Modelling of Columnar-to-Equiaxed Transition and Inclusion Distribution in Continuously Cast Steel Billets

Abstract

Solidification castings can exhibit a columnar or an equiaxed morphology or a combination of both. Since the relative proportions of these two components strongly influence the internal quality of cast product, the study of morphological transition from columnar to equiaxed structure (CET) becomes important. The transition also affects quality parameters like inclusion distribution in castings which has a significant bearing on the properties of cast products. In this work, a combined model for CET and inclusion distribution in continuously cast steel billets is presented. A conduction based transient thermal solidification model is employed in conjunction with Hunt's criterion for CET to predict the evolution of melt temperature, the location of transition and area-fractions of columnar and equiaxed zones across the billet cross-section. A correlation between melt temperature and equiaxed nuclei density is proposed and incorporated in the model to account for a more realistic variation of CET with melt superheat. The model is compared with available experimental data and is used to explore the effect of process parameters on CET and determine the spatial distribution of non-metallic inclusions in the solidified billet.

Mathematical modelling of morphological transition and spot segregation in continuously-cast high carbon steel billets

“…The values have been plotted with and without Equation 13 to demonstrate the influence of N 0 in the model. A reference value of N 0 = 10 7 m -3 is taken for a superheat of 21 K (Chaube et al, 2015). It is evident that the model predictions, without the proposed empirical relation, grossly underestimate the effect of superheat on CET.…”

“…Therefore, for high melt superheats, this phenomenon becomes prominent and can hardly be ignored. Since there are no accurate analytical expressions available in literature describing this behaviour, an empirical correlation has been used (Chaube et al, 2015):…”

“…Available data (Shamsi and Ajmani, 2010) for CC1 billet caster (Tata Steel, Jamshedpur) is used to predict shell thickness profile, columnar front velocity, thermal gradient and CET with the progress of solidification. For better CET predictions, a superheat linked dependency of the number density of effective nuclei -that grow into equiaxed grains -is incorporated into the model through an exponential decay of Hunt's nucleation parameter N 0 (Chaube et al, 2015). With this treatment, the results show a much stronger dependence of CET with superheat even when the thermal gradients and growth velocities are insubstantially modified.…”

Experimental investigation on solidification of GCr15 bearing steel by the simulated continuous casting

“…As known, the solidification structure and segregation of steel are determined by the solidification process, and the superheat and cooling rate of the liquid steel are the key parameters to influent the solidification. One side low superheats lead to a high proportion of equiaxed structure that is recognised as the best for minimising segregation [13][14][15][16][17][18][19][20]. However, Choudhary et al [10] and Pikkarainen et al [11] presented that liquid steels cast at high superheat were generally having finer cast structures as compared to those cast at lower superheats.…”

“…As known, the solidification structure and segregation of steel are determined by the solidification process, and the superheat and cooling rate of the liquid steel are the key parameters to influent the solidification. One side low superheats lead to a high proportion of equiaxed structure that is recognised as the best for minimising segregation [13–20]. However, Choudhary et al.…”