Control of carbides and graphite in Ni-hard type cast iron for hot strip mills

The carbide and graphite formation and redistribution of alloy elements during solidification were investigated on Ni-hard type cast iron (Fe-C-Si-Ni-Cr-Mo) to develop higher quality rolls for hot steel strip mills. By the control of Ni and Si contents of iron, eutectic graphite flakes crystallize even in cast irons containing strong carbide formers such as V, Nb and Cr. The crystallization of Ni-hard type cast iron with V and Nb proceeds in the order of primary γ, γ + MC, γ + M₃C and γ + graphite eutectic. Since the influence of each alloying element on graphite formation is estimated based on the solubility of C in molten iron, the change in graphite forming tendency of residual liquid is evaluated by the parameter expressing the solubility limit of C to molten iron. The amount of graphite increases with the decreasing of solubility parameter. In addition, inoculation with ferrosilicon effectively increases the graphite flakes.