This study investigates the effect of the metallic reinforcement phase on the properties of castings made from medium-carbon steel using the Lost Foam Process. Numerical simulations were conducted using ESI-ProCast v2121.5 software to analyze the influence of sample sizes and reinforcement ratios on cooling rates and crystalline properties. The study included samples with dimensions of 10∅×200 mm and 50∅×200 mm, with reinforcement ratios of 0% and 50%. The results demonstrated that the introduction of reinforcement phase particles significantly enhances cooling rates and improves the crystalline structure, thereby enhancing the mechanical properties of the castings. Statistical analyses, including variance analysis, revealed that the optimal range of reinforcement ratios lies between 12% and 33%. This range offers a balance between solidification speed and the reduction of structural voids while maintaining the fluidity of the molten metal. Ratios exceeding 33% negatively impacted fluidity, hindering the casting process. This study highlights the potential of innovative mold designs that strategically incorporate the reinforcement phase to optimize cooling rates while reducing harmful environmental emissions. The findings contribute to advancing sustainable casting practices by improving product quality and minimizing environmental impacts, providing a solid foundation for future industrial applications................. Keywords: ...............Lost Foam Process, Casting, Polystyrene Foam, Metallic Reinforcement Phase, Metal Matrix Composites, Cooling Rate.