Numerical simulation on inclusion transport in continuous casting mold

Turbulent flow, the transpor＇t of inclusions and bubbles, and inclusion removal by fluid flow, transport and by bubble flotation in the strand of the continuous slab caster are investigated using computational models, and validated through comparison with plant measurements of inclusions. Steady 3-D flow of steel in the liquid pool in the mold and upper strand is simulated with a finitedifference computational model using the standard k-εturbulence rondel. Trajectories of inclusions and bubhles tire calculated by integrating each local velocity, considering its drag and buoyancy forces, A ＂random walk＂ model is used to incorporate the effect of turbulent fluctuations on the particle motion. The attachment probability of inclusions on a bubble surface is investigated based on fundamental fluid flow simulations, incorporating the turbulent inclusion trajectory and sliding time of each individual inclusion along the bubble surface as a function of particle and bubble size. The chunge in inclusion distribution due to removal by bubble transport in the mold is calculated based on the computed attachment probability of inclusions on each bubble and the computed path length of the bubbles. The results indicate that 6%-10% inclusions are removed by fluid flow transport. 10% by bubble flotation, and 4% by entrapment to the submerged entry nozzle （SEN） walls. Smaller bubbles and larger inclusions have larger attachment probabilities. Smaller bubbles are more efficient for inclusion removal by bubble flotation, so Inng as they are not entrapped in the solidifying shell A larger gas flow rate favors inclusion removal by bubble flotation. The optimum bubble size should be 2-4mm.

EXPERIMENTAL RESEARCH AND NUMERICAL SIMULATION OF MOLD TEMPERATURE FIELD IN CONTINUOUS CASTING OF STEEL

Mold is the heart of the continuous casting machine. Heat transfer and solidification in a water- cooled mold are the most important factors during the continuous casting of steel. For studying the temperature distribution of a mold wall, a simulated apparatus of mold was designed and experiments were performed by it. The measured results indicated that the mold wall temperature approaches the temperature of cooling-water. An equivalent thermal-conductivity coefficient was proposed and deduced on the basis of the conclusion of the experiments. This coefficient was applied to solve the heat transfer between the melt and cooling water, and to characterize the heat transfer capacity of the mold. By this equivalent thermal-conductivity coefficient, it is very easy and convenient to numerically simulate the solidification process of continuous casting. And the calculation results are in agreement with the experiments. The effects of custing speed and water flow rate on the mold temperature field were also discussed.

MICROSTRUCTURE ANALYSIS AND MECHANICAL PROPERTIES OF Zn-Al ALLOY ROD PRODUCED BY HEATED MOLD CONTINUOUS CASTING

The new technology of continuous casting by heated mold was used to produce directional solidification ZA alloy lines to eliminate the inter defects of these lines and increase their mechanical properties. The results are as follows: (1) The microstruc-ture of the ZA alloy lines is the parallel directional dendritic columnar crystal. Every dendritic crystal of eutectic alloy ZA5 was composed of many layer eutectic β and η phases. The micro structure of hypereutectic ZA alloys is primary dendritic crystal and interdendritic eutectic structure. The primary phase of ZA8 and ZA12 is β, among them, but the primary phase of ZA22 and ZA27 is a. (2) Through the test to the as-cast ZA alloy lines made in continuous casting by heated mold, it is found that the tensile strength and hardness increase greatly, but the elongation decreases. With the increase of aluminum amount from ZA 5 to ZA 12, ZA22 and ZA27, the tensile strength increases gradually. ZA27 has the best comprehensive mechanical properties in these four kinds of ZA alloys. (3) Heat treatment can decrease the dendritic segregation and improve the elongation of ZA alloy, but make their strength decrease slightly.

Flow and Temperature Fields in Slab Continuous Casting Molds

In order to develop super-board and super-thick slabs, the flow and temperatur fields were studied in slab continuous casting molds under different practical conditions, such as slab dimensions, with-drawing slab speed, design of nozzles, and superheat tempera-ture. The results showed that it is preferred to incline nozzle bores downwards and the submerged depth of the nozzles is best kept be-tween 250-300 mm. In addition, the solidified shell is thicker at the wide face than that at the narrow face, while the thin points alongthe wide face ekist both in the center and in the some area toward each respective end.

