High yield HY-100 steel is a unique alloy and well known for its employment in heavy construction. HY-100 offers good characteristics like ductility, notch toughness, corrosion resistance, and weldability. The physical characteristics and molecular structure of HY-100 steel are also well known; however, there is little known about the effect of high-velocity projectiles impact on this metal alloy’s crystalline structure and material phase. The effects of high-speed velocity impact on the crystalline structure and material phase changes are studied herein experimentally. The results of an impact on the crystalline structure are assessed by impacting HY-100 steel plates (15.4 × 15.4 × 1.27 cm) with Lexan projectiles. A two-stage light gas gun accelerates these projectiles to a velocity of 6.70 km/s at the point of the impact. The impacted plates’ surfaces are prepared as required for inspection by the Electron Back Scatter Diffraction (EBSD) microscope. Ten regions on each impacted plate area are examined and analyzed after impact. These regions are selected from the area immediately under the impact crater to locations that are not physically affected by the impact. Observations of collected EBSD images show that the predominant phase is Body-Centered Cubic (BCC). Moreover, Face-Centered Cubic (FCC) and Hexagonal-Close-Packed (HCP) phases are also indexed. The samples are also post-impact examined for molecular structure allocation changes. The results were then tabulated according to the regions relative to the impact crater. In this study, traces of HCP were found at some locations in all post-impact stages. This study also indicates that the BCC crystalline structure remained the dominant phase structure after impact, and it is valid with all test samples and all levels of shock loading. At this velocity, the damage zone develops within 5 microseconds due to impacting momentum. HY-100 steel materials go through a reversible phase change when subject to elevated temperature and high quasi-static pressure.