Controlled undercooling of liquid iron: Effect of substrate composition and atmosphere

Any solidification structure is influenced by two main phenomena, nucleation and growth of the solid phase. The continuous casting structure is generally composed of three parts: the chill, columnar and equiaxed zones. The amount of each of these phases depends on the relation between the continuous nucleation of solid, and the growth of the already existing nuclei. Because nucleation involves the formation of a new interface, it requires deep undercoolings unless it can occur heterogeneously on an existing surface. In the continuous casting process, the upper part of the mold, close to the mold wall, is deeply undercooled and hence it is the source of nuclei that will eventually form the chill/columnar zone and the equiaxed zones. The amount of undercooling necessary for nucleation depends on the availability of surfaces for heterogeneous nucleation in the continuous casting mold, these surfaces are the slag/metal interface or the inclusions always present in the steel.; The objective of this thesis is to determine the conditions under which the inclusions present in liquid steel can act as heterogeneous nucleants for solidification. The experimental approach consisted of measuring the undercooling of a pure iron droplet in contact with different oxides (different oxide substrates for sessile drop experiments and crucibles for DTA). The idea behind these experiments was to determine which oxides promote the solidification of iron by providing a suitable surface for nucleation and, on the contrary, which oxides and under which conditions the metal can be deeply undercooled.; The conclusions suggest that deep undercoolings are possible at low oxygen contents, provided the oxygen potential is such that substrate decomposition does not occur. Hence, those oxides that are less stable are prone to be unsuitable for deep undercoolings. On the other hand if the oxygen content increases the undercooling also decreases. Under deep undercooling conditions, the instantaneous solidification rates were as high as those measured during the initial stages of the traditional casting process against a cold mold and average rates were significantly higher.; The observations presented in this thesis are very promising for the development of new casting processes that could be described as follows: (1) A process in which the correct selection of oxide inclusions and composition causes deep undercoolings, allowing the achievement of high solidification rates and lower energy consumption. (2) A process in which the correct selection of oxide inclusions and composition, favors nucleation and hence allows for the control of the solidification structure.