Steel material having low surface hardness and excellent low temperature impact toughness, and method for manufacturing same

A method for manufacturing a steel material, comprising operations of:

• reheating a steel slab including, by weight:
• 0.08 to 0.14% of carbon (C), 0.1 to 0.5% of silicon (Si), 1.2 to 1.7% of manganese (Mn), 0.01% or less of phosphorus (P), 0.01% or less of sulfur (S), 0.01 to 0.05% of aluminum (Al), 0.05:
• or less of niobium (Nb), 0.01 to 0.5% of chromium (Cr), 0.01 to 0.25% of nickel (Ni), 0.01 to 0.1% of molybdenum (Mo), 0.01 to 0.05% of vanadium (V), 0.003% or less of titanium (Ti), 0.002 to 0.01% of nitrogen (N), with a balance of Fe and inevitable impurities;
• rough rolling the reheated steel slab;
• subjecting the rough-rolled steel to finish hot rolling at a temperature of Ar3+50° C. or higher;
• air cooling the hot-rolled steel plate to room temperature;
• performing a quenching operation of heating the air-cooled hot-rolled steel plate for 1.3t+30 minutes or more, where t refers to a thickness of steel in millimeters, in a temperature range of Ar3 or higher, then primary cooling the steel plate to a temperature of Ar3 or lower at a cooling rate of (946×t−1032)/60 to 1.5° C./s, where t refers to a thickness of steel in millimeters, and secondary cooling the steel plate at a cooling rate of 11,500×t−1788° C./s or more, where t refers to a thickness of steel in millimeters;
• and
• performing a tempering operation of heating the quenched steel plate for 1.9t+30 minutes or more, where t refers to a thickness of steel in millimeters, in a temperature range of 600 to 700° C., and then air cooling the same to room temperature.