I work with metal alot (CNC plasma cutting) so I have access to alot of sheet steel. I've used some scrap 3/4" mild steel (A36) for these two hanger targets, but they are really only good for handguns IMO. I've shot them with .223 and 762X39 and they make a very nasty hole. They don't go through but they go into the steel at least 1/4".
If two shots were to hit in the same spot, I'm sure it would go through. I've stopped shooting them with rifles to save them for large caliber handguns.
I use the smaller 1/4" steel for 9 MM but it still wants to "cup" the steel after a while. That's ok though, just turn the steel over and you're good to go for a while more.
What you should be making them out of is 3/8" AR400 or AR500 steel. It's a much harder steel but is also much more expensive.
You won't be able to harden the mild steel enough to matter. It's the carbon content of the steel that allows it to be hardened good.
Here's my target setup:
I actually don't like shooting these steel targets anymore though... I've shot at some with A500 and the shrapnel deosn't fly as much. When I would shoot at my mild steel targets, sometimes I would see leaves getting hit halfway in between me and the targets. That kind of freaked me out. Now I use cardboard or paper targets and have started saving old dishes too.