Structural steel having excellent brittle crack propagation resistance, and manufacturing method therefor

1. A structural steel, the structural steel comprising, by wt %, 0.02 to 0.12% of C, 0.01 to 0.8% of Si, 1.7 to 2.5% of Mn, 0.005 to 0.5% of Al, and the balance of Fe and inevitable impurities, wherein an outer surface part and an inner center part thereof are microstructurally distinguished in a thickness direction, the surface part includes tempered bainite as a base structure, fresh martensite as a second structure, and austenite as a residual structure,

wherein an average particle diameter of the tempered bainite is 3 μm or less, excluding 0 μm, an average particle diameter of the fresh martensite is 3 μm or less, excluding 0 μm, and

wherein a Kca value of the surface part based on −10° C. in a temperature gradient ESSO test is 6000 N/mm 3/2 or greater, and a high angle grain boundary fraction of the surface part is 45% or greater.

2. The structural steel of claim 1 , wherein the surface part is divided into an upper surface part on an upper side and a lower surface part on a lower side, and the upper surface part and the lower surface part each have a thickness of 3 to 10% of a thickness of the steel.

3. The structural steel of claim 1 , wherein the base structure and the second structure are included in a volume fraction of 95% or greater in the surface part.

4. The structural steel of claim 1 , wherein the residual structure is included in the surface part at a volume fraction of 5% or less.

5. The structural steel of claim 1 , wherein the center part includes acicular ferrite.

6. The structural steel of claim 5 , wherein an average particle diameter of the acicular ferrite is 10 to 20 μm.

7. The structural steel of claim 1 , wherein the steel further includes, by wt %, one or two or more of 0.02% or less of P, 0.01% or less of S, 0.005 to 0.10% of Nb, 0.001% or less of B, 0.005 to 0.1% of Ti, 0.0015 to 0.015% of N, 0.05 to 1.0% of Cr, 0.01 to 1.0% of Mo, 0.01 to 2.0% of Ni, 0.01 to 1.0% of Cu, 0.005 to 0.3% of V, and 0.006% or less of Ca.

8. The structural steel of claim 7 , wherein the steel has 2% or greater of a Mn equivalent represented by Mn eq of Equation 1 below:

Mn eq =[Mn]+1.5[Cr]+3[Mo]+[Si]/3+[Ni]/3+[Cu]/2+124[B]  [Equation 1]

wherein [Mn], [Cr], [Mo], [Si], [Ni], [Cu] and [B] refer to contents of Mn, Cr, Mo, Si, Ni, Cu, and B, respectively, and refer to 0 when the corresponding steel composition is not included.

9. The structural steel of claim 1 , wherein a tensile strength of the steel is 570 MPa or greater.

10. A method for manufacturing the structural steel of claim 1 , the method comprising:

reheating a slab including, by wt %, 0.02 to 0.12% of C, 0.01 to 0.8% of Si, 1.7 to 2.5% of Mn, 0.005 to 0.5% of Al, and the balance of Fe and inevitable impurities in a temperature range of 1050 to 1250° C.;

rough rolling the slab at a temperature of Tnr to 1150° C.;

first cooling the rough rolled steel to a temperature of Ms to Bs at a cooling rate of 5° C./s or higher, based on a temperature of the surface part of the rough rolled steel;

heat recuperating the steel by maintaining a surface part of the first-cooled steel to a temperature range of (Ac 1 +40° C.) to (Ac 3 −5° C.) by heat recuperation;

finish rolling the heat-recuperated steel; and

second cooling the finish rolled steel to a temperature range of Ms to Bs° C. at a cooling rate of 5° C./s or higher; thereby producing the structural steel of claim 1 .

11. The method of claim 10 , wherein the slab further includes, by wt %, one or two or more of 0.02% or less of P, 0.01% or less of S, 0.005 to 0.10% of Nb, 0.001% or less of B, 0.005 to 0.1% of Ti, 0.0015 to 0.015% of N, 0.05 to 1.0% of Cr, 0.01 to 1.0% of Mo, 0.01 to 2.0% of Ni, 0.01 to 1.0% of Cu, 0.005 to 0.3% of V, and 0.006% or less of Ca.

12. The method of claim 10 , wherein the slab has 2% or greater of an Mn equivalent represented by Mn eq of Equation 1 below:

Mn eq =[Mn]+1.5[Cr]+3[Mo]+[Si]/3+[Ni]/3+[Cu]/2+124[B]  [Equation 1]

wherein [Mn], [Cr], [Mo], [Si], [Ni], [Cu] and [B] refer to contents of Mn, Cr, Mo, Si, Ni, Cu, and B, respectively, and refer to 0 when the corresponding steel composition is not included.

13. The method of claim 10 , wherein the surface part is a region to a depth of 3 to 10% compared to a thickness of the steel from an outer surface of the steel toward a center of the steel.

14. The method of claim 10 , wherein the first cooling is performed immediately after the rough rolling.

15. The method of claim 10 , wherein a starting temperature of the first cooling is Ae 3 +100° C. or lower with respect to a temperature of the surface part of the steel.

16. The method of claim 10 , wherein a temperature of the finish rolling is Bs to Tnr° C.