There is no way to know either the alloy or the hardness, but the indoor range where I shoot has a plate rack. The rack was there when the current operator of the range took over the business eleven years ago, and it was old then. The plates were a full inch thick. They were also badly cratered, resulting in unpredictable ricochets of bullet fragments. (This is all from handgun ammo -- the range has never allowed centerfire rifle because the backstop isn't capable of handling it.)
About three years ago, early during one of the Thursday night competitions, one of the better competitors took a fairly significant wound in the leg from a fragment that bounced back about 20 yards after hitting the plate, and struck the guy in the lower leg. The wound was bad enough that he was taken to the emergency room to be patched up.
At that point, the plate rack was retired and the range operator ordered a set of new plates made from some kind of steel that's made to stand up to gunfire -- possibly/probably the aforementioned AR500, but I don't know. The new plates are only 1/2-inch thick, so they perform much better (the old plates often wouldn't fall when hit squarely with a 9mm bullet), and there isn't a hint of cratering.
Getting the right alloy is crucial.
And I concur -- the word is "steel," without an 'E' at the end, and it isn't a proper name so it doesn't get capitalized.
