Simulating steel semi-solid metal forming close to a real industrial forming is necessary to reduce potential run-time errors and costs. Simulating requires modeling of semi-solid behaviour. In this work a model based on micromechanical analysis with consideration the viscoplastic response along with an enhanced Herschel-Bulkley model has been proposed to predict the flow behaviour of semi-solid M2 high speed steel. This model is introduced to overcome the limitations of the previously used models. The extracted model parameters from the steady state flow stress and step-change of shear rate experiments were then calculated and fitted to the experimental rheology results of continuous cooling. The model was then implemented linking into the computational fluid dynamics software (FLOW 3D Ver.9.3). A T-Shape die used to investigate the flow front of semisolid alloy. Also the extracted parameters at low shear rate in the step change experiments used for simulation of the compression process of M2 tool steel at high solid fraction by the proposed model and in the ABAQUS 6.9 software. The experimental results of compression test shows a well agreement with simulations results indicative of the performance of the proposed model to predict rheological properties and flow behaviour of semi-solid states especially for the tool steel systems in the wide range of liquid fraction.