Steel plate having excellent heat affected zone toughness and method for manufacturing same

Abstract

The present invention relates to a steel plate for use in ships and the like, which has excellent toughness in a heat-affected zone (HAZ), even when a steel material having high strength and high ductility is welded with a certain amount of heat input or more, and to a method for manufacturing same.

1 . A steel material having excellent weld heat-affected zone toughness, comprising by weight:
 0.04 to 0.07% of carbon (C), 1.5 to 1.7% of manganese (Mn), 0.1 to 0.3% of silicon (Si), 0.01 to 0.04% of aluminum (Al), 0.7 to 1.0% of nickel (Ni), 0.05 to 0.30% of molybdenum (Mo), 0.010 to 0.018% of titanium (Ti), 0.01 to 0.03% of niobium (Nb), 0.003 to 0.006% of nitrogen (N), 0.007% or less of phosphorus (P), 0.002% or less of sulfur (S), a balance of Fe, and unavoidable impurities,   wherein a weld heat-affected zone (HAZ) welded with a heat input of 100 to 200 KJ/cm includes a bainite phase of an area fraction of 90% or more in a region from a fusion line (FL) to FL+3 mm.   
     
     
         2 . The steel material having excellent weld heat-affected zone toughness of  claim 1 , wherein an average grain size of prior austenite in the region from the fusion line (FL) to FL+3 mm is 100 μm or less. 
     
     
         3 . The steel material having excellent weld heat-affected zone toughness of  claim 1 , wherein the region from the fusion line (FL) to FL+3 mm has a tensile strength of 610 MPa or more and an impact toughness of 33 J or more at −20° C. 
     
     
         4 . The steel material having excellent weld heat-affected zone toughness of  claim 1 , wherein the steel material comprises bainite in an area fraction of 90% or more (including 100%) and residual acicular ferrite as a microstructure. 
     
     
         5 . A method for manufacturing a steel material having excellent weld heat-affected zone toughness, comprising:
 reheating a steel slab including by weight: 0.04 to 0.07% of carbon (C), 1.5 to 1.7% of manganese (Mn), 0.1 to 0.3% of silicon (Si), 0.01 to 0.04% of aluminum (Al), 0.7 to 1.0% of nickel (Ni), 0.05 to 0.30% of molybdenum (Mo), 0.010 to 0.018% of titanium (Ti), 0.01 to 0.03% of niobium (Nb), 0.003 to 0.006% of nitrogen (N), 0.007% or less of phosphorus (P), 0.002% or less of sulfur (S), a balance of Fe, and unavoidable impurities, in a temperature range of 1100 to 1180° C.;   rough rolling the heated steel slab in a temperature range of 900° C. or higher;   finishing rolling the steel slab in a temperature range of 800° C. or higher after the rough rolling to prepare a hot-rolled steel material; and   cooling the hot-rolled steel material until the temperature at t/4 point of the thickness (t, unit of mm) of the hot-rolled steel material is 600° C. or lower at a cooling rate of 10° C./s or more.   
     
     
         6 . The method for manufacturing a steel material having excellent weld heat-affected zone toughness of  claim 5 , wherein the rough rolling is performed at a reduction ratio of 40% or more. 
     
     
         7 . The method for manufacturing a steel material having excellent weld heat-affected zone toughness of  claim 5 , wherein the finish rolling is performed at a cumulative reduction ratio of 50% or more. 
     
     
         8 . The method for manufacturing a steel material having excellent weld heat-affected zone toughness of  claim 5 , further comprising: welding the hot-rolled steel material after the cooling,
 wherein the welding is performed with a heat input of 100 to 200 KJ/cm.