There's an airfield somewhere in the United States with a metal runway. Where is it?
You are thinking of Bogue Field (KNJM). It is a Marine Corps auxiliary landing field, currently mocked up to look like a carrier deck, and last I heard it was still active and in use to practice carrier landings.

The runway surface is Perforated Steel Planking (PSP - actually I believe the new stuff isn't "steel" anymore but the name remains) - discrete sections that can be laid out to quickly construct temporary landing strips in forward combat zones.



A search on AirNav for "pierced steel plaking" came up with a couple more military fields:

KNXP   Twentynine Palms SELF Airport (Strategic Expeditionary Landing Field) - 
Twentynine Palms, California, USA

2CA9   Castle Dome Army Heliport - 
Yuma Proving Ground (Yuma), Arizona, USA

2CA9 says it's a helipad but it's 3000x65. The rest of the results were all (reasonably sized) helipads.