Steel pipe as opposed to stainless steel can be used for slumping. It will spall, so there will be a need to clean up the flakes of rust after firing. But since there is so much spalling, putting kiln wash or boron nitride is a waste of effort. Each firing will flake off any separator painted onto the metal. Cover the pipe with fibre paper instead - 0.5 mm at least.
You need to advance in temperature slowly as the pipe drains the heat from the glass where it rests. My practice is to advance the temperature at 100C/hr to 100C with a 20 minute soak, followed by 50% increases in rate to 250, and to 500 with 20 minute soaks before proceeding to the next segment. This probably is more cautious than necessary on all but the first segment.