Being an effective sintering enhancer boron is gaining relevance for obtaining high density PM steels. Thermodynamic calculations are an important tool for studying the roll of alloying elements in the formation of a liquid during sintering. In the present work, the system Fe-Cr-B was obtained by combining up to date thermodynamic descriptions for the subsystems Fe-Cr, Cr-B and B-Cr. The calculations were carried out with Thermo-Calc software to predict isothermal sections for the ternary diagram for 1210 and 1250°C. The analysis of the isothermal sections indicates that the solid phases in equilibrium with the liquid are M2B and a-BCC solid solution. The generation of the liquid is based on a eutectic reaction (Lða+(FeCr)2B) involving the mixed borides previously formed. On the other hand, simulations for PM steels with constant boron but higher chromium content allowed realising that the formation of the liquid may be completely inhibited, within the temperature range under consideration, as materials with too high Cr/Fe ratios are used. This study was also supported by selected experiments which were in excellent agreement with the thermodynamic calculations.