Ultra-thick steel excellent in brittle crack arrestability and manufacturing method therefor

A steel comprising:

• by wt %, 0.02 to 0.07% of C, 1.8 to 2.2% of Mn, 0.7 to 1.2% of Ni, 0.005 to 0.02% of Nb, 0.005 to 0.02% of Ti, 0.1 to 0.4% of Cu, 0.01% or less of P, 0.004% or less of S, and a balance of Fe and inevitable impurities, wherein grains that have a high angle grain boundary of 15° or more, as measured by EBSD, have an average grain size of 15 μm or less in the t/4−(3*t)/8 region, where t represents a thickness of the steel, and the same below,
• wherein a microstructure of a test piece collected in the t/4 region of the steel includes a composite structure of acicular ferrite and granular bainite, and further includes island martensite as a second phase,
• wherein the acicular ferrite is included in a ratio of 60 to 80 area %, the granular bainite is included in a ratio of 20 to 40 area %, the island martensite is included in a ratio of greater than 0% 10 area % or less, and
• wherein a naval research laboratory-drop weight test (NRL-DWT) specified in ASTM E208-06 is performed on a test piece collected in the t/4 region of the steel, a Nil-ductility transition (NDT) temperature is −45° C. or lower.