Trim stems from artichokes, leaving about ½” long. Snap off any browned leaves, then remove top fourth of artichokes; discard. If you like, use kitchen scissors to snip off any thorns.
Bring 2” of water to a simmer in a large pot. Fit with a steamer basket and place artichokes inside; season with salt and pepper. Cover and steam until leaves pull out easily and hearts are tender (a paring knife will meet with no resistance), 70–80 minutes. Remove from heat; squeeze lemon over.
Mix butter and garlic in a small bowl; season with salt and pepper. Serve with artichokes.