ΔE 31.4, loose: different enough that you should pick on purpose. Steel (Metallic) is noticeably lighter than Tarnished Steel.
Can I use Steel (Metallic) instead of Tarnished Steel?
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 31). Steel (Metallic) is noticeably lighter than Tarnished Steel.
Can I use Tarnished Steel instead of Steel (Metallic)?
The same difference applies the other way round: Tarnished Steel is noticeably darker.
ΔE is the colour difference between our two swatch estimates: under about 6 is close, over about 15 is loose. Real paints also differ in finish, coverage and how they thin, so test before you commit.