Macro-Micro Modeling of the Dendritic Microstructure of Steel Billets Processed by Continuous Casting

J. M. Cabrera-Marrero, V. Carreño-Galindo, R. D. Morales, F. Chávez-Alcalá
1998 ISIJ International  
Operating data of a steel billet caster such as casting speed steel temperature and cooling conditions in the water spray zone are employed to predict dendritic microstructures of solidified steel. For that semi-empirical equations to calculate primary and secondary arm spacing were derived from uni-directional solidification experiments of steel samples with various compositions. When these equations are combined with a heat transfer model, which involves operating parameters of a caster, it
more » ... possible to predict dendritic microstructures, making possible micro-modeling from macro-processing data. Experimental measurements of primary and secondary arm spacing in fou~commercial steels agreed acceptably well by the predictions performed using this approach. The knowledge of microstructure will ailow to pursue predictions of microsegregation phenomena, The near net shape casting of steel products is increasingly getting important. The benefits of this processing method in comparison with traditional technologies are, among others, diminution of costs, reduction in stand-by times and ability for roliing in line especially in the case of billets or thin siabs. In order to maintain or Improve the mechanical properties achleved by conventional processes, optlmum conditions of continuous casting (CC) processing must be defined. From this point of view, the prediction of the solidification microstructure and its dependence on process variables is an important task. Under industrial conditions, metallic alloys usually solidify in dendritic interfaces. Dendrites are characterized by means of the primary~l and secondary~2 dendritic arm spacing. The latter spaclng is strongly related to microsegregation effects, which explains why it has received more attentlon than the primary one. The dependenceof both )~1 and~2 spacing on the chemical composition and solidification conditions is needed for a correct microstructural prediction whose results will be employed for microsegregation understanding. However, there Is theoretical model describing with enough accuracy the dependenceof the dendrite arm spacing on time (i,e. solidification parameters). Wlth regard to~1 few theoretlcal expressions have been reported,i~3) In all of these~1 is related to the solldification rate R and to the thermal gradlent G in the ) where n= 1/4 and != l/2 after most of the theoretical models. The thermal gradient G is defined as G= (TL-Ts)/vt*, TL being the liquidus temperature, Ts the solidus temperature and T4' the mushy zone wide. The discrepancies between authors in the above equation are mainly focused on the constant N, which in turns depends on the dendrite tip geometry. In relation to~2 several expressions are available in the literature.4~9) During solldification, the secondary arms tend to Increase their spacing owing to different facts, namely: remeltlng of small secondary dendrites, meltlng of dendrite roots or coalescence between dendrites. The first authors in detecting this feature were Kattamis et al.4) and Flemings et a/.,5) and nowadays it is believed to arlse from surface stresses at the liquidsolid interface. The coarsenlng of ).2 dilutes the composition of the liquid phase and, indirectly contributes to the chemical homogenization, No consideration of this effect would predict more mlcrosegregation than the real one. The moreor less complex consideration of the latter phenomena leads to different equations for ).2'6~9) Most of the various theoreticai models reported are based on the fact that )~2 is related to the remaining time in the liquid-solid region, that is, to the local solidification time tf' The relation observed Is of the form of Eq. (2), and again the difference between the dlverse
doi:10.2355/isijinternational.38.812 fatcat:6ahpxtgwevgehjpwyfgr4osiwm