Mild steel is iron that contains less than about 0.25% carbon. This
low carbon content means that it cannot be hardened to any noticeable
degree. If its been work hardened then it can be annealed by heating
to a dull red and allowing it to cool VERY slowly. It will never get
anywhere near the softness of copper or brass. I would think that
any mild steel plate you buy is already as soft as it will ever get.
If used for jewellery its biggest problem is rust. You can reduce
rusting by chemical blacking, or by heating to a dull red and
quenching in old motor oil (not really recommended for safety
reasons). There are various chemicals that can colour steel, mostly
various shades of blue, black and grey.
If you want to use steel for jewellery, use stainless steel; it can
be worked like mild steel and is highly resistant to rust. There are
many types of stainless steel, so you can browse to find the grade
that’s most suitable.
I’ve not personally found a steel that cannot be silver soldered
(silver brazed), but its possible that grades containing lead will
All (non-tool) steels can be drilled and pierced, but some grades of
stainless are notorious for very rapidly work hardening to the
extent that they can blunt saws and drills. Other grades are fine -
you should do your homework.