Progress in the application of cold gas dynamic spraying to repairing continuous casting molds

A new continuous casting mold repairing method--cold gas dynamic spraying （CGDS） is introduced. The study investigates the advantages of the CGDS process regarding repairing operation, such as convenient, in-situ repairation,little heat delivery, microstructural and dimensional stability and other special applications. Microstructure and mechanical properties of the copper alloy coating, nickel coating, ceramic composite coating, and their interface to the substrates ,which are usually used in repairing operation have been researched by means of optical microscopy （ OM）, scanning electron microscopy （SEM）, transmission electron microscopy （TEM） and micro-hardness tests. Experimental results have demonstrated the relative density of the copper alloy coating is as high as 98.7%, and that no obvious difference can be observed between the CrZrCu substrate and the Cu alloy coating in terms of microstructures; thus the interface is quite difficult to be identified. The bonding strength and micro-hardness of the Cu alloy coating reach up to 37 MPa and 310 HV0.2 ,respectively. The interface between the copper alloy coating and the nickel coating is either zigzag or wave shaped, and the cohesion is relatively good. As-sprayed nickel coating is dominated by severely deformed particles,and the relative density is up to 98.5%. Complete recrystallization occurred after annealing at 900℃ for one hour,while its micro-hardness remains as high as 124.1 HV02. All these results have indicated that CGDS is a promising technology for repairing the continuous casting mold and that its future development is prosperous as well.

Effects of Rare Earth Oxide on Viscosity of Mold Fluxes for Continuous Casting

The effects of RE (rare earth) oxide on viscosity of mold fluxes were investigated with a rotary viscometer. The results show that: (1) The viscosity of mold fluxes is remarkably increased by RE oxide addition, especially when the mass fraction of RE oxide is more than 10%. (2) By addition of RE oxide, precipitation of the insoluble particles with high melting point from the molten slag with the decreasing of the temperature leads to the increase of viscosity. Viscosity curve shows that RE oxide is soluble in some extent in mold fluxes. When RE oxide is in a state of supersaturation, the existence of insoluble particles also makes the viscosity of mold fluxes increase. (3) Not only the viscosity of mold fluxes can be reduced, but also the capacity to dissolve and absorb RE oxide can be increased by Li_2O, B_2O_3 and BaO. However, the contents of Li_2O, B_2O_3, and BaO should be controlled to suitable levels. (4) The solidification temperature of mold fluxes can be increased by the addition of RE oxide, which is unfavorable to heat transfer and lubrication of mold fluxes between steel shell and mold.

Study on Meniscus Temperature Fluctuation during Mold Oscillation in Continuous Casting by Modeling Experiments

The temperature of the meniscus in continuous casting was measured during mold oscillation by modeling experiments. It is found that the temperature of meniscus varied periodically along with mold oscillation. Based on the above phenomenon, the mechanism of some techniques, such as, hot top mold, high frequency and low amplitude mold oscillation, soft contact mold electromagnetic continuous casting and so on, which can improve the quality of continuous casting billets, was analyzed. The results show that the decrease of the temperature fluctuation of early solidification shell is their common mechanism.

Effects of transient fluid flow and solidification on the transport of bubbles in a slab continuous casting mold

Transient molten steel flow in a slab continuous casting mold has been calculated using large eddy simulation, considering heat transfer and solidification. The transport of bubbles in the liquid pool of the solidified shell has been considered according to the dispersed phase model. A mathematical model has been used to evaluate the influence of bubble size, casting speed, and adsorption of nonmetallic inclusions on bubble removal and bubble distribution within the solidified shell in the mold. The results show that the ratio of bubbles floating to the top surface decreases with increasing casting speed and decreasing bubble diameter. Nonmetallic inclusion adsorption has a weak effect on the bubble.

Physical modeling and numerical simulation of electromagnetic stirring in a slab continuous casting mold

Through physical modeling and numerical simulation,the flow field in a slab continuous casting mold with electromagnetic stirring is measured under different casting parameters and stirring currents. To qualitatively evaluate the flow field in the mold, two indexes,i, e., mold flux entrapment and velocity uniformity, are proposed. Based on these two indexes, some optimized stirring parameters under different casting conditions can be determined.

Theoretical Analysis of the Solidification of Aluminum Alloy Billetin Air-Slip DC Mold

Based on the heat transfer analysis of Air-Slip DC mold, a numerical model was presented to study the quantitative relationships between critical solidification layer and casting rate, pouring temperature and mold cooling ability etc. The analytical results show that the Air-Slip mold heat transfer condition plays important roles on choices of a casting rate and the pouring temperature, and that the product of billet diameter and casting rate is a certain constant under a certain condition of the mold.

DISTRIBUTION OF THE MAGNETIC FIELD AND ITS CONTROLEFFECT ON THE MOLTEN METAL FLOW IN FLOW CONTROLMOLD

A thermo-simulation experimental continuous caster was established to study the effect of two static magnetic fields imposed the whole width of the mold on the molten metal flow. Low melting point Bi-Pb-Sn alloy and silicon oil were adopted to simulate the molten metal and mold powder respectively. The results show that the control effect of electromagnetic fields on the molten metal flow in the different zones of mold is different. Therefore it is important to study and design new types of magnetic fields to satisfy the request of controlling the molten metal flow in the mold.

