Analysis about forming mechanism of equiaxed crystal zone for 1Cr18Ni9Ti stainless steel twin-roll thin strip

Based on a simple macro/micro mathematical model developed in this paper and its predicted results, the forming mechanism of equiaxed crystal zone for 1Cr18Ni9Ti stainless steel twin-roll thin strip is investigated and analyzed. In the developed model, the latent heat is treated with the enthalpy method, the grid and nodes are divided by the assumed streamlines. Moreover, the heterogeneous nucleation and columnar-to-equiaxed transition (CET) models are also introduced, together with the revising of dendrite growth dynamic model of Kurz–Giovanola–Trivedi (KGT). Finally, with the help of solid fraction, the coupling of macro/micro models is realized by using different grid-sizes and time-steps, macro and micro, together with the columnar dendrite front tracing. The predicted results of mathematical models indicate that, regardless of the solidification types (semi-solid, rolling or ideal type) of 1Cr18Ni9Ti stainless steel thin strip, the thin strips include the equiaxed crystal zone. The forming of equiaxed crystal zone is possibly related to the following mechanisms: (1) the sedimentation and accumulation of free crystals floating in molten pool onto the dendrites solidification fronts and the accumulated crystals growth and (2) the suppression of the growth of dendritic solidification front and preferential growth of fine free crystals in unsolidified layer near the strip center by the abrupt decrease in the heat transfer coefficient on the strip surface after leaving the minimum gap between the rolls.