The material microstructure affects both the mechanical and the magnetic properties of ferromagnetic materials. This behavior can be conveniently used for designing systems for electromagnetic nondestructive material characterization (MC) of specimens. In this paper, we present an efficient numerical model of a MC system, which measures the incremental permeability of a specimen along a hysteresis loop in the presence of the eddy currents induced by a suitable coil. The numerical model solves in an efficient way two coupled 3D volume integral equations in the presence of periodic sources, eddy currents and vector hysteretic media. The results highlight the relationship between the incremental permeability tensor and the measured quantity related to the impedance of the excitation coil