In Infinity, I got the first few ingots from chests (TC Greatwoods and such). After my mob farm was set up, I melted down any and all armor drops (I had to automate repairing them first, which was a different challenge) in a TiCo Smeltery. This provided me with a decent sum of steel as well as a boatload of gold.
That combined with an EnderQuarry in the Nether provided me with all the steel I needed. If I needed more, I would probably use the TE Induction Smelter as noted above, as it is the easiest method, and I have an abundance of coal powder from SAG Milling my coal ore.