In my youth I studied metallurgy and had the opportunity not only to study grain structure and characteristics of different types of steel. I also spent time on the shop floor working with preheating furnaces and studying austenitic effects at different temperatures. We always worked on an austenitic temperature transition starting around 723 degrees C. The effects on steel at uncontrolled and variable high temperatures is unimaginable. (Think twin towers)
If you feel like swimming among crocodiles, this paper will provide you with sleepless nights:
Steel - Austenitic variables.
In my opinion, this vessel is almost totally compromised and should be decontaminated and scuttled. Even if the keel and bottom two decks/tanks, etc could be salvaged, it would not be worth the effort since the scrap value is limited.
IG