I don't think that would make any difference. The problem isn't traces of radioisotopes in the initial steel, the problem is vastly larger amounts of radioisotopes created in the steel under intense neutron bombardment due to absorption in certain stable impurity elements.
And when I say "intense", I mean it: the integrated neutron flux the first wall of a DT fusion reactor would be exposed to over its operational lifespan is comparable to, or even greater than, that experienced by components of a nuclear weapon in the brief interval of a nuclear explosion.
