There's little force acting on a floor plate, even during shooting; a very-hard steel is unnecessary. A harder steel will take a higher polish, and scratch less readily. So, the compromise is in the realm of machinability.
A receiver, bolt or barrel is another matter. You want hardness for anti-wear; but you need toughness to absorb impact loadings. I'm too rusty and lack the reference books to carry this part any farther...
Regards, Art