Nb–Cr complex carbide coatings were produced onto AISI D2 steel by the thermo-reactive diffusion process to improve the surface hardness and corrosion resistance of this tool steel. The carbide coating treatment was performed using molten borax added with ferroniobium, ferrochrome and aluminum at temperatures of 1,223, 1,293 and 1,363 K during 2, 3, 4 and 5 h. The coating layers were characterized by optical and Scanning Electron Microscopy, X-ray diffraction, energy-dispersive X-ray spectroscopy and X-ray fluorescence spectrometry. The coating growth rates were evaluated, and a model of the layer thickness as a function of the treatment time and temperature was established. The hardness of the coating was measured by nanoindentation, and its resistance to corrosion was evaluated with electrochemical tests of potentiodynamic polarization. The produced carbide layers presented a homogeneous thickness as well as an improved hardness and corrosion resistance as compared to the uncoated steel.