BCRider gives good advice.
I own only two steel targets, a spinning Birchwood Casey double steel plate, and a Caldwell Magnum Gong target made of AR550 (if I recall).
The little spinning target is supposedly rated for up to .44 Magnum but I don't buy it, at least not after shooting it with some midrange .454 Casull loads at 25 yards. Which after the second or third shot ripped it out of the ground, and flung it down range a few feet. Upon closer inspection the target is now dented up pretty badly from the impacts of 360gr bullets.
The Caldwell has been much better, although the plate does have holes through the plate to allow the gong to be hung from chains. The chain hooks are soft mild steel, and the gong swings a good deal when hit with full power .454 loads and the plate chewed up the chain hooks pretty badly. Then they stretched out, and the plate falls off the chains when hit. I just finished rebuilding it using some clevises that should be much more durable. I will note that the plate itself is quite durable, the only damage to it is from a friend of mine smacking it with a .223 round shooting 55gr FMJ at 50 yards before I could tell him not to. All it did was leave a little pock mark, no cratering, and no penetration. As long as impact velocity is below 3000fps it shouldn't leave any damage at all, although FMJ and copper solids are not recommended.