Lot of good advice so far. There may be better steels available but these penny pinching days using what you have is not a bad way to go either.
I'll repeat and emphasize, targets must be able to swing and/or fall easily. And I do mean "easily". Too heavy or rigid fixed targets will throw detrius, even whole bullets back at you.
I used to manufacture metallic steel targets and did extensive testing shooting at different steels and mountings. There is a right way and a wrong way. The scar on my elbow is a reminder of one of the wrong ways.
BTW, I used T1A steet. It is a high impact resistant steel used in the mining industry and it served well. Not saying there aren't others just as good or better but that is what I used. No complaints from customers.