In a resealable plastic bag, combine lime juice, soy sauce, scallions, ginger, and red-pepper flakes. Add steak, and seal bag (place in a dish to catch any leaks); marinate in the refrigerator, turning occasionally, up to 1 hour.
Heat grill to high; lightly oil grates. Remove steak from marinade, letting excess drip off (discard marinade); season with salt and black pepper. Place on grill; cover. Cook, turning once, until meat reaches desired doneness, 6 to 8 minutes for medium-rare. Let rest 10 minutes before slicing thinly.