 to 1.5%, Si: 0.3% or less (excluding O), Al: 0.005 to 0.5%, P: 0.02% or less, S: 0.01% or less, Ti: 0.005 to 0.02%, Nb: 0.01 to 0.1%, and N: 0.006 to 0.01%, and

at least one selected from the group consisting of Ca: 0.006% or less, V: 0.03% or less, Ni: 2.0% or less, Cu: 1.0% or less, Cr: 1.0% or less and Mo: 1.0% or less, and a balance of Fe and unavoidable impurities;

heating the steel slab to a temperature in a range of 1050 to 1250° C.;

hot rolling the heated steel slab at a temperature of hot finish rolling of 910° C. or lower; and

performing cooling at a cooling rate of 20° C./Hr or less after the hot rolling.

9 . The method of manufacturing a high-strength steel having excellent toughness in a heat-affected zone of claim 8 , wherein the hot finish rolling is performed at a temperature in a range of 850 to 910° C.

10 . The method of manufacturing a high-strength steel having excellent toughness in a heat-affected zone of claim 8 , wherein the cooling is performed at a temperature in a range of 800 to 435° C. at a cooling rate of 20° C./Hr or less.