Correlation between line speed, microstructural evolution, and mechanical behaviour in traditional and continuous electric furnace annealed low alloy steel

The steel industry faces intense pressure to decarbonize annealing processes, which significantly contribute to CO₂ and NO₂ emissions through fossil fuel combustion in traditional annealing furnaces. This study systematically compares recrystallization behaviour, microstructure evolution, texture development, and mechanical properties of commercial quality (CQ) low-carbon steel strips (0.5 mm-thick) annealed in two different furnaces: traditional annealing furnace (TAF) vs. continuous electric annealing furnace (CEAF) at line speeds of 15–30 mpm as a process parameter. The furnace temperature of TAF is significantly higher than that of CEAF, so the two annealing routes are compared under their respective practical operating conditions rather than as a strictly matched controlled-variable experiment. Specimens annealed in TAF exhibit line-speed-dependent partial recrystallization with modest grain refinement and stable tensile strength. In contrast, CEAF-treated specimens yield superior outcomes up to 25 mpm such as fine equiaxed grains, elevated microhardness (∼15% higher), and enhanced tensile strength (∼12%) attributed to uniform heating that promotes homogeneous nucleation under the CEAF thermal cycle. At 30 mpm, CEAF exhibits incomplete recrystallization with retained rolling strain, elongated grains, and a “spring-back” microstructure reflecting insufficient thermal exposure at the lower effective annealing temperature. GOS/KAM mapping confirms markedly reduced recrystallization fraction in CEAF vs. TAF at 30 mpm, while SEM fractography reveals shear band interaction dominating TAF failure vs. microvoid coalescence in CEAF. These results establish optimal CEAF operating window (≤25 mpm) for superior mechanical performance through refined recrystallization textures, positioning electrified annealing as an emissions-free alternative to traditional processing while delivering enhanced strip quality for automotive applications, subject to the limitation that cross-furnace comparisons are process-dependent and influenced by the temperature difference between TAF and CEAF.