The steadily increasing demands for high performance tools require higher and higher alloyed tool steels which are only producible via a powder metallurgical (PM) route. Main advantages of the PM-route are prevention of macro- and micro segregation, a high content of very fine and evenly distributed carbides as well as a higher flexibility in the choice of the alloying elements. Comparing the microstructure of the niobium alloyed K490 PM MICROCLEAN with other PM - steels it is noticeable that the average carbide size in K490 PM MICROCLEAN is about two times smaller than in common PM steels. Thus, the aim this thesis is to clarify the effect of niobium in the K490 PM MICROCLEAN. Furthermore, the occurring types of carbides in K490 PM MICROCLEAN shall be determined. On the basis of a carbide size distribution the different carbide types of the niobium alloyed K490 PM MICROCLEAN are being compared with the niobium-free PM-Steel S790 PM MICROCLEAN. By measuring the solidification path of these two PM-steels the carbide formation temperature of both steels are determined and compared with thermodynamic calculations. On the basis of the obtained results it was shown that in K490 PM MICROCLEAN contains V-rich, Cr-rich and Mo-W-rich carbides. The thermodynamic calculations in combination with the conducted experiments indicated that niobium only influences the MC carbide formation temperature. Thus, the finer carbide distribution of K490 PM MICROCLEAN is caused by the entire alloying concept.