## 4 Conclusions  
The shock-compression response and spalling behavior of a high-strength low-alloy steel with a complex composition were studied. The free surface velocity curve of the HSLA under impact stress in the range of 3.04-18.66 GPa was measured, and the metallographic analysis of the recovered specimens was carried out. This study can lay the foundation for the development of high-performance alloy steel and promote its application in large construction machinery, aircraft missile casings, armored tank vehicle structures, and protective materials. The main conclusions are as follows:  
1. The entire mechanical behavior of the HSLA under the loading of the plate impact was investigated based on the free surface velocity, from initial elastic deformation to plastic deformation caused by the shock wave, and then to the spalling process under the interaction of the rarefaction wave. When the impact velocity exceeded 757 m/s, an \(\alpha\)\(\rightarrow\)\(\varepsilon\) phase transition occurs within the material that causes a step change in the particle velocity. The calculated phase-transition stress threshold is 13.51 GPa.
2. The shock wave velocity and shock stress of the HSLA were calculated based on the measured particle velocity. It is found that an obvious linear relationship exists between the shock wave velocity \(U_{\mathrm{s}}\) and the particle velocity \(u_{\mathrm{P}}\): \(U_{\mathrm{s}}=3961\) m\(\cdot\)s\({}^{-1}+1.415\)\(u_{\mathrm{P}}\).
3. It is found that the HEL of the HSLA exhibited little change with an increase in impact stress, remaining consistent within the range of 1.32-1.50 GPa. The strain rate and spallation strength first increased and then decreased because of the high strain rate. The spallation strength increased again after the phase transformation, which may be related to microstructural changes caused by the phase transformation.
4. Microscopic observation of recovered specimens showed that the HSLA exhibited multiple spalling layers under low-velocity impact and that the failure mode was mainly a brittle fracture. Under high-velocity impact, the crack opening was larger and clearer, and the failure mode changed from an initial brittle fracture mechanism to a more ductile fracture mechanism.  
Conceptualization, Y.L. and J.L.; investigation, Y.L. and J.L.; writing--original draft preparation, Y.L.; writing--review and editing, Y.L. and J.L. All authors have read and agreed to the published version of the manuscript.  
Figure 9: Metallographic microscope observation of internal structures of recovered specimens subjected to impact velocities of 409 m/s and 938 m/s.  
**Funding:** The work presented in this paper was funded by the Innovation Special Zone Project of the Science and Technology Commission of the Military Commission (grant number 22-TQ01-04-ZT-01-009) and the China Postdoctoral Science Foundation (grant number 2018T110503).  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** The data presented in this study are available on request from the corresponding author upon reasonable request.  
**Conflicts of Interest:** The authors declare no conflict of interest.