These steelmaking slags for refining the melt are lime-based, so are "Basic" slags. "Basic" being a chemical term, as distinct from "Acid" or "Neutral". They are formed of lime (CaO; "burned" / calcined limestone) plus additions like fluorspar (CaF2) for fluidity, etc.
Basic slags can be "oxidising" or "reducing". Refinement with a low-oxygen "reducing" slag is associated with a very "clean" steel with very low non-metallic impurities. An initial oxidising slag removes the most deleterious impurities and is usually the first (and possibly, as in this case, the only) slag formed.
Pursue your own investigation of the topic if it is important to you; some of the terms already mentioned (steelmaking, steel slag, Basic, oxidising, reducing) may be useful search terms.
(R. Smith, 29Dec2017)