The recycling of steel scrap gains increasing attention as global decarbonization efforts intensify, driving the development of sustainable alloy design that prioritizes scrap tolerance. A challenge is the presence of tramp elements such as copper (Cu) and tin (Sn), which can significantly influence material properties even at low concentrations. In this study, eight Fe–Cu and Fe–Cu–Sn alloys are investigated to elucidate the effects of chemical composition on solidification microstructure and micromechanical behavior. The results reveal a pronounced grain refinement effect induced by Cu and Sn in as-cast conditions, reducing grain size from about 130 to 13 μm. Micromechanical testing of boundaries with segregation reveals that: in Fe–Cu alloys, the boundaries are harder than the matrix due to solid solution strengthening by Cu atoms in the ferrite lattice, whereas in Fe–Cu–Sn alloys, the boundaries appeared softer, which is suggestive of the possible formation of (Cu, Sn) precipitates during solidification. These findings advance understanding of the influence of Cu and Sn on microstructure and micromechanical properties of recycled steels. Further elucidating these mechanisms and their dependence on alloying will support the development of steels with extended compositional tolerance, enhancing recyclability and promoting sustainable steel production.