Explosive welding, kind of cool:



Explosion welding - Wikipedia, the free encyclopedia

Anyway even if you remove the steel the surface of the copper is going to look crappy because of the welding involved. Are you sure it is just normal steel and not some kind of exotic mix (is it rusting currently)? Chemical removal will be messy (unless you have access to large amounts of warm seawater and lots of time on your hands to let it rust away).