Steel material, for pressure vessel, showing excellent hydrogen-induced cracking resistance and method for preparing same

A steel material for a pressure vessel, comprising, in percent by weight, 0.10 to 0.25% of carbon (C), 0.05 to 0.50% of silicon (Si), 1.0 to 2.0% of manganese (Mn), 0.005 to 0.40% of aluminum (Al), 0.010% or less of phosphorus (P), 0.0010% or less of sulfur (S), 0.001 to 0.03% of niobium (Nb), 0.001 to 0.03% of vanadium (V), 0.001 to 0.03% of titanium (Ti), 0.01 to 0.20% of chromium (Cr), 0.05 to 0.15% of molybdenum (Mo), 0.01 to 0.50% of copper (Cu), 0.05 to 0.50% of nickel (Ni), 0.0005 to 0.0050% of magnesium (Mg), 0.0005 to 0.0050% of calcium (Ca), 0.0020% or less of oxygen (O), and the balance of Fe and other unavoidable impurities,

• wherein a microstructure comprises, in an area fraction, 30% or less of pearlite and the balance of ferrite,
• wherein the steel material includes non-metallic inclusions containing a Mg—Al—Ca—O composite oxide, and
• wherein the Mg—Al—Ca—O composite oxide contains the Mg—Al—Ca—O composite oxide which satisfies an R1/M1 ratio ranging from 4.0 to 19.0, an R2/A1 ratio ranging from 0.3 to 0.7, and an R3/C1 ratio ranging from 1.9 to 9.0 when a non-metallic inclusion, among the non-metallic inclusions, having a diameter of 5 μm or more is analyzed by EDS,
• where, when it is assumed that the sum of contents of CaO, Al2O3, and MgO in the components of the inclusions included in the steel material is set to 100%, the C1 represents an average value of the CaO contents in the respective inclusions included in the steel material, the A1 represents an average value of the Al2O3 contents, and the M1 represents an average value of the MgO contents, provided that it is meant that R1=C1+A1, R2=C1+M1, and R3=A1+M1.