Improving the quality of steel and the steelmaking process has been a matter of routine for metallurgical engineers and steelmaking companies in a demanding market for quality products at highly competitive price. The chemical and temperature adjustment are made during the secondary refining process, as well as the inclusion modification required to product quality, and also the demand for castability accuracy. Continuous casting process is the most used solidification casting process, in which the flow of pouring liquid metal through the submerged entry nozzle is assured by the correct temperature and the formation of liquid inclusion in the casting temperature. Thermocalc and CEQCSI were the software used in this work to assess the effect of carbon, silicon and sulphur in the castability window of the aluminium vs calcium phase diagrams. They have proved to be highly suitable and effective and the results showed that the chemical elements used directly affected the position of the castability window of carbon steel. An analysis of a 0.2%C billet sample using Scanning Electron Microscopy showed that there is a great heterogeneity of inclusions in aluminium-killed and calcium-treated steel.