At high temperature, FeSi nanograins obtained after partial crystallization of amorphous Fe-Si-Cu-Nb-B precursor are magnetically decoupled and superparamagnetic, due to the disappearance of the magnetism of the residual amorphous matrix. At a transition temperature Ttr typically ranging between 600 K and 850 K, still above the amorphous Curie temperature, a spontaneous polarisation JSsf appears, despite a much lower blocking temperature. Magnetostatic interactions, rendered by the Lorentz field (L.f.), or a residual ferromagnetic coupling between grains through the amorphous medium, have been invoked to explain this behavior. We investigate here the magnetostatic hypothesis. An improvement of the L.f. model is carried out, based on the idea that the field acting on a nanograin is screened by a surrounding soft shell. The permeability of this shell in the vicinity of Ttr is determined, leading to an attenuation factor S = 2.8 and a decrease of Ttr compared to the L.f. approximation. Moreover, a description of the spontaneous magnetization curve JSsf(T) / JS(T) is obtained, with a slope near Ttr much sharper than predicted by the (Ttr - T)1/2 law associated to an invariant molecular field factor. Comparison with experiments shows good agreement.