You could use the Cookie Cutter system to cut a Cookie out over the plate and excavate to expose and remove the steel plate. You could locate the plate center and use the Newcon 12" auger to expose the steel plate and cut a Key shaped hole with a straight tipped cutting torch ( using a torch on a sewer system is risking an explosion!) You could cut the Key shaped hole with a drill to avoid explosion danger. When you have a Key shaped hole you can cut and pull with the Mr. Manhole cutter.
Product Link: https://mrmanhole.com/introducing-the-cookie-cutter-system-for-easy-fast-and-safe-road-penetrations/
Additional Resources: https://mrmanhole.com/introducing-the-cookie-cutter-system-for-easy-fast-and-safe-road-penetrations/