High-strength steel material having enhanced resistance to crack initiation and propagation at low temperature and method for manufacturing the same

1. A high-strength steel material comprising, by weight, carbon (C): 0.01% to 0.07%, silicon (Si): 0.002% to 0.2%, manganese (Mn): 1.7% to 2.5%, Sol. aluminum (Sol.Al): 0.001% to 0.035%, niobium (Nb): 0.03% or less (not including 0%), vanadium (V): 0.01% or less (not including 0%), titanium (Ti): 0.001% to 0.02%, copper (Cu): 0.01% to 1.0%, nickel (Ni): 0.01% to 2.0%, chromium (Cr): 0.01% to 0.5%, molybdenum (Mo): 0.001% to 0.5%, calcium (Ca): 0.0002% to 0.005%, nitrogen (N): 0.001% to 0.006%, phosphorus (P): 0.02% or less (not including 0%), sulfur (S): 0.003% or less (not including 0%), oxygen (O): 0.0025% or less (not including 0%), a balance of iron (Fe), and inevitable impurities, and satisfying relational expression (1),

wherein a microstructure of the high-strength steel material comprises polygonal ferrite and acicular ferrite in a total amount of 30 area % or more, and comprises a martensite-austenite composite phase (MA phase) in an amount of 1.1 to 3.0 area %,

wherein the MA phase has an average size of 2.5 μm or less, when measured at an equivalent circular diameter,

wherein the steel material comprises inclusions, wherein inclusions having a size of 10 μm or more, among the inclusions, has 11/cm 2 or less, and

wherein a weld heat-affected zone has an impact energy value at −40° C. of 200 J to 405 J, and a crack-tip opening displacement (CTOD) value at −20° C. of 0.25 mm or more:

5*C+Si+10*sol.Al≤0.5  Relational expression (1):

where each symbol of the element refers to a value indicating each element content in weight %.

2. The high-strength steel material according to claim 1 , wherein the polygonal ferrite and the acicular ferrite are not hardened by hot-rolling.

3. The high-strength steel material according to claim 1 , wherein the steel material has a yield strength of 480 MPa or more.

4. The high-strength steel material according to claim 1 , wherein the steel material has a tensile strength of 560 MPa or more.

5. The high-strength steel material according to claim 1 , wherein the steel material has a ductile-brittle transition temperature (DBTT) of −60° C. or lower.

6. A method for manufacturing a high-strength steel material according to claim 1 , comprising:

preparing a slab comprising, by weight, carbon (C): 0.01% to 0.07%, silicon (Si): 0.002% to 0.2%, manganese (Mn): 1.7% to 2.5%, Sol. aluminum (Sol.Al): 0.001% to 0.035%, niobium (Nb): 0.03% or less (not including 0%), vanadium (V): 0.01% or less (not including 0%), titanium (Ti): 0.001% to 0.02%, copper (Cu): 0.01% to 1.0%, nickel (Ni): 0.01% to 2.0%, chromium (Cr): 0.01% to 0.5%, molybdenum (Mo): 0.001% to 0.5%, calcium (Ca): 0.0002% to 0.005%, nitrogen (N): 0.001% to 0.006%, phosphorus (P): 0.02% or less (not including 0%), sulfur (S): 0.003% or less (not including 0%), oxygen (O): 0.0025% or less (not including 0%), a balance of iron (Fe), and inevitable impurities, and satisfying relational expression (1);

heating the slab to a temperature of 1000° C. to 1200° C.;

finish hot-rolling the heated slab to at a temperature of 650° C. or higher to obtain a hot-rolled steel sheet; and

cooling the hot-rolled steel sheet to obtain the high-strength steel material:

5*C+Si+10*sol.Al≤0.5  Relational expression (1):

where each symbol of the element refers to a value indicating each element content in weight %.

7. The method according to claim 6 , wherein the cooling the hot-rolled steel sheet performs to a cooling end temperature of 200° C. to 550° C. at a cooling rate of 2° C./s to 30° C./s.

8. The method according to claim 6 , further comprising a tempering operation of heating the cooled hot-rolled steel sheet to a temperature of 450° C. to 700° C., maintaining the steel sheet for (1.3*t+10) minutes to (1.3*t+200) minutes, and cooling the steel sheet (where t is a value obtained by measuring a thickness of the hot-rolled steel sheet in mm units).

9. The method according to claim 6 , wherein the preparing the slab further comprises introducing Ca or a Ca alloy into a molten steel at a final stage of secondary refining operation, and bubbling and refluxing with Ar gas for at least 3 minutes after the Ca or Ca alloy is introduced.

10. A high-strength steel material comprising, by weight, carbon (C): 0.01% to 0.07%, silicon (Si): 0.002% to 0.2%, manganese (Mn): 1.7% to 2.5%, Sol. aluminum (Sol.Al): 0.001% to 0.035%, niobium (Nb): 0.03% or less (not including 0%), vanadium (V): 0.01% or less (not including 0%), titanium (Ti): 0.001% to 0.02%, copper (Cu): 0.01% to 1.0%, nickel (Ni): 0.01% to 2.0%, chromium (Cr): 0.01% to 0.5%, molybdenum (Mo): 0.001% to 0.5%, calcium (Ca): 0.0002% to 0.005%, nitrogen (N): 0.001% to 0.006%, phosphorus (P): 0.02% or less (not including 0%), sulfur (S): 0.003% or less (not including 0%), oxygen (O): 0.0025% or less (not including 0%), a balance of iron (Fe), and inevitable impurities, and satisfying relational expression (1),

wherein a microstructure of the high-strength steel material comprises polygonal ferrite and acicular ferrite in a total amount of 30 area % or more, and comprises a martensite-austenite composite phase (MA phase) in an amount of 1.1 to 3.0 area %,

wherein the MA phase has an average size of 2.5 μm or less, when measured at an equivalent circular diameter,

wherein the steel material comprises inclusions, wherein inclusions having a size of 10 μm or more, among the inclusions, has 11/cm 2 or less, and

wherein the high strength material has an impact energy value at −40° C. of 200 J or more, and a crack-tip opening displacement (CTOD) value at −20° C. of 0.25 mm or more:

5*C+Si+10*sol.Al≤0.5  Relational expression (1):

where each symbol of the element refers to a value indicating each element content in weight %.