Modelling molten flux layer thickness profiles in compact strip process moulds for continuous thin slab casting

Abstract

The important functions promoted by powdered flux added over the liquid steel surface in continuous casting moulds are strongly affected by the thickness of the liquid layer that forms as a result of the heat absorbed. The present work discusses the results of a three-dimensional steady state model, developed to represent the coupled fluid flow and heat transfer phenomena that determine thickness profiles of the liquid flux layer. Since the laminar flow of the liquid slag layer depends on the shearing imposed on it by the turbulent motion of the liquid steel beneath it, and since additionally this motion is strongly influenced by the flow characteristics of the steel stream poured into the mould through the submerged entry nozzle (SEN), separate turbulent flow models for the liquid steel in the SEN and the mould were also developed. The consistency among the models and their accuracy was judged by comparing thickness and temperature flux profiles measured in plant against predicted ones; the comparison showed good agreement. The effects of casting speed, mould width, and flux viscosity and heat of melting on the liquid layer thickness were investigated. The last variable was found to exert the most marked influence. Different from conventional casting moulds, where the liquid layer thickness increases with increasing casting speed, in compact strip process moulds the thickness remains almost constant with increasing casting speed. This difference is well accounted for by the model, which suggests that this behaviour stems from the different slag flow patterns generated in straight, wide moulds and in thin moulds having a central upper funnel shaped section.     

Heat flux transients at the casting/chill interface during solidification of aluminum base alloys

Heat flow at the metal/chill interface of bar-type castings of aluminum base alloys was modeled as a function of thermophysical properties of the chill material and its thickness. Experimental setup for casting square bars of Al-13.2 pct Si eutectic and Al-3 pet Cu-4.5 pct Si long freezing range alloys with chill at one end exposed to ambient conditions was fabricated. Experiments were carried out for different metal/chill combinations with and without coatings. The thermal history at nodal locations in the chill obtained during the experiments was used to estimate the interface heat flux by solving a one-dimensional Fourier heat conduction equation inversely. Using the data on transient heat flux q, the heat flow at the casting/chill interface was modeled in two steps: (1) The peak in the heat flux curve q_max was modeled as a power function of the ratio of the chill thickness d to its thermal diffusivity a, and (2) the factor (q/q_max) X α^0.05 was also modeled as a power function of the time after the solidification set in. The model was validated for Cu-10 pct Sn -2 pct Zn alloy chill and Al-13.2 pct Si and Al-3 pct Cu-4.5 pct Si as the casting alloys. The heat flux values estimated using the model were used as one of the boundary conditions for solidification simulation of the test casting. The experimental and simulated temperature distributions inside the casting were found to be in good agreement. Formerly Assistant Professor with Karnataka Regional Engineering College     

Abnormal transient phenomena in the continuous casting process: Part 2

Abstract

This paper is the second of a series of two describing abnormal transient phenomena observed during online monitoring of a billet continuous casting machine. Special attention is paid in Part 2 to in mould solidification. A mould heat flux drift phenomenon (HFD) has been detected, but only for mould powder basicities larger than 0·8. The HFD is related to a decrease of the heat flux in the lower part of the mould and an increase in both the billet-mould friction force and mould thermocouple variability. Results of tests changing the mould powder grade during casting have provided help in explaining the HFD. The probable reason for the HFD is crystallisation of the glassy slag layer. The heat flux ratio parameter (HFR), defined as the ratio between the heat flux in the lower part of the mould and the heat flux in the upper part of the mould, has proved to be a good tool for judging the casting performance of a mould powder.     

Viscosity of casting fluxes used during continuous casting of steel

Molten flux viscosity of eighteen commercially available casting fluxes has been measured as a function of temperature in the range of 1223 to 1723 K. Results show that, at a constant temperature, the flux viscosity is primarily dependent on the mole fractions of SiO₂ and A1₂O₃. The calculated activation energy for viscous flow at 1573 K varies from 59 to 215 kJ/mol depending on the flux viscosity and the flux basicity ratio. The observed “breakpoints” in the viscosity vs temperature plots are explained in terms of precipitation of crystalline phases. The tendency for crystal precipitation is related to the relative viscosity and the basicity of the flux. Using X-ray diffraction techniques the precipitating crystal phases have been identified. Metallographic structures of quenched and air-solidified casting fluxes are reported.     

Design of mould fluxes for continuous casting of special steels

The melting behaviour of mould powder during continuous casting is an important consideration with respect to caster performance, production rate and steel quality. Two important factors, powder composition and carbon addition, are critical to control the properties and melting behaviour of the mould flux. In this study, the effect of different carbonaceous materials on the melting characteristics of mould powders was evaluated. Correlations were established between the structural factors and chemical reactivity of carbon and melting behaviour of mould flux. In addition, two examples are given of the effect of flux composition on casting performance for specific steels. A flux with reasonable basicity and additives was designed for the casting of heat-resistant steel (Incoloy 800) to reduce surface cracks. Another flux was designed for the casting of non-magnetic steel containing high aluminium by partially replacing SiO₂ with Al₂O₃ to limit aluminium oxidation by SiO₂.     

