Numerical study of grate plate configuration effects on cooling non-uniformity and phase transformation of 82Mn wire rod coils in a Stelmor line

Cooling non-uniformity in the Stelmor air-cooling line causes persistent temperature differences between the overlap-dense and loosely packed regions of wire rod coils, leading to heterogeneous phase transformation and inconsistent mechanical properties. This study investigates the effects of grate plate configuration on cooling non-uniformity and phase transformation of 82Mn wire rod coils using a three-step coupled numerical approach. The plenum-chamber airflow was first resolved to obtain nozzle-outlet velocity distributions under different fan conditions and grate plate configurations. These distributions were applied as boundary conditions to compute the coil-scale flow field and convective heat transfer coefficient distribution. The wire rod cooling curves and phase transformation evolution were then predicted by incorporating forced convection, thermal radiation and transformation latent heat release. The predicted temperature histories agree well with industrial measurements, with a maximum deviation of approximately 30 K (less than 3% of the initial wire temperature of 1193 K). The results show that although the grate plate effectively redirects cooling air towards the overlap-dense edge regions, geometric shielding by the densely stacked wire loops remains the dominant factor limiting local forced convection. Phase transformation starts first in the low-density coil-centre region and is delayed in the overlap-dense regions by about 8 to 10 s. At the cooling line exit, the temperature difference between the overlap region and coil centre reaches 74 K, 72 K and 68 K for the Type-C, Type-B and Type-A configurations, respectively. The variation of only 6 K across configurations demonstrates that adjusting the grate plate hole diameter alone has limited effectiveness in improving cooling uniformity. Further improvement requires coordinated optimisation of grate plate configuration, fan operating conditions and roller speed.