**4. Conclusions**

The effect of tempering on microstructures and mechanical properties of ausrolled–austempered samples was investigated. In summary, after tempering, the bainite ferrite coarsened from 70 ± 9 nm to 144 ± 17 nm to 168 ± 35 nm. The volume fraction of retained austenite was reduced, and the impact energy was slightly enhanced. The hardness and UTS decreased sharply. The hardness decreased from 682 ± 11 HV1.0 to 679 ± 12 to 560 ± 8 HV1.0. The UTS decreased from 2581 to 2520−2045 MPa. The YS and El increased, but not monotonously. The impact energy, YS, and El increased with elevated tempered temperature when the samples were tempered at 200–300 ◦C, but decreased with elevated tempered temperature when they were tempered at 350 ◦C and 400 ◦C. When the tempered temperature exceeded 200 ◦C, the microstructure of ausrolled bainite may have exhibited partial recovery and recrystallization during tempering, so the hardness and UTS decreased, and the YS, El, and impact energy were increased with increasing tempered temperature. The fracture appearance of all the samples after impact testing is brittle fracture.

**Author Contributions:** Conceptualization, T.W. and D.L.; writing—original draft, J.Z.; methodology, Y.L. and Q.Z.; writing—review and editing, D.L. and Y.Y. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported by Hubei Superior and Distinctive Discipline Group of "Mechatronics and Automobiles" (No. XKQ2020002), Hubei Key Laboratory of Power System Design and Test for Electrical Vehicle and the National Natural Science Foundation of China (Grant No. 51801051).

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