Advanced high strength steels (AHSSs) are considered to have a promising future due to the outstanding properties compared with the conventional steel and have been widely adopted as the base materials for the automotive components. Some of the challenges preventing the extensive applications of AHSSs are due the solidification defects, i.e. hot tearing and segregation. In this thesis, a 3D mesoscale and multi-physics model is developed and validated to directly investigate solidification defects for semi-solid steel with dendritic morphology associated with the peritectic transformation. Similar to the prior models [1,2], the current model explicitly considers the solidification behavior of each grain prior to assembling, which allows for the mesoscale simulation within a semisolid containing thousands of grains. Six sub-models are incorporated: (i) microstructure generation model is used to create the fully solidified microstructure of equiaxed grains based on a Voronoi tessellation; (ii) a dendritic solidification module based on an average volume approach is developed for predicting the solidification behavior of a random set of grains, considering the diffusion in different phases along with peritectic transformation. The progressive coalescence to form a solid cluster is predicted by incorporating an interfacial energy determination model; (iii) a fluid flow module is developed for the prediction of both intra-dendritic flow and extra-dendritic flow within the dendritic network induced by solidification shrinkage and deformation; (iv) a semisolid deformation model is used and extended to simulate the semi-solid mechanical behavior of steel using a discrete element method. The solid grains are modeled using a constitutive law and implemented via Abaqus commercial software; (v) a coupled cracking model incorporated with a failure criterion is used and extended to predict the crack formation and propagation in semi-solid steel. This comprehensive model consists of models (i-iv) and considers the interaction between the deformation within the solid phase and pressure drop in the liquid phase; (vi) a one-way coupled solute transportation module is also developed and used to simulate the solute redistribution due to fluid flow and diffusion within the liquid channels assuming the solid grains are fixed. The movement of the solute-enriched liquid in the solute transport model is induced by solidification shrinkage and deformation. The new 3D mesoscale model is then applied to correlate the semisolid behavior during solidification to different physical and process parameters. The results from the dendritic solidification model show the evolution in semi-solid microstructure and consequently liquid film migration. The model is able to predict the solidification of equiaxed grains with either globular and dendritic structure having experiencing primary solidification and the peritectic transformation. The coalescence phenomenon between grains is considered at the end of solidification using Bulatov’s approach[24] for estimating interfacial energy. It is seen that only 0.9% of the grains are attractive based on their orientations within a specific domain, significantly depressing final-stage solidification. The dendritic fluid flow model quantitatively captures both semi-solid morphology and the fluid flow behavior, and provides an alternative to the convectional experiment for the prediction of permeability by using the given surface area concentration. Comparison of the numerical and experimental permeabilities shows a good agreement (within ± 5%) for either extra-dendrite or intra-dendritic flow, and deviation from the conventional Carman-Kozeny equations using simplified Dendritic Sv or Globular Sv are explained in detail. The results quantitatively demonstrate the effect of grain size and microstructure morphology during solidification on the permeability prediction. The localization of liquid feeding under the pressure gradient is also reproduced. Additionally, the fluid flow due to shrinkage and deformation for non-peritectic and peritectic steel grades with dendritic morphology during solidification was captured for the first time. The cracking model allows for the prediction of hot tearing initiation and the progressive propagation during a tensile test deformation and the results are compared with the experimental results conducted by Seol et al.[3]at different solid fractions. Parametric studies of coalescence criteria and surface tension on the constitutive behavior of the semisolid are discussed and the deformation behavior of alloys with different carbon contents under a feedable mushy zone is investigated. Finally, the solute transport model has been applied to the continuous casting process of steel for the investigation of centreline segregation, and results indicate that the grain size has a great impact on the solute distribution and solute partitioning combined with intra-dendritic fluid flow leads eventually to liquid channels enriched with solute. The predicted composition in these discrete liquid channels shows a great match with the experimental measured profile obtained via the microscopic X-Ray fluorescence (MXRF). / Thesis / Doctor of Philosophy (PhD)