The process of creating steel requires heating it with coke in a large furnace. A large volume of atmospheric air is pumped into the furnace, and it is this atmospheric air that introduces the radioactive particles to the mix.
There are vacuum furnaces that lack this atmospheric air, but their process is much more expensive.
> From 1856 until the mid 20th century, steel was produced in the Bessemer process where air was forced into Bessemer converters converting the pig iron into steel. By the mid-20th century, many steelworks had switched to the BOS process which uses pure oxygen instead of air. However as both processes use atmospheric gas, they are susceptible to contamination from airborne particulates. Present-day air carries radionuclides, such as cobalt-60, which are deposited into the steel giving it a weak radioactive signature.
There are vacuum furnaces that lack this atmospheric air, but their process is much more expensive.