Commercial made .22 rimfire steel targets can be 1/2" thick. Thats not that thick. Your steel will take a bunch of hits, but will not be invincible.
You could harder them: heat till non-magnetic, quench quickly in used motor oil, temper in the wife's/mother's oven at 400F for about an hour. (Sorry Mrs. Andyaux
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They will not dent as easy and will "ring" a little better, but when they crack they can shatter.