1. Pearlite-free steel alloyed with niobium has good resistance to brittle fracture — the ductile-brittle transition temperature determined from the work of crack propagationap=2 kg-m/cm2 is −100°. 2. The optimal niobium concentration is about 0.1%. Further alloying with up to 0.35% Nb makes the steel considerably more expensive and leads to only a negligible increase in resistance to crack propagation.