EXPERIMENTAL MEASUREMENT OF MAGNETIC FIELD IN A NOVEL FLOW CONTROL OF MOLD

In order to know the distribution of magnetic field in a novel flow control of mold (NFC Mold) and to provide the experimental data for the electromagnetic structure design and the analysis of flow control in continuous casting mold, the magnetic field in a NFC Mold were measured by Tesla meter of Model CT-3. The method of vector synthesis was adopted in the measurement of magnetic fields. The results showed that the magnetic field in the NFC Mold was composed of two main magnetic areas that were symmetrical. Although there was leaking magnetic flux between the lower surface of the upper pole and the upper surface of the lower pole on the sides, it was restrained by the main magnetic fields effectively. Therefore the NFC Mold was more preferably satisfied to be used in controlling the molten steel flow in continuous casting mold.

Variation of meniscus shape and initial steel solidification under different steel-slag interfacial tension in continuous casting mold of slab

A two-dimensional model was applied to investigate the influence of the interfacial tension between the steel and the slag on the behavior of the meniscus in continuous casting mold of slab.The shape of the meniscus and phenomena near the meniscus were revealed,and the profile of the slag rim and the depth of the solidified meniscus and oscillation marks with different interfacial tension of the steel and slag were compared.With the increase in the interfacial tension,the size of the curved meniscus increased,while the curvature and the height of the local meniscus close to the mold decreased.Besides,the thickness of the slag rim,solid slag and total slag near the meniscus had the tendency to increase,and the bottom of the slag rim became lower and thicker.With the increase in the interfacial tension from 0.1 to 2.5 N/m,the location of the largest heat flux near the meniscus decreased from 10.0 to 2.5 mm above the initial level of the steel,and the largest heat flux was within 3.52-4.58 MW/m^(2).Meanwhile,the largest depth of the solidified meniscus decreased from 3.3 to 2.3 mm,and the depth of oscillation marks decreased,which was conducive to the shallow hook at the subsurface of the slab,and the improvement of surface cleanliness of the slab.

Heat Transfer and Deformation Behavior of Shell Solidification in Wide and Thick Slab Continuous Casting Mold

With the considerations of the behaviors of shell deformation, mold flux film and air gap dynamic distribution in shell/mold gap, a two dimensional slice-travel transient thermo-mechanical coupled model of simulation shell solidification in wide and thick slab continuous casting mold was developed by using the commercial program ANSYS. The evolutions of strand-mold system thermal behaviors, including the air gap formation and the mold flux film dynamical distribution in shell/mold gap and shell temperature field, and the evolutions of shell deformation and stress distribution of peritectic steel solidified in a 2120 mm wide and 266 mm thick slab continuous casting mold were analyzed. The results show that the air gap formation and the thick mold flux film distribution mainly concentrate in the regions 0–21 mm and 0–7 mm, 0–120 mm and 0–100 mm off the shell wide and narrow faces corners, and thus the hot spots are given rise to form in the regions 15–55 mm and 15–50 mm off the shell wide and narrow face corners. The shell server deformation occurs in the off-corners in the middle and lower parts of the mold. The stress evolution in shell surface is tensile stress, while that in shell solidification front is compressive stress.

Vortex Flow Pattern in a Slab Continuous Casting Mold with Argon Gas Injection

An Eulerian-Eulerian two-fluid model was developed to study the vortex flow inside a slab continuous casting mold with argon gas injection. Interracial momentum transfer that accommodated various interracial forces including drag force, lift force, virtual mass force, and turbulent dispersion force was considered. Predicted results agree well vaith experimental measurements of the water model in two-phase flow pattern and vortex flow structures. Three typical flow patterns with different argon steel ratios （ASRs） have been obtained： ＂double roll＂, ＂three roll＂, and ＂single roll＂. The flow pattern inside the mold alternates among the three types or it may attain some intermedi ate condition. With increasing ASR, the positions of vortices move from the submerged entry nozzle to the narrow face of the mold, and the sizes of vortices are reduced gradually. The rotating directions of vortices are all from high velocity area to low velocity area. Two mechanisms of vortex formation on the top surface have been suggested, i. e. , congruous shear flow and incongruous shear flow.