Numerical simulation of fluid flow and solidification in continuous slab casting mould based on inverse heat transfer calculation

Abstract

A mathematical model based on an inverse heat transfer calculation was built to determine the heat flux between the mould and slab based on the measured mould temperatures. With K–? turbulence model, a mathematical model of three-dimensional heat transfer and solidification of molten steel in continuous slab casting mould is developed. Solidification has been taken into consideration, and flow in the mushy zone is modelled according to Darcy’s law as is the case of flow in the porous media. The heat flux prescribed on the boundaries is obtained in the inverse heat conduction calculation; thus, the effect of heat transfer in the mould has been taken into consideration. Results show that the calculated values of mould temperature coincide with the measured ones. Results also reveal that the temperature distribution and shell thickness are affected by the fluid flow and heat transfer of slab which is governed by the heat flux on the mould/slab interface.     

Controlling Radiative Heat Transfer Across the Mold Flux Layer by the Scattering Effect of the Borosilicate Mold Flux System with Metallic Iron

The present study proposes a countermeasure for regulating total heat flux through the mold flux layer by designed mold flux with additive metallic iron particles. The heat flux through the B₂O₃-CaO-SiO₂-Na₂O-CaF₂-Fe system was investigated using the infrared emitter technique to evaluate total flux density across the mold flux film. Both scanning electron microscope (SEM) and X-ray diffraction analysis were employed in order to identify the morphological and compositional changes of the crystalline phase, according to increasing iron contents in the mold flux. It was confirmed that the crystalline layer of studied mold fluxes does not have a meaningful effect on the total heat flux density due to the similar structure and fraction of the crystalline phase. The extinction coefficient was measured for glassy mold fluxes using an ultraviolet/visible and a Fourier transformation-infrared ray spectrometer in the range of 0.5 to 5 μm. For analyzing the scattering behavior of iron particles on the extinction coefficient, the number density and diameter of particles were observed by an automated SEM (auto-SEM). With these data, Mie scattering theory is adopted to define the scattering behavior of dispersed iron droplets in glassy matrix. It was found that the theoretical scattering coefficient demonstrated about 1623 to 3295 m^?1, which is in accordance with the experimental results. In doing so, this study successfully achieves the strong scattering behavior that would contribute greatly to the optimization of overall heat flux through the mold flux film during the casting process.     

Laboratory study of heat transfer through thin layers of casting slag: minimization of the slag/probe contact resistance

The heat flux densities through thin layers of casting slag have been measured with a soft cooling heat flux probe which made smooth probe/slag interfaces. Thus the contact resistance was avoided. The evaluated effective thermal conductivities were applied, together with the data on the contact resistance, to compute the “system conductivities” existing in the continuous casting “system” water cooled copper/slag/strand. The refractive index and absorption spectrum were measured and used to deduce functions for the radiative and conductive (phonon) conductivities. Although precise values could not be obtained, due to the many assumptions involved, the data indicate that the conductive conductivity does not change drastically at high temperatures and on melting.     

Correction on: Laboratory study of heat transfer through thin layers of casting slag: minimization of the slag/probe contact resistance

The heat flux densities through thin layers of casting slag have been measured with a soft cooling heat flux probe which made smooth probe/slag interfaces. Thus the contact resistance was avoided. The evaluated effective thermal conductivities were applied, together with the data on the contact resistance, to compute the “system conductivities” existing in the continuous casting “system” water cooled copper/slag/strand. The refractive index and absorption spectrum were measured and used to deduce functions for the radiative and conductive (phonon) conductivities. Although precise values could not be obtained, due to the many assumptions involved, the data indicate that the conductive conductivity does not change drastically at high temperatures and on melting.     

Analysis of basic network structure of continuous casting mould powder in electromagnetic field

ABSTRACT

In the continuous casting process, the mould powder plays the role of insulator and heat insulator in the crystallizer, prevents molten steel from oxidizing, absorbs non-metallic inclusions, and controls lubrication and heat transfer between the casting billet and the mould. It is an important functional material that promotes the quality of a billet and ensures that the continuous casting process proceeds smoothly. Here in we analyse the changes in the microstructure of the protective flux from the aspect of the infrastructure of the flux slag. Additionally, using the classical molecular dynamics simulation method, the structure of the CaO–SiO₂–Al₂O₃–MgO slag system was simulated in the presence of magnetic fields of various strengths. The magnetic field was found to have the following effects on the slag structure. The participation of the basic elements Si, Al, O, etc. in bond formation is independent of magnetic field strength. The magnetic field causes changes in the peaks, coordination numbers, and peak widths of the radial distribution functions of bonds such as Si–O, Al–O, and Mg–O. The greater the magnetic field strength, the more disordered the ionic clusters are, and the greater is the decrease in slag viscosity. Although the effect of the magnetic field influences the structure of the slag, there is not much change in the way the molecules and atoms are stacked in the slag.     

