Refinement of the Solidification Structure of Austenitic Fe-Mn-C-Al TWIP Steel

Abstract

The solidification structure of austenitic Fe-22Mn-0.6C-1.6Al TWIP steel with and without Ce inoculation is compared on the macro- and microscale. The expansion of the equiaxed zone, decrease of equiaxed dendrite size and thinning of the dendrite arm are observed in Ce-inoculated TWIP steel. The refining mechanism for the expanded equiaxed zone and the decreased equiaxed dendrite size is the Ce2O3 particles acting as the heterogeneous nucleation sites. With increasing Ce content, the effective density of Ce2O3 particles of around 1 μm size increases initially and then decreases, while the total density always increases. The average diameter of Ce2O3 particles in TWIP steel increases as the amount of Ce increases. The mechanism for the thinning of the secondary dendrite arm is mainly by the solute effect, including the increase in both the undercooling in front of the dendrite tip and the drag effect on solid-liquid interface migration. In addition, the correspondence between the dendrite and grain in Fe-Mn-C-Al TWIP steel under as-cast condition is revealed. It is observed that the primary dendrite interface completely matches the austenite grain boundary. The crystallographic relationship follows 〈100〉d//〈100〉g and {100}d//{100}g (d and g represent the dendrite and grain, respectively) in both the columnar and equiaxed zones. The refinement of the equiaxed austenite grain in Fe-Mn-C-Al TWIP steel is purely the result of dendrite refining during solidification.