This work presents a semi-empirical model framework able to predict the microstructurual development of the steel 15-5 PH during a multi-step forging and heat treatment process as well as the resulting yield strength, fracture toughness and ductile-brittle transition temperature. The model framework will aid computational process development of large structural aircraft parts, by only relying on the output of a finite element simulation of the forging process and the heat treatment parameters. It will be able to predict the performance at different positions of the final component in order to identify critical positions. Furthermore, by identifying the influence of the different process steps and parameters, the production process can be improved regarding e.g. process stability or efficiency. The microstructural constituents included in the model framework are niobium carbonitrides (NbC), the austenite grain size and the resulting substructure size of the martensitic matrix, and reverted austenite. Another constituent important for the mechanical properties, which is only considered intrinsically in this work are copper precipitates. In order to model the microstructural development during forging, laboratory-scale experiments using a Gleeble physical simulation set-up as well as industrial-scale die-forgings have been conducted. Grain growth during pre-heating in-tandem with NbC solution as well as grain refinement via recrystallization and NbC precipitation due to deformation and the effect of NbC clustering will be studied and modeled. The microstructural developement during annealing and aging will be investigated using dilatometry, X-ray diffraction, optical light microscopy, electron backscatter diffraction and atom probe tomography. Furthermore, tensile testing with in-situ high energy X-ray diffraction will be used identify the mechanical stability of reverted austenite. Finally, a set of microstructural conditions will be tuned and tensile testing, Charpy-V testing J-Integral testing will be used to determine the influence of the microstructure on the yield strength, ductile-brittle transition temperature and fracture toughness. Calibration and R^2 analysis of the model frameworks sub-models indicate a high accuracy.