The Fe-Cr-B-C alloy is a new wear-resistant boron cast iron alloy developed from high-chromium cast iron. This alloy is inexpensive, easy to process, and exhibits excellent wear resistance and good formability, making it suitable for the manufacturing of wear-resistant parts with high dimensional accuracy. The Fe-Cr-B-C alloy has great potential for application and is gradually replacing chromium wear-resistant alloys. In recent years, studies have shown that after composition optimization, the Fe-Cr-B-C alloy can be directly used in the as-cast state without subsequent heat treatment, resulting in a significant decrease in cost. Thus, optimization of the composition of the Fe-Cr-B-C alloy is of great significance for the development of wear-resistant materials. The strength-toughness and wear resistance of the boron cast iron mainly depend on the characteristics of the B-rich precipitates. Reasonable control of the B addition can optimize the characteristic of the B-rich precipitates, thereby improving the service properties of the as-cast Fe-Cr-B-C alloy. However, the role of B in the Fe-Cr-B-C alloy has been scarcely investigated. Therefore, the effects of B content on the solidification behavior, as-cast microstructure, hardness, impact toughness, and wear resistance of the Fe-Cr-B-C alloy were examined in this study. The results show that with increasing B content, the liquidus temperature and formation temperature of precipitates significantly decrease, the formation range of precipitates expands, and the solidification temperature range first increases and then decreases. At a B content of 0.0006% (mass fraction), the solidification of the Fe-Cr-B-C alloy proceeds as follows: L→δ→γ dendrite→primary Nb(C, B)→eutectic [γ + Cr₇C₃]. After solidification, the dendrite arm comprised of martensite, and the interdendritic region was composed of residual γ and trace amounts of Nb(C, B), [γ + Cr₇C₃]. With the increase in the B content to 0.51%, the growth of γ dendrites was significantly hindered, resulting in the refinement of the dendritic structure. The solidification process changed to L→γ dendrite→primary (Fe, Cr)₂(B, C)→primary Nb(C, B)→eutectic [γ + (Fe, Cr)₂(B, C)]. After solidification, martensitic transformation occurred in both the interdendritic region and dendrite arms, and a continuous boron-carbide network was formed along the interdendritic region. With the further increase in B content to 2.89%, a large amount of boron-carbide was formed at the initial stage of solidification, which not only caused the disappearance of the dendritic structure but also consumed most of the B atoms, seriously reducing the hardenability of γ matrix and inhibiting its martensite transformation. The solidification process changed to L→primary γ→primary (Fe, Cr)₂(B, C)→eutectic [γ + (Fe, Cr)₂(B, C)]→peritectic [γ + (Fe, Cr)₂(B, C) + (Fe, Cr)₃(C, B)]. The alloy with a B content of 0.0006% possesses the highest impact toughness, and moderate Rockwell hardness and wear resistance. The alloy with a B content of 0.51% possesses the highest Rockwell hardness, optimal wear resistance, and moderate impact toughness. The alloy with a B content of 2.89% possesses the lowest Rockwell hardness and impact toughness, and the poorest wear resistance. The change in boron-carbide characteristic and the martensitic transformation of matrix are the main reasons for the significant differences in strength-toughness and wear resistance among these alloys. The obtained results provide a theoretical basis for optimizing the composition and improving the wear resistance of the as-cast Fe-Cr-B-C alloy.