High-strength steel plate for pressure vessel having excellent toughness after post weld heat treatment and manufacturing method thereof

1. A high-strength steel plate for a pressure vessel having an improved low temperature toughness after a post weld heat treatment (PWHT), the high-strength steel plate comprising:

by wt %, 0.02-0.15% of carbon (C), 0.05-0.50% of silicon (Si), 1.0-2.0% of manganese (Mn), 0.005-0.1% of aluminum (Al), 0.015% or less of phosphorus (P), 0.0015% or less of sulfur (S), 0.01-0.03% of niobium (Nb), 0.01-0.03% of vanadium (V), 0.01-0.03% of titanium (Ti), 0.005% or less of chromium (Cr), 0.005% or less of molybdenum (Mo), 0.02-0.50% of copper (Cu), 0.05-0.60% of nickel (Ni), 0.0002-0.0010% of boron (B), 0.0035-0.0065% of nitrogen (N), and a balance of iron (Fe) and other inevitable impurities; and

a microstructure including a complex structure of ferrite having an area fraction of 35-40% and a balance of bainite, and the bainite having a packet size of 15 μm or less,

wherein the steel plate has a tensile strength of 515-655 MPa, and an impact toughness of 50 J or more at −46° C. after a post weld heat treatment (PWHT).

2. The high-strength steel plate of claim 1 , wherein the microstructure includes carbide/nitride, and the carbide/nitride is Nb(C,N), V(C,N) and M 7 C 3 .

3. The high-strength steel plate of claim 2 , wherein the Nb(C,N) has an average size of 100 nm or less and is included in a fraction of 0.01-0.05%, the V(C,N) has an average size of 30 nm or less and is included in a fraction of 0.01-0.03%, and the M 7 C 3 has an average size of 25 nm or less and is included in a fraction of 0.01-0.02%.

4. The high-strength steel plate of claim 1 , wherein the steel plate has a carbon equivalent (Ceq) of 0.42 or less, the Ceq being expressed by the following Relational Formula 1,

Ceq=C+Mn/6+(Cr+Mo)/5+(Ni+Cu)/15  [Relational Formula 1]

where C, Mn, Cr, Mo, Ni, and Cu refer to contents (wt %) of the respective elements.