If something is that hard to remove from my cast iron pans, I usually fill the pan with water and let it boil until it comes off easier. Scraping is done with a wood or plastic only, nothing metal. So far, it's worked for me. I understand cast iron is making a comeback. I read somewhere that modern 'no-stick' pans can give off nasty fumes if you overheat them.
Years ago, I found two really disgusting cast iron pans way back under the kitchen sink in one of the houses I bought. It looked like someone burned dinner in them 20 years ago and just shoved them in there and forgot them. They weren't damaged, just filthy, and they were Griswolds! I just had to find a way to clean them.
Somewhere I read an article about using 'electrolysis' to clean old cast iron pans. It seemed simply enough.
Hang them in a large non-conductive container (I used a plastic trash can). Fill with water and add some washing soda. Hook up to a battery charger to it, one to the pan and the other to another conductor. One is a cathode, the other is an anode, though I don't remember which is which. Turn it on, and after some time, the crud floats away. At the time, I had no battery charger, but I do have a Miller stick welder. So I turned it to it's lowest setting and hooked it up. Within half an hour the water in that trash can was close to boiling, and a layer of scum covered the water. I don't remember how long I left it go, but when I pulled it out, it was absolutely spotless and a bright shiny light gray. Virgin cast iron! I just re-seasoned them and use them at least twice a week.