I have read that dry ice down the tube may provide enough thermal shrinkage on the steel to allow it to come off but just checking to see if anyone has any better ideas.
Sorry for this :
Carbon and steel, have nearly the same expansion rate
to about 300-deg ! And just for a referencing ; so does
cement. It's one of those things builders found out when
they started using steel re-bar to strengthen cement buildings.
For a straight taper :
Next time out ; undersize your mandrel by 1/8", and use a foam or wax
to bring it up to full diameter. The foam can be removed
with acetone ; the wax with heat, but use a Low-temp wax,
or lower max-temp than that of your epoxy.