Extremely thick steel plate for steam drum having excellent surface quality and lamellar tear resistance, and manufacturing method for same

A steel material comprising:

• in % by weight, C:
• 0.2 to 0.3%, Si:
• 0.05 to 0.5%, Mn:
• 1.0 to 2.0%, Al:
• 0.005 to 0.1%, P:
• 0.01% or less, S:
• 0.015% or less, Nb:
• 0.001 to 0.02%, V:
• 0.001 to 0.03%, Ti:
• 0.001 to 0.03%, Cr:
• 0.01 to 0.3%, Mo:
• 0.01 to 0.12%, Cu:
• 0.01 to 0.4%, Ni:
• 0.05 to 0.4%, Ca:
• 0.0005 to 0.004%, a balance of Fe, and other unavoidable impurities,
• wherein Ceq, according to the following Relationship 1, satisfies a range of 0.5 to 0.6,
• a ferrite and pearlite composite structure having an average grain size of 20 μm or less is contained as a base structure, and a hard structure fraction in a surface layer portion, which is a region from a surface to 10 mm in a thickness direction, is 5 area % or less,
• a porosity in a central portion, which is an area of 3/8t to 5/8t, where t is a steel material thickness in mm, is 0.1 mm3/g or less,
• and among precipitates observed in a steel material section after post-weld heat treatment (PWHT), fine VC precipitates with a diameter of 5 to 15 nm are 5 or more per 1 μm2,
• Ceq=[C]+[Mn]/6+([Cr]+[Mo]+[V])/5+([Ni]+[Cu])/15 [Relationship 1]
• wherein, in Relationship 1 above, [C], [Mn], [Cr], [Mo], [V], [Ni] and [Cu] indicate contents in wt % of C, Mn, Cr, Mo, V, Ni and Cu included in the steel material, respectively, and when these components are not intentionally added, 0 is substituted.