Abstract
Medium carbon V-microalloyed steel continuously cooled from the austenitization temperature at still air, with predominantly acicular ferrite structure, has been investigated by means of four-point bending of notched Griffiths–Owens’s type specimens at liquid nitrogen temperature. Local fracture stress and plastic strain were determined by using finite element analysis and fracture surface examination using scanning electron microscope. It was revealed that cleavage fracture initiation, which takes place close to the notch root in the narrow zone of high plastic strains, is not related to any broken coarse second phase particles. It was assumed that microcracks nucleate by strain induced fracture of pearlite nodules. Two effective surface energy values of 24 and 42 J/m2 were estimated according to the Griffith’s equation, indicating the influence of crystallographic orientation between neighboring grains at the origin of fracture. Lower value was attributed to fracture of coarse ferrite–pearlite units with similar crystallographic orientation and higher value to fracture propagation through fine acicular ferrite matrix.