Liquid steel flow in continuous casting machine: modelling and measurement

Abstract

Single phase (liquid steel) and two-phase (liquid steel and argon bubbles) three-dimensional computational fluid dynamic and heat transfer models were developed for the continuous casting machines of ArcelorMittal. The computational domains include tundishes, slide gates, submerged entry nozzles and moulds. The effects of buoyancy, tundish design, tundish practices, nozzle design and caster practices on flow structure were investigated. Mathematical modelling is discussed in detail. In addition, submeniscus velocity measurements in the slab caster mould are performed with the method of torque measurement. A consumable probe is inserted into the liquid steel meniscus from the top of the mould through mould powder and slag layer. The liquid steel flow applies a drag force to the probe, which then generates a torque. This torque value is measured and then converted back to velocity. The concept and challenges of the technique are discussed, and the effects of casting parameters on mould flow structure are investigated. Product quality in relation to real time meniscus velocity measurements is also discussed.     

Transient fluid flow and superheat transport in continuous casting of steel slabs

The turbulent flow of molten steel and the superheat transport in the mold region of a continuous caster of thin steel slabs are investigated with transient large-eddy simulations and plant experiments. The predicted fluid velocities matched measurements taken from dye-injection experiments on full-scale water models of the process. The corresponding predicted temperatures matched measurements by thermocouples lowered into the molten steel during continuous casting. The classic double-roll flow pattern is confirmed for this 132×984 mm slab caster at a 1.52 m/min casting speed, with about 85 pct of the single-phase flow leaving the two side ports of the three-port nozzle. The temperature in the top portion of the molten pool dropped to about 30 pct of the superheat-temperature difference entering the mold of 58 °C. About 12 pct of the superheat is extracted at the narrow face, where the peak heat flux averages almost 750 kW/m² and the instantaneous peaks exceed 1500 kW/m². Two-thirds of the superheat is removed in the mold. The jets exiting the nozzle ports exhibit chaotic variations, producing temperature fluctuations in the upper liquid pool of ±4 °C and peak heat-flux variations of±350 kW/m². Employing a static-k subgrid-scale (SGS) model into the three-dimensional (3-D) finite-volume code had little effect on the solution.     

Effects of Basicity and B2O3 on the Crystallization and Heat Transfer Behaviors of Low Fluorine Mold Flux for Casting Medium Carbon Steels

An investigation was carried out to study the effects of basicity (CaO/Si₂O) and B₂O₃ on the crystallization and heat transfer behaviors of low fluorine mold flux for casting medium carbon steels. The double hot thermocouple technique (DHTT) was employed to study the crystallization behavior of mold flux with a different basicity and B₂O₃ content, under the simulated thermal gradient as in a real caster. The infrared emitter technique (IET) was also applied for the study of heat transfer behavior of the above mold fluxes. By combining the results of IET and DHTT, this article indicated that the increase of basicity would decrease the general heat transfer rate of mold flux, as it tends to promote crystallization of mold flux apparently, while B₂O₃ has the opposite function. The combined effects of basicity and B₂O₃ could be used to adjust the general crystallization and heat transfer properties of low fluorine mold flux for casting medium carbon steels, which would provide an instructive way for the design of Fluorine free mold flux for casting medium carbon steels.     

Numerical simulation of solidification in an aluminum casting

This work describes the computer simulation of the solidification pattern in an aluminum casting. The Finite Element Method (FEM), adopting nonmoving mesh, was used to calculate the transient thermal field during phase transition. Integral averaging techniques were employed to calculate both the nonlinear heat conductionK(T) and specific heatC _p (T) functions for the elements of the domain; a temperature dependent functionC _p (T) accounts for the energy of the phase change. A complete computer program, with pre- and postprocessors, for Computer Aided Design of castings is also presented. Anad hoc experiment has been performed to verify the accuracy of numerical results.     

Solidification modelling of microstructure in twin-roller thin strip casting of stainless steels

The influence of process parameters on the dendritic microstructure of thin strips cast by the twin-roll method is analyzed in the framework of a one-dimensional solidification model and compared with experimental results. As a relevant characteristic the secondary dendrite arm spacing Λ₂ as a function of the distance x from the roll surface is investigated. The difference between the local dendrite arm spacing near the strip surface and the strip centre, respectively, increases with the strip thickness and only depends on the casting temperature to a small extent. An increase in the strip/roller heat transfer coefficient due to a rising casting velocity or possibly enhanced roll-separating forces leads to a decrease in the dendrite arm spacing. The effect of a sudden decrease in heat transfer during the solidification process, on the Λ₂(x) characteristics, e.g. by a local separation of the solidified shell from the roller surface, is discussed.     

