Have you actually read the article? The last few paragraphs address this:
> N.B: A brief note on the claim carbon nanotubes exist in crucible steel:
> The only articles that "found" carbon nanotubes was published as a brief communication to Nature, i.e not a peer reviewed article, not a full article. This was in 2006, and was only a few pages in length.
> It later found its away into a conference paper by the same authors, still not a peer reviewed article. This was 2 pages in length. These findings should be considered preliminary.
> The method used (dissolving crucible steel in acid and seeing what remains) revealed stands of carbon, but carbon dissolves VERY readily into steel. Crucible steel is typified by cementite spheroids, which often stretch into rods during forging as they are deformed. If you dissolve cementite in acid, removing the iron component, you are left with carbon.
> This does not mean there was an intact carbon nanotube in the core of the cementite rod - and even if it DID mean that, it would have negligible impact on performance because it is *encased* in cementite, which itself is in a soft matrix of pearlite or sorbite.
> But don't take my word for it. Other academics, including those who have been instrumental in understanding crucible steel (namely John Verhoeven) doubt the findings.
> " John Verhoeven, of Iowa State University in Ames, suggests Paufler is seeing something else. Cementite can itself exist as rods, he notes, so there might not be any carbon nanotubes in the rod-like structure."
> "Another potential problem is that TEM equipment sometimes contains nanotubes, says physicist Alex Zettl of the University of California"
https://www.nature.com/articles/444286a