Pressure vessel steel having excellent hydrogen induced cracking resistance, and manufacturing method therefor

1. A pressure vessel steel having high resistance to hydrogen induced cracking, the pressure vessel steel comprising, by wt %, carbon (C): 0.06% to 0.25%, silicon (Si): 0.05% to 0.50%, manganese (Mn): 1.0% to 2.0%, aluminum (Al): 0.005% to 0.40%, phosphorus (P): 0.010% or less, sulfur (S): 0.0015% or less, niobium (Nb): 0.001% to 0.03%, vanadium (V): 0.001% to 0.03%, titanium (Ti): 0.001% to 0.03%, chromium (Cr): 0.01% to 0.20%, molybdenum (Mo): 0.05% to 0.15%, copper (Cu): 0.02% to 0.50%, nickel (Ni): 0.05% to 0.50%, calcium (Ca): 0.0005% to 0.0040%, and the balance of iron (Fe) and inevitable impurities,

wherein the pressure vessel steel has a microstructure comprising bainite having a dislocation density of 5×10 14 to 10 15 /m 2 in a volume fraction of 80% or greater and the balance of ferrite (excluding 0%), and

wherein after post weld heat treatment (PWHT), the microstructure of the pressure vessel steel comprises Nb(C,N) or V(C,N) carbonitride having a diameter of 5 nm to 30 nm in an amount of 0.001% to 0.002%.

2. The pressure vessel steel of claim 1 , wherein the bainite comprises acicular ferrite.

3. The pressure vessel steel of claim 1 , wherein after PWHT, the pressure vessel steel has a tensile strength of 550 MPa or greater.

4. A method for manufacturing a pressure vessel steel having high resistance to hydrogen induced cracking according to claim 1 , the method comprising:

preparing a steel slab, the steel slab comprising, by wt %, carbon (C): 0.06% to 0.25%, silicon (Si): 0.05% to 0.50%, manganese (Mn): 1.0% to 2.0%, aluminum (Al): 0.005% to 0.40%, phosphorus (P): 0.010% or less, sulfur (S): 0.0015% or less, niobium (Nb): 0.001% to 0.03%, vanadium (V): 0.001% to 0.03%, titanium (Ti): 0.001% to 0.03%, chromium (Cr): 0.01% to 0.20%, molybdenum (Mo): 0.05% to 0.15%, copper (Cu): 0.02% to 0.50%, nickel (Ni): 0.05% to 0.50%, calcium (Ca): 0.0005% to 0.0040%, and the balance of iron (Fe) and inevitable impurities;

reheating the steel slab to a temperature of 1150° C. to 1200° C.;

rough rolling the reheated steel slab at a temperature of 900° C. to 1100° C.;

finish hot rolling the rough-rolled steel slab at a temperature of Ar3+80° C. to Ar3+300° C. to manufacture a hot-rolled steel sheet;

cooling the hot-rolled steel sheet to a temperature of 450° C. to 500° C. at a cooling rate of 3° C./s to 200° C./s; and

cooling the cooled hot-rolled steel sheet to a temperature of 200° C. to 250° C. by a stack cooling method and then maintaining the hot-rolled steel sheet for 80 hours to 120 hours.

5. The method of claim 4 , wherein the rough rolling is performed at a reduction ratio of 10% or greater in each of last three passes and a total reduction ratio of 30% or greater.

6. The method of claim 4 , wherein the cooling of the cooled hot-rolled steel sheet by the stack cooling method is performed at a cooling rate of 0.1° C./s to 1.0° C./s.