Ultrahigh-strength steel for welding structure with excellent toughness in welding heat-affected zones thereof, and method for manufacturing same

A method of manufacturing a steel sheet for a welded structure having a weld heat-affected zone (HAZ), the method comprising:

• preparing a slab comprising, by wt %, carbon (C):
• 0.05% to 0.15%, silicon (Si):
• 0.1% to 0.6%, manganese (Mn):
• 1.5% to 3.0%, nickel (Ni):
• 0.1% to 0.5%, molybdenum (Mo):
• 0.1% to 0.5%, chromium (Cr):
• 0.1% to 1.0%, copper (Cu):
• 0.1% to 0.4%, titanium (Ti):
• 0.005% to 0.1%, niobium (Nb):
• 0.01% to 0.03%, boron (B):
• 0.0003% to 0.004%, aluminum (Al):
• 0.005% to 0.1%, nitrogen (N):
• 0.001% to 0.006%, phosphorus (P):
• 0.015% or less, sulfur (S):
• 0.015% or less, iron (Fe) as a residual component thereof, and inevitable impurities, wherein the Ti and N component contents satisfy Formula 1 below, the N and B component contents satisfy Formula 2 below, and the Mn, Cr, Mo, Ni, and Nb component contents satisfy Formula 3 below;
• heating the slab to a temperature of 1100° C. to 1200° C.;
• hot finish rolling the heated slab at a temperature of 870° C. to 900° C. to form a hot finish rolled steel sheet;
• and
• cooling the hot finish rolled steel sheet to a temperature of 420° C. to 450° C. at a cooling speed of 4° C./s to 10° C./s to form a hot rolled steel sheet, wherein the hot-rolled steel sheet has a microstructure including, by area fraction, acicular ferrite in an amount of 30% to 40% and bainite in an amount of 60% to 70%, and the microstructure further comprises TiN precipitates having a size of 0.01 μm to 0.05 μm and a density of 1.0×103/mm2 or more, and being dispersed at an interval of 50 μm or less,
• 3.5≤Ti/N≤7.0 [Formula 1]
• 1.5≤N/B≤4.0 [Formula 2]
• 4.0≤2Mn+Cr+Mo+Ni+3Nb≤7.0 [Formula 3].