In a blender, combine lime juice and zest, cilantro, curry powder, and scallions. Puree.
Place steak in resealable bag and pour half the marinade in bag to coat the steak. Reserve the other ½ cup of marinade. Let steak marinate for 30 to 45 minutes.
Heat a grill or grill pan to medium-high. Clean and lightly oil hot grill. Remove steak from marinade, shaking off excess. Season with salt and pepper.
Grill steak until browned, 8 to 10 minutes per side for medium-rare (cooking time depends on thickness of steak). Transfer steak to a cutting board and let rest 5 minutes before thinly slicing against the grain.
Season reserved marinade with salt and pepper, and serve with sliced steak.