###### Abstract  
The shock and spalling behavior of a compositionally complex high strength low-alloy steel (HSLA) was studied using plate impact testing. The free surface velocity of the specimen in the range of 194-938 m/s was measured by a displacement interferometer system for any reflector (DISAR). The Hugoniot elastic limit (HEL), spallation fracture and microstructural evolution of the HSLA under an impact stress of 3.04-18.66 GPa were analyzed. Shock Hugoniots were obtained from the measured particle velocities and calculated shock velocities. The velocity curves show clear signs of HEL and velocity fallback, indicating a transition from elastic to plastic and spalling behavior. When the impact velocity exceeds 757 m/s, the particle velocity rises to the peak and then increases again, indicating that an \(\alpha\)\(\rightarrow\)\(\epsilon\) phase transition occurred, with a threshold of 13.51 GPa. It was found that the impact velocity is linearly related to the particle velocity of the HSLA. As the impact stress increased, the HEL remained within the range of 1.32-1.50 GPa, while the spalling strength presented an upward trend with the increasing impact stresses. Metallographic analysis shows that the impact failure is dominated by brittle fracture at lower velocities, while more ductile fracture occurs at higher velocities.  
high-strength low-alloy steel (HSLA); eletroslag remelting (ESR); plate impact test; Hugoniot elastic limit; spall strength +
Footnote †: journal: Medical Engineering, Nanjing University of Science and Technology, Nanjing 210094, China  
requirements for the metallurgical quality of steel are also getting higher and higher [14]. Generally speaking, the residual elements S, P and the five harmful impurity elements Pb, Sb, Bi, Sn, As, etc. in the steel are easy to segregate and precipitate at the grain boundary, and form a low-melting point eutectoid with Fe, which reduces the strength of grain boundaries and affects the hot workability of steel [15; 16]. Therefore, it is worthy of attention to reduce the content of impurities, such as S and P, the gas content and the content of nonmetallic inclusions in HSLA, thereby improving the comprehensive mechanical properties of the steel. Electrosplag remelting, as a secondary melting technology, can improve the purity of steel and improve the low-magnification structure of steel [17; 18]. The REO-MgO-CaO-Al2O3-CaF2 new five-element rare earth slag system was selected, and rare earth deoxidizer was added to the slag pool during the electroslag remelting process to form a certain reducing atmosphere in the electroslag furnace, and finally improve the toughness and toughness of the steel and fatigue performance.  
High-strength low-alloy steels provide perfect candidates for major improvements in the automotive, aerospace and military defense fields due to their acceptable impact toughness, ultra-high strength, and excellent weldability [19; 20; 21]. In this study, a series of plate impact experiments were carried out to investigate the shock wave structure, HEL and spallation strength in a compositionally complex HSLA. The transition from elastic to plastic behavior under impact flyer loading was analyzed, and the phase transition and spallation of HSLA were determined from the measured free-surface particle velocities and metallographic microscopy of the recovered specimens.