Atoms in the atmosphere

Apparantly one way salvage operators make money is to lift steel armour plates from warships sunk before the first atomic bombs were detonated. The atom bombs apparently relesed material that finds it way into the steel produced today and by virtue of the date of manufacture and destruction of these ships their steel contains none of these radioactive particles, the steel is therefore good for use in sensitive scanning devices.
What exactly was released into the atmosphere and why does it find its way into the steel produced today?

WAG: the carbon isotope distribution is different? That would seem to fit.

My roommate, a former engineer on a nuclear sub, confirmed the value of pre-atomic-age forged steel. The metal from battleships laid before then is called OBS in material lists, for “old battleship steel”.

As to what is in our environment that contaminates production: since the early bombs only had 1 kg. of the fissionable material actually fission, the rest got pulverized and dispersed to the winds. Also, lots of soil was exposed to radiation and kicked up to stratospheric heights, where it was dispersed globally.

In a related topic, scientists are raising lead ingots from grecco-roman cargo ships sunk in the Mediterranian Sea thousands of years ago. Apparently, sitting on the bottom of the seaa, the ingots are no longer in contact with background radiation…cosmic sources, artificial sources,
the very uranium and or plutonium from which the lead decayed, anything at all.
This makes the aincient lead a superior sensor shield.