Only ones I can think of are all aluminium... Only steel one I remember hearing about is one that Dr. Lector fabricated for himself (I think he made a couple, I saw it in the DAR-22 R&D thread). I don't think I've heard of an aftermarket steel one, but I could be wrong. Any particular reason you want one? I think most of the aftermarket ones are aluminium, but their design makes up for the barrel droop when free-floating a bull barrel... Only other concern I could see would be wear (which seems to be a non-issue even after thousands of rounds) and stripping the receiver threads for the rail (just be careful when attaching rails).
Some of the early ones might be made from steel... I don't know the history of the 10/22, but you might check that out and then see if you can track down an original one from the 60's or 70's.