Two medium-carbon microalloyed steels with a predominant acicular ferrite microstructure were investigated in this study in order to determine the initial micro-crack formation mechanism and the role of acicular ferrite structure in cleavage fracture. In order to ensure cleavage fracture, samples were investigated at -196 °C for uniaxial tension and four point bending fracture. Previous investigations have shown that cleavage fracture for steels with a predominant acicular ferrite microstructure has not been initiated by the fracture of coarse TiN particles as in ferrite-pearlite, bainite, or martensitic microalloyed steels. The average maximal thickness of cementite plates measured in this work is 0.798 µm and 0.966 µm, for V and TiV steel, respectively. The corresponding stress values required for their fracture according to Griffith's equation are 1970 MPa and 1791 MPa, respectively. Estimated values of the effective surface energy for the V steel with an average cementite volume fraction of 3.8% range from 40 Jm-2 to 86 Jm-2, and for the TiV steel with an average cementite volume fraction of 18.3% range from 55 Jm-2 to 82 Jm-2. The fracture of coarse cementite plates was found to not to be responsible for the cleavage fracture initiation in case of both steels.
Keywords: ABAQUS; MIPAR; acicular ferrite; cleavage fracture; finite element analysis; forging steel.