The constitutive behaviour of a metastable austenitic stainless steel during metal forming and hardening is described with a macroscopic model. The steel undergoes a strain-induced transformation during forming. Depending on the annealing conditions, the steel also transforms isothermally as opposed to athermal martensite. This transformation can also take place immediately after plastic deformation, as a result of the residual stresses. The martensite phase of this steel shows a substantial ageing response, about 1000 N/mm2. A constitutive model based on the data from literature as well as on measurements is presented which describes the stress–strain behaviour along with the phase transformation. The work hardening is based on dislocation density as internal state variable. With this information, a new model is constructed that describes the isothermal or stress-assisted transformation, the strain-induced transformation, the work hardening as well as the ageing behaviour of the steel. The model can be implemented in a FEM code.