In general, springs are made from steels that are quite suitable for making blades. Almost any spring will make a decent blade if heat treated appropriately. As springs, they are a bit soft for supreme edge holding, though they are "hard enough" to make a halfway decent knife without changing the hardness by heat treating.

Beating a cold spring flat, from it's arched condition is very difficult, and silly, and not good for the steel's microstructural condition.