© 2020 The microstructure and tensile properties of thermo-mechanically processed ferritic steels alloyed with Mo, Cr, V and Nb were investigated using scanning transmission electron microscopy, atom probe tomography and uniaxial tensile loading. The microstructures consisted of predominantly polygonal ferrite (~4 ± 3 µm average grain size) and a high number density of nano-sized, disc-shaped interphase precipitates (~2-4 nm diameter and 1 ± 0.4 nm thick). Interphase fiber carbides were also present in V, Cr and Nb -alloyed steel. Cluster formation was observed both between the interphase precipitates in rows and in ferrite matrix between the rows. The individual contributions to yield strength arising from different operating strengthening mechanisms were estimated. For the first time, the operation of coherency and modulus strengthening mechanisms for nano-sized interphase precipitates in this class of steels was considered. The major contributions were due to grain boundary strengthening and nano-sized precipitation, in the order of ~280 MPa and ~395 MPa, respectively. Significant cluster strengthening (~150-170 MPa) was also evident.