Steel plate having high strength and excellent impact toughness after deformation, and method for manufacturing same

A steel plate, comprising by weight %:

• 0.04 to 0.08% of C, 0.1 to 0.35% of Si, 1.4 to 1.8% of Mn, 0.01 to 0.035% of sol.Al, 0.2 to 0.5% of Ni, 0.1 to 0.3% of Cr, 0.05 to 0.15% of Mo, 0.015 to 0.035% of Nb, 0.005 to 0.02% of Ti, 0.002 to 0.006% of N, 0.01% or less of P, 0.003% or less of S, with a balance of iron (Fe) and other inevitable impurities,
• wherein an R value defined in Relational expression 1 below is 0.85 to 1.35,
• a microstructure at a point equal to ¼ of a thickness of the steel plate consists of by area fraction, 40 to 60% of acicular ferrite, 40 to 60% of bainite, and 3% or less of a sum of residual cementite and MA,
• an average grain size of acicular ferrite at the point equal to ¼ of the thickness is 20 μm or less, and
• a microstructure in a portion 8 mm directly below a surface portion comprises by area fraction, 5% of a hard phase,
• wherein the hard phase is an MA phase,
• R
• =
• [
• Ni
• ]
• +
• 3
• [
• Mo
• ]
• +
• 2
• [
• Cr
• ]
• [
• Relational
• ⁢
• expression
• ⁢
• 1
• ]
• where [Ni], [Mo], and [Cr] are a weight percent of each element.