In this study, the thermal behavior characteristics of flange case interior were analyzed by the numerical method. The boundary conditions at the inlet port of flange case were the heat flux and heat flux time. As the heat flux time at inlet of case increased, the temperature values gradually increased, and the degree of increase was very small. If the heat flux of the melted iron increases to 2,000,000 W/m2, the temperature change at the case interior will occur largely, causing heat deformation. As a result, in order to reduce thermal defects at the case interior, the heat flux and heat flux time of the melted iron should be set within 500,000 W/m2 and 5 seconds, respectively.