 In this paper, the authors examined the effect of the LM-GB content on the mechanical properties of EA-4T high-speed axle steel. They found that increasing the volume fraction of GB structure leads to refinement of the LM structure and improvement of its hardness and strength. Additionally, they observed that the micro-hardness of LM follows the Hall-Petch relationship with the Lath-Martensite packet size. Based on these findings, a predictive model for the mechanical properties of EA-4T steel was developed, which suggests that when the LM fraction is between 40% and 70%, a superior combination of strength, ductility, and toughness can be achieved. This article is authored by Yin Zhang, Yu Chiao, Guan Ji Huang, and others.