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Abstract

Disclosed are an ultra-thick steel sheet may have excellent strength and impact toughness, and a manufacturing method thereof. The ultra-thick steel sheet comprises, in percent by weight (wt %), 0.06 to 0.1% of C, 0.3 to 0.5% of Si, 1.35 to 1.65% of Mn, 0.015 to 0.04% of AI, 0.015 to 0.04% of Nb, 0.15 to 0.4% of Cr, 0.005 to 0.02% of Ti, 0.3 to 0.5% of Ni, 0.002 to 0.008% of N, 0.01% or less of P, 0.003% or less of S, and the remainder being Fe and inevitable impurities, and a microstructure, by area fraction, comprises 80% or more of polygonal ferrite with an average grain size of 40 μm or less and the remainder of pearlite with an average grain size of 20 μm or less.

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