In this work a material model for hardening development in sheet metals during forming processes involving loading path changes is formulated. In particular, such hardening development is due to the formation and interaction of dislocation microstructures in the material, resulting in an evolution in the size, center and shape of the yield surface. Such yield surface evolution is accounted for in the current model with the help of an evolving structure tensor. The model is intended for an air hardening steel and takes therefore thermomechanics into account in particular phase transformations from ferrite to austenite and from austenite to martensite. As numerical examples a tension shear test and a heating-cooling sequence are simulated.