Is 16MnCr5 (SS 2511) available as flat steel?

Yes, Livallco stock flat steel in 16MnCr5 (SS 2511), can also replace 16NiCrS4. Many wholesalers stock only round steel in this grade, but if the end product is square, there is much to be gained by asking Livallco. Livallco can also offer sheet metal in this grade.