I would try ironing it but make sure you don't iron the map directly, put something over it like a dish cloth and then
apply the iron. Let me know if this works.

Once a piece of paper is folded (creased), it will never be the same... folding breaks the paper fibers and once broken nothing will mend them. Ironing will, of course, help flatten the creases. But you'll always see them. That's why large pictures (posters, etc.) are always shipped rolled in a large tube or sent flat between stiff cardboards.

Just frame your map and enjoy it, knowing most maps are folded!