Hello,
Yes they can be used for knifes, the high end planer blades are made of some pretty good steel. Before you forge them though make sure that they don't have chrome or nickle in them I have never heard of a good planer blade that has those metals in the steel but with all the junk coming from china it is better to be safe than sorry. Annealing is an ok process for mild steels that you are planing on cold forging, but for higher carbons I just cut my working shape, and heat it up; be careful to keep a good temp in the part you are forging as higher carbon steels can crack very easily when hit at a temp too low. also when you finish the knife make sure to harden and temper the metal, mild steels don't need this process (unless you case harden them in vermiculite) but high carbons steels do, as the crystal structure of the carbon is what enhances the hardness of the steel, by quenching you form max hardness, or close to, and tempering relaxes this super hard metal into a more reasonable hardness for what ever you are planing to make, thus reducing the likely hood of your blade shattering. hope this helps