EVOLVEMENT MECHANISM OF SURFACE OSCILLATION MARKS ON ROUND BILLET DURING SOFT-CONTACT ELECTROMAGNETIC CONTINUOUS CASTING

In the steel industry, the continuous casting product is usually a pre-product before being fed to hot rolling mill, which brings in high cost and pollution and low productivity. So a new technology is demanded urgently to produce surface defect free billets and slabs. The success of electromagnetic casting (EMC) technology utilized in aluminum casting to improve surface quality enlightens the metallurgical researchers to apply this novel technology into production of steel. An experiment using high frequency magnetic field (26 kHz) was carried out on a vertical type caster with a segmented copper mold in our laboratory. Carbon structural steel (0.20% C, mass fraction) was used as experimental material. A special power was used to provide high frequency current to an induction coil surrounding the segmented mold. Moreover, the distribution of magnetic field and meniscus shape of Sn-Pb-Bi alloy in the mold were investigated and used to explain the phenomena happened in the initial solidification area in the case of EMC. The result shows that oscillations marks (OMs) can be totally suppressed and the surface of round billets with EMC technology becomes extremely smooth compared to the billets without EMC technology when the output power comes to an optimal one. According to the conventional mechanism of OM formation, the pressure generated in the flux channel due to the relative movement between solid flux rim and molten steel pool plays an important role in the formation of OMs. In the case of EMC, the solid flux rim becomes smaller as temperatures of mold and liquid steel in the initial solidification area increase because of Joule heat generated by the high frequency magnetic field, and the flux channel turns wider as Lorentz force is imposed on the liquid steel. As a result, the flux pressure becomes smaller, which makes the billets have better surface quality. However, when the powder exceeds the optimal value, the distribution of initial solidification starting point along the mold perimeter behaves in a, wavy pattern. Simultaneously, the free surface of molten steel fluctuates more greatly, resulting in formation of OMs. As a conclusion, the formations of wavy OMs on the round billet surface are resulted from non-homogeneity of magnetic field between slit center and segment center as well as fluctuation of liquid steel level.