Abstract: Disclosed are a method for heat-treating a surface of a spheroidal graphite cast iron, particularly, heat-treating an uppermost surface of spheroidal graphite cast iron, and spheroidal graphite cast iron heat-treated by the same. The method may include first heat treating for forming ferrite and second heat treating for oxy-nitriding. The spheroidal graphite cast iron heat-treated includes an oxidation layer and a compound layer having a thickness of about 15 to 30 ?m, which may be uniform. The method of the heat-treating may decrease the pearlite fraction in the uppermost surface of spheroidal graphite cast iron and increase the ferrite fraction by forming ferrite, thereby forming a compound layer having a thickness of about 15 to 30 ?m during an oxy-nitriding heat treatment.

Abstract: Disclosed herein is high-strength special steel containing about 0.1 to 0.5 wt % of carbon (C), about 0.1 to 2.3 wt % of silicon (Si), about 0.3 to 1.5 wt % of manganese (Mn), about 1.1 to 4.0 wt % of chromium (Cr), about 0.3 to 1.5 wt % of molybdenum (Mo), about 0.1 to 4.0 wt % of nickel (Ni), about 0.01 to 0.50 wt % of vanadium (V), about 0.05 to 0.50 wt % of titanium (Ti), and the remainder of iron (Fe) and other inevitable impurities.

Abstract: Disclosed herein is high-strength special steel containing about 0.1 to 0.5 wt % of carbon (C), about 0.1 to 2.3 wt % of silicon (Si), about 0.3 to 1.5 wt % of manganese (Mn), about 1.1 to 4.0 wt % of chromium (Cr), about 0.3 to 1.5 wt % of molybdenum (Mo), about 0.1 to 4.0 wt % of nickel (Ni), about 0.01 to 0.50 wt % of vanadium (V), about 0.05 to 0.50 wt % of titanium (Ti), and the remainder of iron (Fe) and other inevitable impurities.