The microstructure of steel is responsible for the macro-behavior of steel or in other words steel's material properties. Surface microcracks and internal flaws can be introduced to steel microstructure during solidification. The soundness of the steel is altered by formation of flaws and microcracks in casting production line. Additive elements to molten steel and chemical element and alloys in the iron slag may produce indigenous and exogenous inclusions in the microstructure. The presence of the inclusions in the microstructure changes the cleanliness of the steel and affects the material properties of the steel products. This paper is a review of literatures on the sources of formation of inclusions and flaws in the as-cast steel and the effect of these defects on the microstructure properties.