In this article, a constitutive model for quench-hardenable boron steel is presented. Three sets of boron steel blanks are heat treated such that their as-treated microstructures are close to fully martensitic, bainitic and ferritic/pearlitic, respectively. Hardness measurements show that the resulting blanks cover the full scope of possible hardness values, from 165 HV in the ferritic/pearlitic range to 477 HV in the fully hardened state. These three main grades provide the input data for a constitutive model consisting of an extended Swift hardening law and a stress triaxiality and Lode angle dependent fracture criterion. The hardening behavior of each grade is determined using standard tensile tests at quasi-static strain rates. The strain-based fracture criterion is calibrated using four different flat fracture samples. The behavior of intermediate hardness grades is approximated by piecewise linear combination of the three calibrated constitutive models. A newly developed tapered tensile test specimen featuring a hardness transition zone in the gauge section is used to verify the model at hand. A four point bending test of a top hat section of intermediate hardness is used to verify the model for complex loading conditions.