A new multiscale model is presented for the first time to perform the slip, slip-twin induced plasticity phenomenon in low-C expandable tubular steel, with distinct amount of Mn, Si and other impurities, using finite element method. An elastic-plastic deformation of expandable steel is modeled using crystal plasticity theory by considering slip only, and combined slip and twin deformation modes. Effects of grains orientation, slip and twin deformation mechanism, work hardening, and number, distribution and relative volume fraction of different phases present in expandable tubular steel are taken into consideration in the developed model. The inclusion of different phases, either single or multiple, is made possible through a simple homogenization technique. The upper scale finite element model consists of different representarive volume elelemnts (RVE) representing different single or combined phases. Each RVE is composed of 500 to 7000 crystals. Averaging the behavior of entire RVEs in the finite element model allowed to bridge the gap between single crystal and actual specimen behavior. Numerical implementation is done through user defined subroutines in commercial finite element analysis software, ABAQUS. Simulations are done for single/polycrystal as well as single and multiple phases present in expandable tubular. Results are validated through published experimental and numerical data. The effect of grain orientation, and loading condition on stress-strain behavior, slip and twin systems activity, and overall specimen behavior such as wall and length reductions and expansion forces are investigated. Further to substantiate the theoretical understanding of expandable tubular, an analytical model is developed, whose results agree very well with available experimental and finite element results. Various studies are done to extract useful data needed for various elements of this research work as well as to gain useful insight into the material behavior. These mechanical strength and microscopic studies include strength properties, compositional analysis, crystallographic orientations, grain sizes and morphology, distribution of phases, and ductile-to-brittle transition. The results showed that the current tubular material experiences severe degradation in load bearing capacity after expansion, which restricts its expansion limit to 15 18% in order to have adequate safety margin for down-hole application. A comparative study with commercially available similar type of steel showed that twin based steel have a promising future for expandable application, which constitutes major contribution of this research work as stated before.