Fill a large pot with enough water to reach the bottom of a steamer basket. Add 1 teaspoon salt and bring to a boil.
Meanwhile, using a serrated knife, cut off the top third of the artichokes; using a vegetable peeler, peel the stems. With kitchen shears, snip off any sharp tips from the remaining leaves. Rub all the cut surfaces with half a lemon.
Spread the artichokes open with your hands. Place stem-side up in the basket with the 2 lemon halves, cover, and steam until a paring knife inserted in a stem meets little resistance, 25 to 35 minutes.