STEEL MATERIAL FOR HIGH-HEAT-INPUT WELDING

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Provided is a structural steel plate suitable for high heat input welding with a welding heat input of more than 300 kJ/cm and suitable for various structures in fields such as shipbuilding, architecture, and civil engineering. Specifically, the structural steel plate has a chemical composition containing, in percent by mass, 0.03% to 0.08% carbon, 0.01% to 0.15% silicon, 1.8% to 2.6% manganese, 0.008% or less phosphorus, 0.0005% to 0.0040% sulfur, 0.005% or less aluminum, 0.003% to 0.03% niobium, 0.005% to 0.030% titanium, 0.0050% to 0.0080% nitrogen, 0.0003% to 0.0025% boron, and optionally at least one of vanadium, copper, nickel, chromium, molybdenum, calcium, magnesium, zirconium, and a rare earth metal, and having a Ceq (IIW) of 0.33 to 0.45, the balance being iron and incidental impurities. The structural steel plate has a prior austenite grain diameter of 200 µm or less and a martensite-austenite constituent area fraction of 1.0% or less in a heat-affected zone microstructure adjacent to a fusion line after high heat input welding with a welding heat input of more than 300 kJ/cm.