Numerical simulation of mold rotation and its effect on carbides in HSS ESR ingot

High-speed steel contains a large amount of carbides, the shape and distribution of which have an important influence on its quality. To improve the distribution of carbides in M2 high-speed steel, the temperature field and the shape of the metal pool during the mold-rotation process were investigated in detail using a numerical simulation. Moreover, the effect of the mold-rotation speed on the electroslag remelting process was investigated using a rotating bifilar electroslag remelting furnace under laboratory conditions. The morphology and distribution of carbides in an ESR ingot were observed using an SEM, and the composition of carbides was analyzed through an electrolytic extraction experiment. Results show that with increase in mold rotation speed, the high-temperature zone of the slag pool moves from the core to the edge. Moreover, the temperature distribution becomes uniform. The depth of the metal pool becomes shallow, and the thickness of the two-phase region decreases, which results in a short local solidification time and small secondary dendrite spacing. Correspondingly, with the increase in the mold rotation speed, the slag skin of ESR ingot becomes thin and more uniform than earlier. The cooling intensity of the mold on the ESR ingot is high, and the carbide network begins to break and become thin. The morphology of carbides changes from flake to fine rod. XRD analysis determines whether the mold rotates or not, carbides always comprise M₂C, MC, and M₆C. However, the content of M₂C increases and the contents of MC and M₆C decrease with the increase in mold-rotation speed. The main reason for the improvement in the carbide structure is that the mold rotation decreases the metal pool depth and two-phase zone thickness, which improves the solidification conditions and reduces the element segregation.