**5. Conclusions**

From the present investigation, the following conclusions can be drawn:

For the same thermomechanical processing schedule, the as-received samples show different microstructures, depending on carbon and niobium contents. The Nb-free alloys mainly consist of polygonal ferrite and some carbon enriched constituents, while the Nb-bearing alloys exhibit acicular microstructures with significantly reduced amounts of polygonal ferrite.

Based on equilibrium thermodynamic relations, it appears that strain-induced NbC can precipitate in the roughing temperature range in high Nb steels.

Depending of the amount of strain, the suppression of some recrystallized austenite grains would be expected to cause duplex grain structures, which is believed to be responsible for inferior mechanical properties.

**Author Contributions:** Conceptualization, A.J.D.; methodology, A.J.D. and R.A.A.; validation, A.J.D. and R.A.A.; formal analysis, R.A.A. and A.J.D.; writing—original draft preparation, R.A.A.; writing—review and editing, A.J.D.; supervision, A.J.D.; project administration, A.J.D.; funding acquisition, A.J.D. All authors have read and agreed to the published version of the manuscript.

**Funding:** The Basic Metals Processing Institute, Department of Mechanical Engineering and Materials Science, University of Pittsburgh thanks its industrial sponsors for partially funding this study. In addition, the United States Steel Corporation (U.S. Steel) should be thanked for providing the experimental materials and processing. Finally, one of the authors (Almatani), was supported by a scholarship from King Abdulaziz City for Science and Technology (KACST), Saudi Arabia.

**Acknowledgments:** The authors would like to thank the Basic Metals Processing Institute, Department of Mechanical Engineering and Materials Science, University of Pittsburgh and its industrial sponsors for partially funding this study. In addition, the United States Steel Corporation (U.S. Steel) should be thanked for providing the experimental materials and processing. Finally, one of the authors (Almatani), was supported by a scholarship from King Abdulaziz City for Science and Technology (KACST), Saudi Arabia.

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

## **References**


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