I've been told by an informed source that rebar is quite passable for a working blade and is *usually* medium carbon. However, this is a bone of contention from smith to smith. You wouldn't want to make an heirloom out of it and you'll end up sharpening it more than you would tool steel, though. The best thing about rebar is it's generally free is *usually* constant quality and if nothing else is great from practicing on. I've got 2 blades here that I've forged I need to finish made from it.
You can make quite a few stick tangs from that bunch!