100 yards for a high-power centerfire is pushing it- what are you calling "hardened steel"- if it's AR-500 you probably won't crater the steel, but like I said- steel core at that close range you need to check it.
Anything other than AR-500 and you're looking for a potential injury-again.
Even if you hang the plate correctly from the back side so it angles downward- if it's not AR-500 and it craters (and it will), any bullets hitting a crater will fragment and ricochet uncontrollably.