Application of advanced experimental techniques to elucidate the strengthening mechanisms operating in microalloyed ferritic steels with interphase precipitation

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 (similar to 4 +/- 3 mu m average grain size) and a high number density of nano-sized, disc-shaped interphase precipitates (similar to 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 similar to 280 MPa and similar to 395 MPa, respectively. Significant cluster strengthening (similar to 150-170 MPa) was also evident. (C) 2020 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.