For the largest areas of bainitic ferrite; \* Δ is the difference between the experimental and total calculated yield stress values.

For both steels, the measured yield stress decreased with an increase in strain during processing. In the CrNbV-steel this corresponds to the decrease in grain size strengthening contribution (in trend and value). However, in the MoNbV-steel the grain size and precipitation strengthening contributions increased with strain. Obviously, in the MoNbV-steel these contributions did not compensate for the decreased solid solution/cluster strengthening. This indicates a significant role of solid solute atoms and atom clusters as strengthening agents in bainitic microalloyed steels. With an increase in dislocation density, associated with bainitic microstructure, a potential number of dislocation-atom/cluster interaction sites may increase faster than the number of dislocation-precipitate interaction sites, due to a very high density of atoms/clusters. Thus, the role of solute concentrations increases in bainitic microstructures compared to the ferritic.

In the CrNbV-steel coarser martensite could be expected to decrease elongation for the higher strain processing. However, an opposite trend was observed: the elongation has increased in the CrNbV-steel for the higher strain schedule. This could have been related to more intensive dislocation generation in the CrNbV-steel during tensile testing, leading to more homogeneous slip and delated local micro-crack formation. The dislocation generation in the CrNbV-steel would be associated with the increased number density of >20 nm particles for this processing condition. Nano-precipitates were reported to stimulate dislocation generation [84,85].

Based on these a final conclusion can be made with respect to the design of steel composition: if a bainitic steel is required to exhibit superior strength, Mo should be used as a microalloying element, because it increases the solubility of other microalloying elements and increases the solid solution and cluster strengthening effects; if a bainitic steels is required to exhibit higher ductility, Cr can be used to facilitate bainite transformation but allow precipitation to take place.

#### **5. Conclusions**

Comparative study of Mo and Cr effects on microstructure and mechanical properties of newly developed NbV-microalloyed bainitic steels has shown the following:


**Acknowledgments:** This project was financially supported by the ARC Research Hub for Australian Steel Manufacturing and, in particular, by Bluescope Steel Ltd. (Melbourne, Australia). The microscopy was carried out using JEOL JSM-7001F FEGSEM (supported by grant No. LE0882613) and JEOL JEM-2011 TEM (supported by grant No. LE0237478) microscopes at the Electron Microscopy Centre at the University of Wollongong.

**Author Contributions:** Andrii Kostryzhev has conceived the idea, conducted the microstructure characterisation and analysis of the microstructure-properties relationship, and has written the paper. Navjeet Singh and Liang Chen carried out the sample processing in Gleeble. Navjeet Singh conducted the tensile testing. Chris Killmore and Elena Pereloma carried out the overall project management, participated in discussion of the results, and contributed to the paper writing.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


© 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).
