Abstract: The conventional production of carbon black (CB) as a rubber reinforcing filler is energy-intensive, emits large amounts of CO₂, and causes severe dust pollution. Simultaneously, the iron and steel industry generates significant quantities of solid wastes, i.e., steel slag and calcium-based desulfurization ash, whose disposal poses critical environmental challenges. Developing eco-friendly alternative fillers from these byproducts is urgently required. Steel slag micropowder is rich in calcium oxide (CaO), silicon dioxide (SiO₂), and iron oxide (Fe₂O₃), whereas calcium-based desulfurization ash (FGD) mainly consists of CaO and sulfur trioxide (SO₃). Both these substances contain inorganic active components that are capable of interfacial interactions with rubber composites to offer certain reinforcing effects. Consequently, partially replacing CB with steel slag and calcium-based desulfurization ash has become a crucial topic for research. This study focuses on the high-value resource utilization of ferrous metallurgical solid wastes. It systematically investigates the partial replacement of N220 CB by 600-mesh steel slag powder (SS), raw calcium-based desulfurization ash, and its hydrothermal conversion product—calcium-based desulfurization ash whisker (CSW)—in styrene-butadiene rubber (SBR) composites. Unlike the irregular, impure FGD, the hydrothermally synthesized CSW exhibits a well-defined, high-aspect-ratio acicular morphology with high purity and crystallinity, which may improve interfacial interactions. Composites were prepared with varying SS/FGD or SS/CSW ratios and total CB replacement levels of 15%, 22.5%, and 30% by mass. The vulcanization characteristics, mechanical properties (tensile and tear strength), microstructure, and thermal stability were systematically analyzed to elucidate the reinforcement mechanisms and optimization pathways. The results show that using SS/FGD blends to replace CB significantly accelerated the vulcanization rate, and improved the processing fluidity. However, the mechanical properties deteriorated markedly with increasing replacement ratio due to poor filler dispersion and interfacial debonding. By contrast, the SS/CSW blends exhibited a substantially superior performance. When 15% mass fraction of CB was replaced by a 1∶1 mass ratio of SS to CSW (composite SC І–1), the composite achieved optimal properties: a tensile strength of 17.41 MPa and tear strength of 90.53 kN·m^–1, closely approximating those of the neat CB reference (SBR–0). Moreover, the scorch time (t₁₀) was significantly prolonged, indicating enhanced processing safety without sacrificing curing efficiency. Microstructural analysis revealed that the fillers were uniformly dispersed in the rubber matrix at this optimal substitution level. Notably, the high-aspect-ratio CSW acted as “microfiber bridges” across the interface, effectively hindering crack propagation and dissipating fracture energy through fiber pull-out and breakage. When the replacement ratio exceeded 22.5%, severe filler agglomeration occurred, creating stress concentration points and interfacial voids that led to a marked decline in the mechanical performance. Thermogravimetric analysis further demonstrated that the 15% mass fraction replacement composite (SC І–1) maintained a thermal stability comparable to that of SBR–0, with a decomposition temperature of approximately 400 ℃, while the residual char increased with higher filler loading, imparting improved flame retardancy. In conclusion, this study confirms that an appropriate combination of SS and hydrothermally derived CSW can effectively replace a portion of CB as a reinforcing filler without compromising key properties. The optimal formulation (15% mass fraction replacement, SS∶CSW = 1∶1) balances the mechanical performance, processability, and thermal stability. This work provides a theoretical foundation and technical support for the green transformation of the rubber industry by reducing the reliance on fossil-fuel-based fillers and promoting circular economy principles.