Effects of Microstructure and Particle Population on Void Damage Evolution in Complex Phase Steel
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This thesis aims to provide a detailed description of the microstructure and void damage evolution in a grade of hot-rolled complex phase (CP) sheet steel produced by ArcelorMittal Dofasco (AMD). The evolution of microstructure was studied through a series of dilatometer experiments, and the effects of matrix microstructure and TiN particle population on void damage evolution were studied through interrupted hole tension testing and micro-computed tomographic (microCT) imaging of both commercially produced sheets and sheets heat treated to produce polygonal ferrite (PF) and granular bainite (GB) model microstructures. Continuous cooling transformation (CCT) diagrams illustrate the phase transformations which produce PF, GB and lath bainite (B) in the grade of CP steel sheet under study. While the level of deformation achieved in industrial hot rolling processes is unattainable in laboratory-scale experiments, interpretation of CCT diagrams enables the understanding of how different microstructures develop in commercial CP steel sheets. In the grade of CP steel under study, large 'primary' voids were found to nucleate at TiN particles through either fracture of large (>100 µm) TiN particles or decohesion of the particle-matrix interface at smaller TiN particles. Voids develop ellipsoidal shapes in specimens deformed to moderate to high levels of plastic strain, with growth occurring primarily in the direction of tensile loading. A population of fewer, larger voids developed in deformed hole tension specimens is associated with a distribution of fewer, larger TiN particles or a high level of TiN particle clustering. In sheets containing large TiN particles, a GB model microstructure was found to result in a population of fewer, larger voids than those developed in PF sheets. The onset of coalescence was observed at high levels of plastic strain, with void damage development culminating in one or more large coalescence events. Coalescence mechanisms are dependent on void spacing and orientation with respect to the loading direction, both of which are influenced by the matrix microstructure and TiN particle population. A microstructure consisting of PF and GB containing a uniform distribution of small TiN particles is proposed to produce a combination of high strength and stretch flangeability.