Sorry for the slow reply – this one slipped by me.
I don’t think steel and aluminum tariffs will have any impact on the TSP I Fund because most of the I Fund’s component economies have been exempted already (the notable exception being Japan).
There are a lot of moving parts, but I expect it to be roughly a net wash for the I Fund. Some countries will export less to the US, so steel/aluminum producers in those countries will be hurt. But producers in exempted countries will now face less competition and so will be able to raise prices on steel being exported to the US. And foreign companies which import steel will be able to get it cheaper from non-exempt countries than they could before, improving their profit margins.
So in the end, all steel and aluminum imported to the US will be more expensive (a minor drag on the US economy), while EU and other exempted countries will see benefits both importing and exporting.