Effect of Niobium Modification on Solidification and Crystallization Mechanism of Medium-Carbon Cast Steel

The effect of niobium modification on refinement of primary austenite, shrinkage characteristic, and solidification behavior of medium-carbon cast steel melted in a medium-frequency induction furnace is studied. It is established that the modification with 0.1 wt.% niobium increases the fluidity of the steel liquid, enhances the feeding capacity of the cast steel, turns the dispersed shrinkage porosity into concentrated shrinkage cavity, changes the coarse dendrites to fine ones, even in equiaxed grain structures, and dwindles the primary austenite grain size greatly. Using electron microscopy and energy-dispersive analysis, it is discovered that niobium combines with carbon to form solid phase particles of NbC. These particles are chemically stable at high melting temperatures and facilitate the primary austenite nucleation effectively by non-spontaneous nucleating. A model of matching between the crystal lattices γ-Fe and NbC is suggested. The mechanism of NbC heterogeneous nucleation consists in that the primary austenite grows on {111}_γ-Fe along to the closest-packed plane {111}_NbC in crystal orientation \({\langle 011\rangle }_{\gamma -\mathrm{Fe}}\Vert {\langle 112\rangle }_{\mathrm{NbC}}\) ${⟨011⟩}_{\gamma -\mathrm{Fe}}\Vert {⟨112⟩}_{\mathrm{NbC}}$ , and the mismatch \({\updelta }_{{\langle 111\rangle }_{\mathrm{NbC}}}^{{\langle 111\rangle }_{\gamma -\mathrm{Fe}}}\) ${\mathrm{\delta }}_{{⟨111⟩}_{\mathrm{NbC}}}^{{⟨111⟩}_{\gamma -\mathrm{Fe}}}$ of the crystal planes is only 9.79%.