Splat, if you’re just removing rust and accuracy/concentricity isn’t an issue, spin the pipe slowly in the lathe and power sand it (as someone suggested, cover the bed to catch the debris).
Steadies only work on stock that’s already round so it sounds like your pipe is only approximate!
If you need to remove metal accurately over the entire length you’d need to support between centres on a mandrel or threaded plugs, with the headstock plug longer so you can drive on it and with a shoulder so it doesn’t disappear up the pipe on first cut! But it doesn’t sound like you need this level of precision (just mentioning so you know for the future).
Mal