Display omitted •The optimal temperature range for hot deformation of 10CrMoWNb steel was established.•A constitutive Arrhenius-type model of 10CrMoWNb steel was obtained.•The Nb7B4C4 phase was identified as a reason of the hot fracture of the steel. High-Cr ferritic–martensitic steels are important materials for use in nuclear reactors. This study describes a development activity for this category of steels involving the investigation of the hot deformation behaviour and microstructure evolution during hot deformation of 10CrMoWNb steel. Hot compression and tension tests were performed in the temperature range of 900–1350°C by using a Gleeble 3800 thermomechanical simulator. The results indicate that the flow stress and ultimate tensile strength increase with a decrease of the deformation temperature and an increase of the strain rate. Based on the experimental true strain-true stress data, the modified Arrhenius-type constitutive model was established for a form of 10CrMoWNb ferritic–martensitic steel. The hot plasticity properties of the 10CrMoWNb steel increase with temperature up to 1275°C due to dynamic recrystallisation processes in the austenite phase. The reduction of area decreases when the temperature is higher than 1300°C and is zero at 1350°C for all strain rates because of the liquid phase appearance in the structure of the steel.