Application of PMO technology to improve quality of ø 600 mm vertical continuous casting 8418 die steel large round billet

To alleviate solidification structure defects and macrosegregation in ø600 mm large round billets of 8418 die steel produced by vertical continuous casting, this study adopts a combined method of numerical simulation and industrial experiments to investigate the regulatory effect and internal mechanism of Pulsed Magneto-Oscillation (PMO) on the flow field, temperature field, solidification structure and macrosegregation of continuous casting billets. Numerical simulation results show that PMO can induce an upper and lower dual-circulation flow structure near the coils in the secondary cooling zone. Under the working condition of 500k_U A (where k_U represents the equipment coefficient of PMO), the peak flow velocity of the upper circulation reaches 91.29 mm/s, and that of the lower circulation is 38.58 mm/s. The intense convection promotes circulating heat exchange of high-temperature molten steel between the solid-liquid interface and liquid core, realizing cooling of the liquid core and reheating of the billet surface, which optimizes the heat transfer environment during solidification. Industrial experimental results indicate that PMO promotes the nucleation of molten steel at the front of the solid-liquid interface to form a "crystal rain", thereby refining the solidification structure of the billets. The dual-circulation flow further accelerates heat dissipation at the billet center, reducing the area fraction of central cracks from 10.86% to 3.22%. Meanwhile, PMO effectively improves the compositional homogeneity of billets, decreasing the maximum carbon segregation index from 1.31 to 1.19 and significantly mitigating the segregation fluctuation of Cr, Mo and V elements at the central and 1/2 radius positions of the billets. The research demonstrates that PMO technology can synergistically optimize the solidification structure, central cracks and macrosegregation of ø600 mm 8418 die steel large round billets fabricated by vertical continuous casting, providing a reliable technical support for the production of high-quality large-section high-alloy die steel round billets.