Recent steel developments for dynamically high loaded closed die forgings illustrate a trend towards fine grained microstructures, which are formed directly from the forging temperature and which do not need any further heat treatment. In a literature survey, typical forging steels and their most important influencing parameters were compared. Experiments were performed using three MnB-steel grades with different silicon contents. In order to estimate of the thermodynamic data and transformation diagrams, as well as to choose the settings for the test parameters, the software JMatPro was utilized. Using the testing machine Gleeble 3800, isothermal transformation-tests at different tem-perature levels in the bainitic region were conducted. The transformation kinetic was meas-ured by dilatometry. In addition, tensile and hardness tests were done after the isothermal transformation. The results provide a first overview on the formation of a fine microstructure and the ex-pected mechanical properties. An influence of silicon was present, however the effect was rather small. At the transformation temperatures just above the martensite start temperature (at about 400 °C) strength values of almost 1000 MPa could be reached. Therefore, significant improvements of the fatigue endurance can be presumed.