Hi there,
To add to the other posts...
Another problem with plain steel (bolt or nail) is that the max saturation level
isnt that high so you need a lot more turns to make a transformer.
Also, there are eddy currents which eat up power and heat up the core.
Nothing says it better though than a picture. Below is a picture of a
BH curve of a material that is designed just for the sake of making
transformers and inductors, and if you look real close and zoom in
on the very center of that curve you'll see another curve in blue,
and notice how much smaller that blue curve is than the larger
big 'S' curve is. Well, the big curve is the magnetic material,
and the very small blue curve is the plain steel bolt or nail.
Quite a difference.
I've also included a 5x zoomed view of the small blue curve...
**broken link removed**
This is just an illustration, as in real life the blue curve could be much smaller too,
as small as 200 times less than the big curve, which says that a bolt or nail
saturates 200 times easier than a good grade of magnetic steel. This is
partly because the steel is specially made to be used with magnetic fields.
Total saturation basically means the steel core disappears, magnetically,
and the construction, even though the core is still inside the wire coil,
turns back into an AIR CORE COIL until the current level drops back down
to some much smaller level where the steel can again become magnetically
active.