I made a small curbside find today, a 3" stack of sheet steel pieces
with a "FREE" sign taped to them. So into the truck they went. The
pieces are 6-10" wide by 30-36" long and range in thickness from 1/16"
to 1/4". Unfortunately, most of the thicker stuff is a bit curved,
bowed the long way anything from 1/2" to 1" at the center. I'd guess
from stress caused by shearing them to size.

Any suggestions how to flatten them out?

Tove