## 4 Conclusions  
The recently developed ultrahigh-strength press-hardening steel was studied in this paper. The steel was tempered at different temperatures in the range of 150 \({}^{\circ}\)C to 400 \({}^{\circ}\)C, and subsequently tested for mechanical properties. Microstructural characterization along with XRD measurements and fracture surface inspection were conducted. The following findings were made:  
1. A hardness greater than 650 HV/58 HRC was obtained with a tensile strength exceeding 2200 MPa. Reheating at 980 \({}^{\circ}\)C prior to quenching resulted in a prior austenite grain size of 12.3 \(\upmu\)m and no quench-cracking was detected after the water quenching. Therefore, milder quenching media are not necessary to avoid quench cracking in the case of the 10 mm thick plate studied.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Material**} & **Crystallite Size** & **Microstrain** & **Dislocation Density** & **Retained Austenite** \\  & **[Å]** & **[\%]** & **[\(\times\)10\({}^{15}\) m\({}^{-2}\)]** & **[\%]** \\ \hline WQ & 231 & 0.590 & 8.3 & 3.6 \\
150 \({}^{\circ}\)C & 277 & 0.611 & 7.1 & 2.8 \\
200 \({}^{\circ}\)C & 369 & 0.577 & 5.1 & 2.8 \\
300 \({}^{\circ}\)C & 338 & 0.382 & 3.7 & - \\
400 \({}^{\circ}\)C & 352 & 0.281 & 2.6 & - \\ \hline \hline \end{tabular}
\end{table}
Table 2: Microstrain, crystallite size, dislocation density, and retained austenite content based on the X-Ray diffraction (XRD) measurements.  
Figure 10: Dislocation densities of the tested steels (WQ tempering temperature set as 25 \({}^{\circ}\)C).  
2. Tempering had a significant effect on both the mechanical properties and microstructure of the steel. The most notable changes in the microstructure were the amount and size of the precipitates formed during tempering. The water quenched variant showed only minor traces of auto-tempering, but transition carbides began to form when tempering at 150 \({}^{\circ}\)C and 200 \({}^{\circ}\)C. Tempering at 300 \({}^{\circ}\)C and 400 \({}^{\circ}\)C altered the microstructure from martensite to more ferrite with cementite.
3. The hardness and tensile strength reductions were not substantial for the low-temperature tempered variants (150 \({}^{\circ}\)C and 200 \({}^{\circ}\)C). Further increases in tempering temperature resulted in a decrease of hardness, strength, and uniform elongation. The yield-to-tensile strength ratio increased with increasing tempering temperatures. Low-temperature Charpy-V impact toughness energies increased with tempering until the tempered martensite embrittlement (TME) range was reached.
4. Fracture surface inspection showed that the shear lip formation increased with increasing tempering temperature for the impact toughness samples tested at room temperature, with the highest fraction of ductile fracture in the specimens tempered at 400 \({}^{\circ}\)C. However, tempering at 300 \({}^{\circ}\)C or above resulted in decrease in impact toughness when tested at \(-40\)\({}^{\circ}\)C due to martensite temper embrittlement.
5. X-ray diffraction (XRD) analysis revealed a decrease of dislocation density with the increasing tempering temperature, as expected. The retained austenite content was approximately 3% for the quenched and low-temperature tempered steels but after tempering at 300 \({}^{\circ}\)C and 400 \({}^{\circ}\)C there were no traces of retained austenite, i.e., the decomposition of austenite had occurred.  
Supervision, D.P., J.K.; writing-original draft, O.H.; writing-review and editing, D.P., A.K., J.K., T.L.; XRD analyses, J.H.; fracture surface analysis, S.P.  
This research has been done within the program Steel Ecosystem for Focused Applications (SteFA). We gratefully acknowledge financial support from Business Finland and the companies participating in the program.  
The corresponding author would like to express his gratitude for the support provided by the University of Oulu Graduate School through the Advanced Materials Doctoral Program (ADMA-DP).  
The authors declare no conflict of interest.