In this work, the change in steel grade in a tundish of 20.3 tons with two exits employed from low to medium carbon grade steels is analyzed by numerical simulation, using multiphase flow modeling conditions in an isothermal state and non-isothermal, considering heat losses by radiation in the slag opening zone. The Tundish Volume Fraction (TVF) method determines the overlapping steel up to 10, 25, and 50% of the remaining steel. For non-isothermal cases, the temperature gradient over the incoming and old steel was 17 K. The model is validated through plant temperature measurements and steel heat losses at the tundish exits. The results indicate that heat transfer substantially modifies the fluid-dynamic behavior of steel. Likewise, the short circuit phenomenon is visible in the case of a 50% reduction of steel attributed to a non-reported elsewhere flow memory loss phenomenon once the tundish work level was recovered during the simulation under isothermal conditions. On the other hand, the intermixed steel is reduced as the amount of old steel remaining in the tundish is reduced; in addition, for the same case under isothermal conditions, the calculation of this amount is higher by a value close to 3 times that calculated under non-isothermal conditions.