The MgO content in ladle slags is addressed. It is important to tend to, but not to exceed the MgO saturation limit during slag induction. Exceeding this limit will have an adverse effect on the refining properties of slag. The heating temperature of the slag and metal in a ladle furnace is studied. It is shown that the MgO saturation limit varies during steel processing. Taking this variation into account allows adjusting the target content of this oxide when calculating the amount of injected MgO-containing materials, ensuring low corrosivity of homogeneous slag to the lining, while maintaining its refining properties.