Multi-scale degradation behavior of metallurgical coke in CO₂–N₂ atmospheres spanning blast-furnace temperature and CO₂ concentration ranges

Coke degradation in blast furnaces critically limits high-efficiency, stable operation. Temperature and CO₂ concentration vary greatly across blast furnace zones, and standard single-point tests fail to capture coke's full-service performance. Here, twelve temperature–CO₂ combinations spanning the ranges encountered in a blast furnace were imposed in CO₂–N₂ atmospheres to study the evolution of the coke reactivity index (CRI) and coke strength after reaction (CSR), while multi-scale characterization revealed degradation mechanisms. Coke performance evolved non-monotonically: CSR declined gradually at 900–1100 °C, steeply at 1100–1300 °C, reached a minimum at 1300 °C with 15% CO₂, then rebounded to 71.88% at 1400 °C, while CRI dropped from 37.85% to 16.42%. This inflection point arises because high-temperature graphitization strengthening surpasses CO₂ solution-loss damage, giving an apparent recovery of strength relative to the 1300 °C condition. Mechanistically, we establish a dynamic competition between solution-loss degradation and graphitization restructuring, and construct a multi-scale cascade model: chemical transformation→ carbon skeleton ordering → pore-morphology response. Accordingly, an evaluation scheme in which the performance indices are referenced to the corresponding blast-furnace regions is proposed, identifying the root of the cohesive zone as the critical degradation region, with reference CSR values and microstructural parameters, supporting condition-matched coke design and BF optimization.