I am talking about the step-up / step-down transformers for EU, like ULV-LV, LV-MV, etc. They all require a machine casing, wires (any copper up to MV-HV, annealed copper for HV-EV and higher), and two double iron plates, which can be one of various types of iron like normal iron, wrought iron, pig iron, cast iron, meteoric iron, and so on but does not accept any steel like steel or meteoric steel. I interpreted the recipe components to correspond with the windings and core that a real life transformer would have, and since steel has a carbon content intermediate of normal iron and pig iron, it makes sense “lore-wise” for steel to be a valid material for such a core.