Sen Luo

Heat Transfer and Central Segregation of Continuously Cast High Carbon Steel Billet

A numerical model of heat transfer was developed to investigate the heat transfer of continuously cast billet with the aid of surface temperature tests by ThermaCAM™ researcher and nail shooting experiments. The effects of secondary cooling practice and casting speed on the solidification process and central segregation of carbon were investigated as well with the actual central segregation tests. The results show that the surface center and billet center temperatures exhibit a different pattern during solidification, and the solidified shell thickness is presented as an “S” type. With the increase of secondary cooling intensity and the decrease of casting speed, the end points of the solidus line and the liquidus line move forward, and the central segregation level of carbon decreases. The optimal casting condition is suggested for continuously cast high carbon billet with F-EMS (final electromagnetic stirring).

Characteristics of solute segregation in continuous casting bloom with dynamic soft reduction and determination of soft reduction zone

Abstract A mathematical model accounting for the discharge of solute enriched liquid from the mushy zone with soft reduction by one pair of pinch rolls was established, and the ratio of solute concentration in the mushy zone before and after soft reduction, named as solute segregation ratio, is proposed to describe the characteristics of solute segregation. The theoretical analysis of solute segregation ratio at different soft reduction positions shows that the start point of the soft reduction zone should not be greater than the minimum optimum soft reduction position and the end point of the soft reduction zone should not be less than the maximum optimum soft reduction position corresponding to minimum solute segregation ratio. The soft reduction industrial practices of steel YQ450NQR1 and steel 37Mn2 were carried out with the soft reduction parameters determined by the model, and results showed that the inner quality was improved.

Theoretical model for determining optimum soft reduction zone of continuous casting steel

Abstract A theoretical model for determining the optimum soft reduction zone of continuous casting steel was developed. According to the theoretical analysis of the solute segregation behaviour in the mushy zone with the soft reduction by one pair of pinch rolls, there is an optimum soft reduction position for each element of steel, and all optimum soft reduction positions of different solute elements should be included in order to eliminate the centreline segregation effectively. The theoretical analysis shows that the carbon content has a great effect on the optimum soft reduction zone, and the cooling rate could affect both the optimum soft reduction positions of the segregation prone elements (P, S) and the optimum soft reduction zone of continuous casting steel. Plant trials of steel SWRH82B with soft reduction were carried out to validate the theoretical model, and the results show that the solute element segregation in the centreline of the strand has been improved with the optimum soft reduction zone.

Mechanism of Macrosegregation Formation in Continuous Casting Slab: A Numerical Simulation Study

Solidified shell bulging is supposed to be the main reason for slab center segregation, while the influence of thermal shrinkage rarely has been considered. In this article, a thermal shrinkage model coupled with the multiphase solidification model is developed to investigate the effect of the thermal shrinkage, solidification shrinkage, grain sedimentation, and thermal flow on solute transport in the continuous casting slab. In this model, the initial equiaxed grains contract freely with the temperature decrease, while the coherent equiaxed grains and columnar phase move directionally toward the slab surface. The results demonstrate that the center positive segregation accompanied by negative segregation in the periphery zone is mainly caused by thermal shrinkage. During the solidification process, liquid phase first transports toward the slab surface to compensate for thermal shrinkage, which is similar to the case considering solidification shrinkage, and then it moves opposite to the slab center near the solidification end. It is attributed to the sharp decrease of center temperature and the intensive contract of solid phase, which cause the enriched liquid to be squeezed out. With the effect of grain sedimentation and thermal flow, the negative segregation at the external arc side (zone A1) and the positive segregation near the columnar-to-equiaxed transition at the inner arc side (position B1) come into being. Besides, it is found that the grain sedimentation and thermal flow only influence solute transport before equiaxed grains impinge with each other, while the solidification and thermal shrinkage still affect solute redistribution in the later stage.

Numerical Simulation and Experimental Study of F-EMS for Continuously Cast Billet of High Carbon Steel

A final electromagnetic stirring model was developed for billet continuous casting of high carbon steel using the commercial software ANSYS and CFX, and the numerical model was validated by the magnetic flux density measured under a Teslameter CT-3. The magnetic flux density and fluid flow in the liquid pool at the location of final electromagnetic stirring (F-EMS) were calculated by the present numerical model. Meanwhile, the plant trials were carried out to determine the optimum current intensity and frequency of F-EMS for the continuously cast billet of high carbon steel. The numerical results show that, through increasing the current intensity by 100 A, the corresponding increases of magnetic induction intensity, tangential electromagnetic force and flow velocity at the solid/liquid interface in the strand are 0.025 T, 1933 N/m3 and 6.9 cm/s, respectively. Moreover, the industrial trial results showed that for the continuously cast billet of 60 steel, the optimum current intensity and frequency of F-EMS, which is 8.2 m from the meniscus, are respectively 380 A and 6 Hz. With the optimum F-EMS parameters, the significant improvement of center segregation of billet is achieved, and the center carbon segregation index in billet reaches 1.04.

