It would depend on the type of steel. Steel used in automobile fenders is malleable at highway temperatures. Steel used in automobile springs is not malleable, but is prone to shattering at highway temperatures. One is manufactured by cold stamping, the other is either cast or hot-rolled. Both are malleable at some temperature, depending on the impurities, carbon percentage for example.
Well... yes and no...
The are different types of steel, as supplied by the mill... but it is likely that the fender material would have originally been "harder" and "stronger" than that used for springs. The sheet metal used in automotive bodies is typically cold-rolled steel, which, although still malleable for stamping/forming purposes, is still harder and stronger than the hot-rolled equivalent. Part of the reason for this is that cold-rolled typically has a nicer, smoother surface finish suitable for painting and is theoreticly less prone to scratching. Cold rolled is manufactured from hot-rolled stock, essentially an added step in the process to obtain this enhanced property in additon to tighter tolerances on metal thickness.
OTOH, springs are likely made from some alloy of softer, hot-formed stock -- perhaps hot-rolled sheet or plate for leaf springs, some kind of heavy wire or rod for coil springs. As supplied from the mill, it would be softer and more malleable, making it easier to stamp/bend/form to the desired shape. The "springy-ness" would be imparted to these parts through heat treatment after they were made, which also affects related properties such as tensile strength, hardness, etc. etc.
Castings??? Nyahhh! (wish you hadn't mentioned that, you were pretty much in the ballpark otherwise. Not bad, anyway!)