The surging demand for high-quality rotor shafts or similar components with large format in heavy industries poses new challenges to steelmakers. Due to the inherent shortcomings and limitations of the existing technologies, they do not work properly for these components. For example, the conventional ingot casting (IC) technology is limited by the ingot height and has lower yield and higher operational costs; the developed continuous casting (CC) technology is restricted by the cross-section of the strand. Therefore, a new casting concept, semi-continuous casting (SCC), which attempts to combine the advantages of IC and CC, is recently proposed for steel production. Although a similar casting process, i.e., direct-chill casting, is mature in producing non-ferrous alloys, the application of the SCC in the production of quality steel requires addressing certain challenges due to the low thermal conductivity and high pouring temperature of steel. Additionally, the understanding of the solidification process regarding the formation of the as-cast structure and macrosegregation during the SCC process is still unknown. In this thesis, a three-phase columnar-equiaxed solidification model, which was previously proposed by Wu and Ludwig, was extended and used to investigate the formation of the as-cast structure and macrosegregation during the SCC process of steel. Firstly, the three-phase multiphase solidification model was extended: 1) the fragmentation of columnar equiaxed, which serves as a main source of the equiaxed grains, is considered; 2) a coupling scheme between the flow field and electromagnetic field is established; 3) phenomena of remelting and destruction of the equiaxed grains are considered when the grains are exposed to the superheated region. Secondly, the well-established model was validated by comparing the simulation results with the benchmark experiments at the laboratory scale and the continuous casting of steel billet at the industrial scale. 1) The evolutions of the as-cast structure and macrosegregation of Sn-10wt.% Pb alloy under the effect of different types of forced convection, which is powered by a travelling magnetic field (TMF), were systematically studied by Hachani et al., in 2015 at the SIMAP Laboratory in Grenoble, France. Four different experiments were investigated: (i) without TMF; (ii) TMF in the same direction as natural convection; (iii) TMF in the opposite direction as natural convection; (iv) TMF periodically reversed with respect to natural convection. The TMF is found to play an important role in homogenizing the temperature and increasing the macrosegregation intensity. Good simulation-experiment agreements in terms of temperature field, as-cast structure and macrosegregation were obtained. 2) The effect of the mold electromagnetic stirring (M-EMS) on the formation of as-cast structure and macrosegregation of a steel billet (195 mm × 195 mm) was investigated. The M-EMS tends to accelerate the superheat dissipation in the mold region, leaving the liquid core out of the mold region largely undercooled. Additionally, the fragmentation rate is sensitive to the M-EMS implementation. The calculated macrostructure showed a satisfactory agreement with the as-cast structure. Finally, the extended three-phase model was used to simulate the solidification process of the SCC under the effect of EMS. The solidification principle during the SCC process was deeply investigated. As an outlook, the model can be used to perform systematic parameter studies, instead of costly pilot/field-casting trials, towards the optimization of the SCC process. The computational capacity (hardware) is still a limiting factor for this purpose.