Analysis of Thermal Behavior for Beam Blank Continuous Casting Mold

Finite element models of steady heat conduction for cross section of beam blank mold were developed by using ABAQUS software. The effect of mold grinding thickness, cooling water velocity, diameter of restrietor rods and water channel design on hot face temperature was analyzed in detail. Attention was focused on the peak temperature and temperature uniformity along hot face. The results showed that the peak temperature of existing mold, about 337.2 ℃, is located in the fillet, and two valleys of hot face temperature are found in flange corner and junction of wide face and narrow face, respectively. Decreasing mold thickness, increasing cooling water velocity and increasing diameter of restrictor rods can all reduce peak temperature and improve temperature uniformity along hot race at the expense of lower overall temperature. Redesigning the water channel can decrease peak temperature and thermal gradient of mold without lowering overall temperature of hot face. In particular, the small hole design can improve temperature uniformity across hot face and obtain the best advantage.

Large Eddy Simulation for Unsteady Turbulent Field in Thin Slab Continuous Casting Mold

The unsteady turbulent flow during the continuous casting of steel is important, because it influences critical phenomena that affect steel quality. Unsteady three-dimensional flow in the mold region of the liquid pool during continuous casting of steel slabs has been computed using realistic geometries starting from the submerged inlet nozzle to the mold. The cassette filter function was used to deal with unsteady Navier-Stokes equation, and then the turbulent flow in the thin slab CCM was simulated with the large eddy simulation method combined with the Smagorinsky sub-grid scale model in this paper. And the model was verified by the Particle Image Velocimetry (PIV) experimental results which was got from a relate scientific literature. In this thesis, by means of LES, the flow characteristics in the thin slab CCM were acquired, such as the vortex distribution, the formation of the large eddy coherent structures, development, shedding and fracture process. In the same time, the turbulent asymmetric distribution was revealed even the nozzle in the centre position. Interactions between the two halves cause large velocity fluctuations near the meniscus. And the vortex is located at the low velocity side adjacent to the SEN. Along with the unsteady time development, the unsteady turbulent large vortex structures of the liquid steel in the CCM presented periodic bias flow distribution, and the period is about 20 seconds.

Analysis of Non-uniform Mechanical Behavior for a Continuous Casting Mold Based on Heat Flux from Inverse Problem

The distortion of mold plates plays an important role in the formation of surface cracks on continuously cast steel products. To investigate the non-uniform distortion of a mold, a full-scale stress model of the mold was de veloped. An inverse algorithm was applied to calculate the heat flux using the temperatures measured by the thermo- couples buried inside the mold plates. Based on this, a full-scale, finite-element stress model, including four copper plates, a nickel layer and water slots in different depths, was built to determine the complex mechanical behavior of the continuous casting mold used to produce steel slabs. The heat flux calculated by the inverse algorithm was applied to the stress model to analyze the non-uniform mechanical behavior. The results showed that the stress and distortion distributions of the four copper plates were not symmetrical, which reflected the non-uniform distortion behaviors of copper plates, water slots, nickel layer and the corner region of the mold. The gap between the mold and the slab was increased because of the corner distortion, which was very important for the heat transfer of initial solidifying shell, and it may be a major reason for the slow cooling of the slab corner.

Multiphase Flow and Thermo-Mechanical Behaviors of Solidifying Shell in Continuous Casting Mold

The metallurgical phenomena occurring in the continuous casting mold have a significant influence on the performance and the quality of steel product.The multiphase flow phenomena of molten steel,steel/slag interface and gas bubbles in the slab continuous casting mold were described by numerical simulation,and the effect of electromagnetic brake（EMBR） and argon gas blowing on the process were investigated.The relationship between wavy fluctuation height near meniscus and the level fluctuation index F,which reflects the situation of mold flux entrapment,was clarified.Moreover,based on a microsegregation model of solute elements in mushy zone with δ/γ transformation and a thermo-mechanical coupling finite element model of shell solidification,the thermal and mechanical behaviors of solidifying shell including the dynamic distribution laws of air gap and mold flux,temperature and stress of shell in slab continuous casting mold were described.

Turbulent Flow and Transport of Bubbles in Slab Continuous Casting Mold Under Electromagnetic Brake

Large eddy simulation model with magnetohydrodynamics(MHD)is developed to study the effect of electromagnetic braker(EMBr)on molten steel flow and transport of argon bubbles in the continuous casting process. The large eddy simulation with Smagorinsky sub-grid scale model is used to calculate the turbulent variables.The results indicate that the magnetic force can optimize the molten steel flow pattern in the braking area.The flow is asymmetric, especially in the lower of the mold,even if with applied electromagnetic effect.The flow deviation of the molten steel in the mold is inevitable existence.More bubbles tend to release from the top surface with LMF-EMBr,while,FCMF-EMBr has little effect on the overall removal fraction of bubbles,but it affects the trajectory of bubbles.