Steel for pressure vessels having excellent resistance to hydrogen induced cracking and manufacturing method thereof

1. A steel for pressure vessels, 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): 0.01 to 0.50%, nickel (Ni): 0.05 to 0.50%, calcium (Ca): 0.0005 to 0.0040%, oxygen (O): 0.0010% or less, and the balance of iron (Fe) and inevitable impurities,

wherein a microstructure comprises 30% or less of pearlite and 70% or more of ferrite by area fraction,

wherein a Ca—Al—O complex inclusion is included to satisfy Relational Expression 1 below,

0.02≤ S 1/ S 2≤0.1  Relational Expression 1:

where S1 is a total area of Ca—Al—O complex inclusions having a size of 6 μm or more, measured by a circle equivalent diameter, and S2 is a total area of all Ca—Al—O complex inclusions,

wherein the Ca—Al—O complex inclusion is not fractured, and

wherein the steel comprises (Nb, V) (C, N) precipitates in an amount of 0.01 to 0.02% by area after a post weld heat treatment (PWHT), and an average size of the (Nb, V) (C, N) precipitates is 5 to 30 nm.

2. The steel for pressure vessels of claim 1 , wherein the steel further comprises N: 20 to 60 ppm by weight.

3. The steel for pressure vessels of claim 1 , wherein the steel has tensile strength of 485 MPa or more after the post weld heat treatment (PWHT).

4. The steel for pressure vessels of claim 1 , wherein the steel has a CLR of 10% or less after the post weld heat treatment (PWHT).

5. A manufacturing method of the steel for pressure vessels of claim 1 , the method comprises, preparing a 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): 0.01 to 0.50%, nickel (Ni): 0.05 to 0.50%, calcium (Ca): 0.0005 to 0.0040%, oxygen (O): 0.0010% or less, and a balance of iron (Fe) and inevitable impurities;

heating the slab to 1150 to 1300° C.;

size rolling the heated slab to a temperature in a range of 950 to 1200° C. and then cooling to obtain a bar having a thickness of 80 to 180 mm;

heating the bar to 1150 to 1200° C.;

finish hot rolling the heated bar to a temperature in a range of (Ar3+30° C.) to (Ar3+300° C.)

and then cooling to obtain a hot-rolled steel plate having a thickness of 5 to 65 mm; and

performing a normalizing heat treatment, heating the hot-rolled steel plate to 850 to 950° C., maintaining for 10 to 60 minutes, and air cooling to room temperature.

6. The manufacturing method of the steel for pressure vessels of claim 5 , wherein the slab further comprises N: 20 to 60 ppm by weight %.

7. The manufacturing method of the steel for pressure vessels of claim 5 , wherein the preparing the slab comprises, adding Metal Ca Wire to molten steel after secondary refining such that an amount of Ca addition is 0.00005 to 0.00050 kg/ton at an addition rate of 100 to 250 m/min; and a clean bubbling of blowing an inert gas into the molten steel into the Metal Ca Wire is added at a blowing amount of 10 to 50V min for 5 to 20 minutes.

8. The manufacturing method of the steel for pressure vessels of claim 7 , wherein the Metal Ca Wire is composed of a Ca alloy and a steel material surrounding the Ca alloy, and the thickness of the steel material is 1.2 to 1.4 mm.

9. The manufacturing method of the steel for pressure vessels of claim 7 , wherein blowing of the inert gas is performed through an inert gas blowing point in a ladle.

10. The manufacturing method of the steel for pressure vessels of claim 5 , wherein a grain size of austenite of the bar after the size rolling is 100 μm or more.

11. The manufacturing method of the steel for pressure vessels of claim 5 , wherein the step of cooling the hot-rolled steel plate to room temperature is performed by multi-stage loading until the steel plate is cooled from the temperature of 200° C. or higher to room temperature.