To have a steel target that will survive shooting rifle rounds you will need to have at least AR500 target faces. Even then the closer you are the shorter the safe life span will be of the target face.
Some rounds will do more damage then others, 62 grain green tip 5.56 seem to do quite a bit of damage at close range (inside 50 yards), while the 55 grain FMJ .223 doesn't do near as much.
Short answer is if you want to shoot at steel safely you really need to have at least AR500 and respect the manufactures safe distance practices.