A theoretical analysis on the perforation of Weldox 460E steel plates struck by flat-nosed projectiles is presented using a previously developed model within a unified framework. This model contains a dimensionless empirical equation to describe the variation of energy absorbed through global deformation as a function of impact velocity. The study further investigates the energy absorption mechanisms of Weldox 460E steel plates, with particular focus on the “plateau” phenomenon, i. e., limited increase in ballistic limit with increasing plate thickness. This phenomenon is explained and compared with results from previously studied 2024-T351 aluminium plates. The model predictions agree well with experimental data for Weldox 460E steel plates impacted by flat-nosed projectiles, including: relationship between global deformation and impact velocity, ballistic limit, residual velocity, and critical conditions for the transition of failure modes. Moreover, the model effectively predicts the “plateau” phenomenon observed in intermediate plate thickness range. It is also found that the indentation absorption energy contributes a significantly larger fraction of the total absorption energy in Weldox 460E steel plates perforated by flat-nosed projectiles than in 2024-T351 aluminium plates, due to the differences in material properties.