most arc furnaces are AC ..and are used by the "mini-mills" rather than the large mills who still make steel using blast and basic oxygen furnaces
iron from the blast furnace is converted to steel by pumping huge quantities of oxygen into the molten iron
which coincidentally release large quanities of carbon monoxide....which is why we have to wear CO monitors when playing around most things in the BOF or above the blast furnace
I do recall seeing 25 Hz stuff but all the newer equipment uses VFDs or DC drives
