Low alloyed martensitic TRIP assisted steels, obtained by Quenching and Partitioning heat treatment, attract attention in materials science as well as in the steel industry. Their microstructure promises advanced properties at high strength levels, though many questions arise regarding the interaction between the heat treatment parameters and the metallurgical mechanisms. Medium carbon steel containing significant amount of silicon is extensively characterized in this study for the application of Quenching and Partitioning heat treatment. In addition to the mechanical properties, which can be achieved, the beneficial carbon partitioning and the counteracting mechanisms bainite formation and cementite precipitation are of particular interest within this study. The essential carbon partitioning from martensite to austenite is investigated by analytical and numerical simulation and experiments also. X-ray diffraction and dilatometry are used to track the way of carbon in-situ while the Quenching and Partitioning heat treatment is applied. The expected microstructure of low alloyed martensitic TRIP assisted steels is derived from the well-known martensitic transformation kinetics and used to estimate the dimensions and shapes of retained austenite within. The results are validated by Transmission Electron Microscopy and used for the carbon partitioning simulations. Furthermore, the counteracting carbide precipitation in retained austenite is analyzed by numerical precipitation simulation and dilatometric experiments. The change in length caused by Quenching and Partitioning is derived from the changes in the crystallographic structure and affirmed by dilatometry and X-ray diffraction. Experimental evidence is found that the cementite precipitation is occurring from retained austenite at high carbon levels, caused by partitioning and high dislocation densities, established by the prior martensitic phase transformation. The effect of bainite formation below martensite start temperature is reviewed and confirmed by dilatometry. The reaction is taken into account by new combination of bainite start regression and the Constrained Carbon Equilibrium model, which allows prediction of reasonable partitioning temperatures avoiding bainite formation. Finally, the findings are used to develop appropriate Quenching and Partitioning heat treatment of 42SiCrB steel, avoiding counteracting mechanisms while full carbon partitioning takes place. Outstanding mechanical properties, characterizing a new steel family, are achieved. Due to the TRIP effect, analyzed by means of X-ray diffraction, the retained austenite contributes remarkably to the stress-strain behavior. Low alloyed martensitic steels feature high strength and high uniform- and total elongation due to the significant retained austenite volume fractions, exceeding 20 % in this study.