[JohnMakin]

mostly because steel is used for things like particle accelerators that are understandably very sensitive to contamination - as a sibling comment noted this is not much of an issue anymore.

[ceejayoz]

That's the why, not the how.

The how is impurities from the air are introduced during the forging process (https://en.wikipedia.org/wiki/Bessemer_process / https://en.wikipedia.org/wiki/Basic_oxygen_steelmaking), not the mining process.

[JohnMakin]

Yea - the parent was confused about the why.