FWIW, last summer one of the students in a class I was teaching had some new steel plate armor. He wanted to shoot them and "see what happens".... ok, it was his idea. I know I have accidentally left a can of paint sitting in front of steel targets, and already know what inevitably happens. Highly unscientific testing followed.
There was some kind of a coating on the steel, almost looked like the stuff used as bed liner in the back of my truck. Shot it with some Black Hills 77 grain OTM ammo out of a 10.3" Colt 6945. There was a small dent in the plate, and a circle of the coating about 2 inches across blown away. The stuff wasn't very think at all, just a coating like it had been sprayed or maybe rolled on. He was happy as hell, since the bullet didn't penetrate, so, good, I guess.
A local PD just had some dude come and donate a bunch of plate carriers with steel plates. I guess if I had the choice between getting shot, and taking a hit on a steel plate, I'd pick the plate. I don't think there's much doubt that there are better options. I'd also like to see any verifiable instances injury from frag or ricochet.