New processes have been developed in the last years to produce near net shape steel products by continuous casting. This work deals with the analysis of the effects of liquid core reduction during solidification in the microstructure of thin slabs obtained by continuous casting processes. This analysis involves experimental works and numerical simulation. Equipment able to simulate the liquid core reduction during solidification was projected and constructed. This instrumented simulator consists of a water-cooled mould with a moveable sidewall driven by a hydraulic piston. A numerical method , based on finite elements, able to simulate the solidification of thin slabs submitted to liquid core reduction and to predict the microstructure formation was developed. Experiences and numerical simulations were carried out using steels with different compositions, with and without liquid core reduction in order to analyze the influence of the solidification parameters in the microstructure formation. The alloy temperature during solidification was monitored by thermocouples positioned within the slab. Samples from the as cast slabs were examined after polishing and etching to characterize the microstructure. Comparisons between experimental and numerical results were carried out showing good agreement. The numerical model was applied to simulate different cases of solidification of thin slabs with liquid core reduction.