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

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≤S1/S2≤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.