I'm trying to install a new single kitchen faucet but the old ond will not budge there is rust on the under side which I've sprayed with CLR there is a semi circle flat disc around the under pipes to the faucet but there is no bolt as my new faucet
What to do?

Take long, sharp chisel and try to split the nut in half.

I think that if you use a screwdriver against the side of that flat disk that the pipe will slide through and tap it with a hammer or some light tool the disk will break loose and slide off and you will then be able to lift the faucet out.

If is is not made with a place for the pipe to slip into the use an electric drill with a 5/16 or 1/4 inch bit and drill a hole up through the nut, after you get through the nut start leaning the drill and as it leans in the nut it will cut the nut to the point that you can crack it with a screwdriver or chisel.