Abstract
Effect of bainite volume fraction on tensile properties of ferrite-bainite steels was investigated experimentally and analytically. Ferrite-bainite steels with different bainite volume fractions having the similar mechanical properties for each constituent phase were prepared and the tensile properties were investigated. Lower yield ratio (YR) and higher n-value were obtained for the steels with bainite volume fraction of 16 to 35%, while the ferrite or bainite single phase steels exhibited relatively higher YR and lower n-value. In order to investigate microscopic deformation behavior of ferrite-bainite steels, an axisymmetric unit cell model based on a regular array of second-phase particles arranged on a BCC lattice was applied. Finite element analyses were carried out to investigate the macroscopic and microscopic response of unit cells with morphological features based on the actual steels. Macroscopic deformation behavior of ferrite-bainite steels calculated by the unit cell model showed good agreement with experimental results. In addition, the effect of bainite volume fraction on YR and n-value was well simulated by the unit cell model. Microscopic investigation on the unit cells revealed that significant strain concentration existed in the ferrite phase near the ferrite/bainite boundary for ferrite-16% and 35% bainite steel, which had higher experimental n-value. However, further increasing of bainite volume fraction caused plastic deformation of bainite phase and the strain concentration was decreased, resulting in lower level of n-value for ferrite-58% bainite steel. Effect of bainite morphology on strain hardening behavior of ferrite-bainite steels was also discussed based on analytical study using the unit cell model.
