I am not sure why you need to change the frequency for ferrous metals. At a college in Melbourne they had a hobby foundry course and they used a 100 Kw power supply at 3 Khz to run three different furnaces. One was 100 Kgs another was 70 Kgs, these two were only used for converting steel to cast iron. The smaller 50 Kgs was only used to melt brass and bronze. Only one furnace at a time could be used to melt any metal. I watched the furnace operator do hundreds of melts for cast iron and a few for bronze and the frequency was never changed to suit ferrous or non ferrous metals. I did ask him why that power supply used 3 Khz and he said that frequency was determined by furnace size, the larger the furnace the lower the frequency and the smaller the furnace high frequencies were needed. When ferrous metals are heated to past their curie point they become non magnetic and melt the same way as non ferrous metals do. He also said that if he filled the furnace with cast iron chips from a machine shop the chips would heat up but will never get to the melting point because 3 Khz would only melt larger chunks in that furnace. I did try to make a induction heater using 50 Hz and it worked heating up thick pieces of steel but when I tried to heat up thin pieces of sheet metal it would not heat them up. So it is possible to make an induction furnace using 50 Hz to melt metals but would be impossible to do in a backyard setting for many reasons to do with using a low frequency.