Simulation of heat transfer and strain behaviour in bloom casting mould

Abstract

An optimum casting model was developed to simulate the effect of mould flux on bloom heat transfer and strain behaviour based on a 3D MiLE method, and the influence of casting speed and superheat on bloom heat transfer and lubrication were also investigated. The simulation results showed that solidified shell thickness growth conforms to a Square Root Law, and that the model predicted results are basically in agreement with previous data in the literature, and provide confidence in model. The bloom temperature distribution range in the corner area is smaller than that in the mid-face, and the corner regions form a high cracking risk zone. The hot tearing indicator and effective stress in the corner area are significantly greater than that in the mid-face, so the corner area is the dangerous zone of cracking; The mould flux lubrication in the bloom mid-face is better than for the bloom corner region, due to a higher shell temperature and a fluid slag; The increasing of casting speed can delay air gap formation of the bloom corner area, improving the lubrication conditions, but when the casting speed is changed, it is also necessary for the mould flux viscosity and crystallization temperature be changed also. Increasing the superheat has little influence on the completely solidified distance of liquid flux in the bloom corner area.     

Comparison of four methods to evaluate flow fields in CSP casting mould

Abstract

Commercial flow analysis tools are widely used in industry, but their capacity, efficiency and accuracy to solve complex flows are not so clear. In the present paper, first, three-dimensional computations are performed to calculate the flow field in the mould region of compact strip production (CSP) casting using a conventional finite volume method program, CFX4, with laminar model, standard k–? model, renormalisation group k–? model and low Reynolds number k–? model with default model parameters for handling turbulence. Next, these were compared with the flow pattern in a 1 : 1 scale water model by other investigations and quantitative comparisons were made for the flow velocities. Finally, the four numerical simulation models were evaluated in detail. Each method has its own merits and disadvantages, so model verification is always necessary for a highly accurate numerical analysis of complex flows. Under the situation of lacking experimental data, the standard k–? may be a good choice to evaluate liquid steel in a CSP mould.     

Influence of oxide scales on heat transfer in secondary cooling zones in the continuous casting process,part 1: heat transfer through hot-oxidized steel surfaces cooled by spray-water

For the cooling of steels in the continuous casting process it is necessary to know the heat transfer from the solidifying strand to the cooling water to enable calculation of the secondary cooling zone. Previous investigations have only determined this variable for non-oxidizing metallic surfaces. For many steels cast in practice, however, the formation of oxide layers prevents a direct transfer of the previous results. In the present research the influence of the oxide layers on the heat transfer has been investigated for spay-water cooling. Results have shown that heat transfer in the range of stable film boiling is determined for a constant spray-water temperature in the same way as for non-oxidizing metals, i.e. using the water mass flux density ·_s only. The changed surface qualities resulting from the oxide formation cause the Leidenfrost temperature, however, to shift considerably to higher values.     

Analysis of variability and non-uniformity of mould heat extraction for round billet continuous casting in plant trials

Abstract

Maintaining a stable and uniform heat transfer from steel shell to mould is important to produce high quality casting billet. In the present paper, a large amount of measured data of heat flux and temperature for round billet continuous casting mould from a plant trial has been analysed to shed light on the variability and non-uniformity of mould heat transfer around the perimeter. The results show that the variability and non-uniformity of heat extraction from the steel through the mould is affected slightly by operational parameters, such as pouring temperature, casting speed, meniscus, electromagnetic stirring current, but strongly by the steel carbon content and mould powder type. The installation of the mould in caster machine determines the magnitude of non-uniformity of heat transfer to a great extent. The relative root mean square (rRMS) of mould heat flux, presenting the variability and non-uniformity of mould heat transfer around the perimeter in transverse section, has wider range of variation and higher mean value compared with that of temperature. When the abnormality of heat transfer happens, such as deposit, the non-uniformity of mould heat transfer is also studied.     

Break temperatures of mould fluxes and their relevance to continuous casting

Abstract

The break temperatures of mould fluxes are important since they help to control the horizontal heat transfer and lubrication between the steel shell and the mould, and consequently affect occurrences of longitudinal cracking and sticker breakout in continuous casting. Break temperatures T _br have been determined for both steady state and dynamic measurement of the viscosity, and equations relating T _br to chemical composition have been obtained for both cases. It has been found that T _br can be affected by (i) cooling rates and (ii) fluorine losses during the measurements.