I just hit the flange with a hammer until a section breaks off than just peel up the lead and okum and pull the flange up. NOTE: wear safety glasses and be aware that flying C/I IS shrapnel. Also if the pipe breaks instead of the flange, you are in for even more work.
Hitting Cast Iron soil piping with a hammer is NOT exactly a smart thing to do UNLESS you KNOW you have easy access below just in case the piping cracks. Your best bet is as follows.
Very carefully, YOU can melt out the old caulked joint AFTER airing (venting) out the sewer piping so it doesn’t explode from various sewer gasses
You can also use a Cast Iron inside cutter and cut just below the caulked joint THEN install another LONGER caulked joint Extra Heavy Cast Iron Floor flange JUST make sure you line up the bolts correctly for your ruffing measurements.
Then using oakum and molten lead pour a new joint LIKE Professionals do. A Quick set” flange will not work properly on Extra Heavy Cast Iron or Galv steel/wrongt
BANGING the HELL out of OLD Brittle Cast Iron piping is a THING old guys do to create work.
Since they found the way to make a FIRE (stone age LOL) we learned to melt the poured joint and LIFT the old flange up using either an old screw drivers of 2 chisels ANYTHING to pry it up and off.
Try to THINK like a mechanic (Journeyman) Guys I know its hard
Busting a cast iron pipe embedded in concrete is NOT very cost effective for the victims we work for.