I "think" Mossberg 500 tubes ARE steel???
I think that Mossberg makes steel tubes for use on their Model 590 series.
These are just upgraded Model 500 guns, so the tube should fit.
Mossberg apparently now uses a Loctite bonder to hold the threaded magazine tube in the receiver.
You have to WARM the area around the tube and where it's screwed into the receiver to about 300 degrees to break the bonder down, then grip the tube close to the receiver with a synthetic strap wrench to unscrew it.
Note I said WARM. DO NOT over heat, since you can ruin the gun. 300 degrees is not that hot, so be careful.