Influence of Forced Flow on the Dendritic Growth of Fe-C Alloy: 3D vs 2D Simulation

A 3D parallel cellular automaton-finite volume method (CA-FVM) model was used to simulate the equiaxed dendritic growth of an Fe-0.82 wt pct C alloy with xy-in-out and xyz-in-out type forced flows and the columnar dendritic growth with y-in-out type forced flow. In addition, the similarities and differences between the results of the 3D and 2D models are discussed and summarized in detail. The capabilities of the 3D and 2D CA-FVM models to predict the dendritic growth of the alloy with forced flow are validated through comparison with the boundary layer correction and Oseen–Ivanstov models, respectively. Because the forced flow can pass around perpendicular arms of the dendrites, the secondary arms at the sides upstream from the perpendicular arms are more developed than those on the upstream side of the upstream arms, especially at higher inlet velocities. In addition, compared to the xy-in-out case, the growth of the downstream arms is less inhibited and the secondary arms are more developed in the xyz-in-out case because of the greater lateral flow around their tips. Compared to the 3D case, the 2D equiaxed dendrites are more asymmetrical and lack secondary arms because of the thicker solute envelope. In the 3D case, the columnar dendrites on the upstream side (left one) are promoted, while the middle and downstream dendrites are inhibited in sequence. However, the sequential inhibition starts on the upstream side in the 2D case. This is mainly because the melt can pass around the upstream branch in 3D space. However, it can only climb over the upstream tip in 2D space. Additionally, the secondary arms show upstream development, which is more significant with increasing inlet velocity. The level of development of the secondary arms is also affected by the decay of the forced flow in the flow direction.

Numerical Simulation of Dendritic Growth of Fe-C Binary Alloy with Natural Convection

Embedded the body force induced by the thermal and solutal gradient into the momentum conservation equation, a 2D CA-FVM model is developed for the numerical of dendritic growth with natural convection, and a numerical simulation is performed for the dendritic growth of Fe-0.82wt%C alloy in the presence of natural convection. The results show that the dendrite tip growth velocity is larger at the initial stage and rapidly decreases to steady-state value. The natural convection induced by the thermosolutal buoyancy is weak and promotes two vortexes flowing around the dendrite from the bottom to the top. With the further growth of dendrite, the natural convection is enhanced and four vortexes is developed between the dendrite arms. Also, the natural convection carries the heat and solute from the bottom to the top, resulting in promoting the downward dendrite branch growth and inhibiting the upward dendrite branch growth. Consequently, the asymmetries of the dendrite morphology, temperature and solute profiles are developed.

Performance Optimization and Evaluation of a 3D CA-FVM Model for Dendritic Growth of Fe-C Alloy

Because of the tremendous computational cost of 3D (three dimensional) calculation of the dendritic growth, the parallel approach and the block-correction technique (BCT) are adopted to improve the efficiency of codes. Meanwhile, the accuracy of the codes is evaluated by comparing the present prediction with the analytical solutions to the fluid flow problem, LGK analytical results and the experimental measured columnar dendritic morphology and secondary dendritic arm spacing (SDAS, λ2). The results show that the parallel Jacobi code with once 2D iteration in 3D BCT is proved to be the most efficient one among the codes compiled in the present work, accordingly is employed to simulate the 3D dendritic growth of alloys. The calculated velocities agree well with the results from the analytical equations. The predicted steady growth velocities of the equiaxed dendritic tip of Fe-0.82wt%C alloy by the present CA model agree with the 3D LGK analytical model as the anisotropy parameter is 0.04. Moreover, the present CA model shows some capability to predict the columnar dendritic growth during the directional solidification process of Fe-1.48wt%C alloy.

The Effect of Phosphorus and Sulfur on the Crack Susceptibility of Continuous Casting Steel

Based on the coupled macro-heat transfer and micro-segregation model for continuous casting wide-thick slab, the effect of phosphorus and sulfur on crack susceptibility was investigated from the peritectic reaction zone, the brittle temperature range, the thermal strain and the difference of deformation energy. The results show that the crack sensibility of continuous casting steel, especially the hypo-peritectic steel, intensifies with the improvement of phosphorus and sulfur in steel matrix. Compared with phosphorus, sulfur does a more powerful effect on the crack susceptibility of steel. The peritectic reaction zone, the brittle temperature range and the thermal strain are capable to characterize the influence of phosphorus and sulfur on the crack susceptibility of steel. However, the calculated difference of deformation energy decreases with increasing phosphorus and sulfur, which is inconsistent with the experiment results. Therefore the difference of deformation energy is inappropriate to be applied to characterize the impact of phosphorus and sulfur to crack susceptibility of steel.