Sometimes a piece of rebar will make a really nice knife. Sometimes it won't. The issue is that there's not much requirement for consistent quality in rebar manufacture. As long as it all meets a certain minimum standard, it is fine for the intended purpose of reinforcing concrete.
I suspect rebar will make an acceptable blade more often than not, but do you really want to test each knife to an extreme before you know whether it is any good?
If you are looking for cheap steel for blades, there are other things you can use. Go talk to the local (ie, not a big chain operation) overhead garage door installer. Tell them you are new to bladesmithing and would be willing to pay scrap prices for old springs. Especially the large 7-8" ones.
The difficult thing with those springs is getting them straight. I have a video that shows how I approach it:
https://www.youtube.com/watch?v=Ld1LROiBL9UThere is another way if your forge won't allow for straightening a ring like I do. You can cut each ring of the spring into 3 or 4 pieces (or more, if you want to) and straighten from there. It is faster and easier than making long bars if you just want to make one or two knives. If you want to make a lot of stuff, as I do in the video above, being able to make the whole ring into a 2' bar is very useful. Still, I have made a fair number (around 25-30) of blades by cutting the rings into short segments and not trying to straighten them up before I start forging.
The small door springs like most home garage doors use will also make very good knives, but small. A lot of my small carving tools are from the small springs.
If you can't get anyone to sell you the old garage door springs, you can buy door springs new for not too much money. About $60 for a pair of 32" springs (the wire is about 1/4") will make literally hundreds of blades. Small ones, but very good ones.
Another source of steel that works very well is "music wire." I have no idea what it is meant for in the musical world, but it is a spring tempered rod of up to 1/4" thickness and usually 3' length. Makes a fine blade. Again, though, you buy new and it does cost more than rebar.
Make friends with the auto shop teacher at a local technical school if you can. I did. I am expecting to get a couple of sets of car springs in the fall when he has students tear down a car. Failing that, go to an automotive junkyard and ask to buy mismatched springs that they can't sell as a set. Offer half price. Some car body shops will do a fair amount of replacing springs. It never hurts to ask about old springs. Again, offer scrap prices.
Failing all that, new steel barstock isn't terribly expensive. I have been treated well by Admiral Steel, though that was over 15 years ago the last time I ordered from them.
-Patrick
