In my opinion, you'd be money ahead to just go ahead and spend a few bucks on some decent KNOWN blade steel that's already been annealed and prepped for making a knife. By the time you go through the effort of cleaning up the steel, annealing it, trying to straighten it out and do anyother prep work on it (which may even involve burning up a few drill bits and wearing out some other tools), you still may not even know the "exact" makeup of the steel and will have no way of knowing if you're getting a truly optimum heat treat or performance from the steel.
Known blade steel is going to respond consistently and predictably. Free steel is nice, but unless you know exactly what it is and how it will perform, chances are it's going to cost you more in the long run. If not money, then surely time and headaches.