The most widely excavated iron artifacts used as weapons or farm tools from central southern regions of Korea were subjects of non-metallic inclusion analysis through metallographic examination, microhardness measurement, and scanning electron microscopy with energy dispersive X-ray spectroscopy. Through metallographic interpretation and study of the analyzed results, the steel manufacturing and iron smelting using heat processing in the iron artifacts excavated from the central southern region of the ancient Korean peninsula was studied, and the analysis of the non-metallic inclusions mixed within the metallic structures was interpreted as the ternary phase diagram of the oxide to infer the type of iron ores for the iron products and the temperature of the furnace used to smelt them. Most of the ancient forged iron artifacts showed $Al_2O_3/SiO_2$ with high $SiO_2$ contents and relatively low $Al_2O_3$ contents for iron ore, indicating t hat for $Al_2O_3$ below 5%, it is presumed that magnetic iron ores were reduced to bloom iron (sponge iron) with direct-reduction process for production. The temperature for extraction of wustite for $Al_2O_3$ below 1% was found to be $1,020{sim}1,050^{circ}C$. Considering the oxide ternary constitutional diagram of glassy inclusions, the steel-manufacturing temperature was presumed to have been near $1,150{sim}1,280^{circ}C$ in most cases, and minimum melting temperature of casting iron part excavated in Daeseong-ri. Gyeonggi was near $1,400^{circ}C$, and it is thought that hypoeutectic cast iron of about 2.3% carbon was casted and fragility of cast iron was improved by decarburizing in solid state.