**5. Conclusions**

In this study, the quasi-static and dynamic mechanical properties of D6A steel were investigated by using a material testing machine and rotating disk Hopkinson tension bar. From the microscopic point of view, the influence of grain refinement on the macroscopic mechanical properties of materials such as strength and work hardening ability was discussed, and the reasons for the decrease in strain rate sensitivity with the decrease in grain size was analyzed simultaneously. The parameters of the classical J–C model were fitted based on the experimental results. The relevant results can provide certain reference value for promoting the practical application of FG D6A steel, as follows:

1. Grain refinement and cementite precipitation are the main reasons for the strength improvement of FG D6A steel. High-density grain boundaries and hard precipitates improve the strength of the material by hindering dislocation movement. In addition, the investigated steel shows obvious strain rate effect, which is considered to be

the result of grain crushing that further enhances the fine-grain strengthening effect during dynamic loading.


**Author Contributions:** Conceptualization, X.F.; methodology, X.F., X.Z., Z.Y. and Y.S.; software, Z.Y.; investigation, X.F., X.Z., Y.S., R.X. and X.H.; writing—original draft preparation, Z.Y.; writing—review and editing, X.F. and Y.S. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the National Natural Science Foundation of China, gran<sup>t</sup> number 12102413.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China with Grant No. 12102413. The authors also gratefully acknowledge the experimental support from the Chinese Academy of Engineering Physics and Northeastern University.

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