Fill a large pot with about 2 inches of water. Set a steamer basket in pot (make sure water doesn't seep through holes). Bring to a boil, then reduce to a rapid simmer. Stand artichokes upright in steamer and season with salt. Top each with a lemon slice. Cover pot and steam until bottoms of artichokes are very tender when pierced through the center with a tip of a paring knife, 35 to 50 minutes.