Investigation of mould thermal behaviour by means of mould instrumentation

Abstract

The thermal behaviour of the continuous casting mould has a critical influence on strand surface quality, casting productivity and operating safety. voestalpine Stahl GmbH has long been interested in the field of research into mould behaviour, and began with the investigation of heat flux in 1995, and then of thermal variability in 1999. Since 2000, an online mould thermal monitoring system for heat flux density, thermal variability and friction has been installed. The heat flow density in the mould is determined by measuring the inflow and outflow temperatures and the throughflow volume of the primary cooling water. Temperature as measured by a thermocouple based breakout detection system in the mould copper plate is used to investigate the thermal variability. These online measured values have been employed to examine the influence of casting parameters, steel analysis and casting powder on the heat flux and thermal variability, and the relationship between these variables and cracking. The knowledge gained through these wide ranging plant based investigations has been used as a major tool in the diagnosis of problems (such as breakout and sticking), for optimisation of the process, particularly in the field of casting powders, and for control of slab quality. In particular, this knowledge has been incorporated into online mould thermal monitoring.     

Mould thermal monitoring: a window on the mould

Abstract

Corus R,D&T at Teesside Technology Centre has developed over a number of years a mould thermal monitoring (MTM) system based on an array of thermocouples in the mould copper plates. The system is installed on the Corus slab casters in the UK, on slab casters at Outokumpu (UK), Sidmar (Belgium) and Kosice (Slovakia) and, at the time of the 4th European Continuous Casting Conference, was also on the medium thickness slab caster at Tuscaloosa (USA), which was sold to Nucor in 2004. The MTM system was also under development on the thin slab caster at Trico (USA) before plant closure (subsequently bought by Nucor), and aspects are currently being developed on the Corus DSP (direct sheet plant) thin slab caster at IJmuiden (The Netherlands). While a prime function is detection and prevention of sticker type breakouts, the MTM system allows real time assessment of thermal conditions, provides a valuable input for online grading and, most important, enables modifications to mould powder practices to be assessed. The present paper briefly outlines recent developments to the MTM system and presents examples of the use of the system to assist mould powder developments.     

Experimental investigation into radiative heat transfer characteristics for mould fluxes containing transition oxides

Abstract

Infrared transmittance of glassy and crystalline mould fluxes was measured using a Fourier transform infrared spectrometer at room temperature. Radiation heat transfer from the steel shell to the mould was calculated by a model. The results indicate that transition metal oxides MnO, FeO and TiO₂ have a marked negative effect on infrared transmittance and radiation heat flux of glassy samples. With MnO, FeO and TiO₂ added, the reductions of radiation heat flux in glassy samples are 19–25%, 34–36%, 6–29% respectively. X-ray diffraction results indicated that the crystalline phase in transition oxides free samples was mainly Ca₄Si₂O₇F₂. After transition oxides MnO, FeO and TiO₂ added, Mn₂SiO₄, Fe₂SiO₄, CaTiO₃, Ca₂SiO₄ and other minor phases were also precipitated in mould fluxes. On account of strong refraction and scattering, the negative effect on radiation heat flux in crystalline samples was much larger than that in the glassy ones.     

Combination of inverse problem and neural network for thermal behaviour calculation of mould process based on temperature measurements in plant trial

Abstract

Heat transfer between mould and strand has a critical influence on billet quality, caster productivity and operating safety. It is very important to obtain the correct distributions of temperature and heat flux, and many studies are made on the calculation methods of heat transfer between strand and mould, aiming to reduce the computation time and improve the calculation accuracy. In the present paper, based on measured data of temperature and heat flux during round billet continuous casting, the calculation method which combines the online measurement data and numerical simulation was investigated. Through identifying the local thermal resistance and its distribution between the mould and the strand by an inverse heat transfer model, the heat flux and shell thickness profiles were calculated. To avoid the iterative solution by inverse model, a faster alternative model using an artificial neural network was developed to predict the thermal resistance from the measured temperature. After training, there is an exact correspondence between the observed temperature values and the thermal resistance. The calculation results obtained by the combination of neural network and numerical simulation can correctly reflect the characteristics of non-uniform heat transfer around the mould circumference, which provides a worthwhile and applicable method for online calculation and visual technology of heat transfer and solidification in continuous casting mould.     

Modelling of steel shrinkage and optimisation of mould taper for high speed continuous casting

Abstract

In high speed continuous casting, optimisation of mould taper is key for intensifying heat transfer and for improving the quality of the cast products. Mathematical modelling has been carried out by combining heat transfer, steel shrinkage and parabolic continuous taper model in order to optimise the mould taper profile. These models have been assembled to a set of software, the inputs of which include the steel grade, casting speed, casting temperature, length and the cross-section of mould tube, while the outputs consist of surface temperature of the strand, thickness of solidified shell, thermal linear expansion coefficient, steel shrinkage, distortion of the mould tube wall, the actual air gap, total taper and the continuous taper profile. Optimum mould taper has a parabolic profile which is tapered inwards that changes continuously along the length of the strand in order to achieve reduction in air gap while avoiding distortion of the mould tube.     

Study on cascaded whole-leaf spring oscillation mechanism for mould in continuous casting

Abstract

A design method of a cascaded whole-leaf spring mechanism is proposed, which is a new oscillation guidance device for the mould in continuous casting. Then its prototype designed in this paper is produced in the lab, of which kinematics and dynamics simulations are carried out based on the rigid–flexible coupling virtual model. Simulation curves of the displacement and velocity of the mould are almost consistent with the ideal ones, which verifies the model built in this paper is rational. Furthermore, natural frequencies and mode shapes of the mechanism are calculated by dynamics simulation, and forces applied on leaf springs and revolute joints are analysed and effects of the basic parameters on these forces are also studied, which establish the basis for further studies and next application of this mechanism.     

Dynamic pressure in mould flux channel during mould non-sinusoidal oscillation

Abstract

Based on the lubrication theory of mould flux, a mathematical model of dynamic pressure in mould flux channel was developed, and the distribution of dynamic pressure and its variation during non-sinusoidal oscillation were investigated. The effects of casting speed and non-sinusoidal oscillation parameters, including the degree of non-sinusoidal operation (non-sinusoidal factor), amplitude and frequency of oscillation on the dynamic pressure in the mould flux channel, were studied. The results indicate that the maximum negative pressure is decreased, and the maximum positive pressure is increased with increasing non-sinusoidal factor. The optimum value of non-sinusoidal factor is ～0·2. With increasing amplitude and frequency of oscillation, both the negative and positive pressure are increased; moreover, the increment of positive pressure is obviously greater than that of negative pressure; especially when the oscillation frequency is increased, the increment of negative pressure is very little. When the casting speed is enhanced, the negative pressure is increased, but the positive pressure is decreased. Therefore, if the casting speed is increased, the oscillation amplitude needs to be increased, as well as the oscillation frequency needs to be decreased properly. With these adjustments, the positive pressure in mould flux channel is nearly unchanged. The actions of strand demoulding and cracks welding are kept effective. Moreover, the negative pressure in mould flux channel is increased properly, which causes the flux consumption to increase, so the mould lubrication is improved. Finally, the strand surface quality is improved greatly, and breakout can be avoided. The applicability of the optimised non-sinusoidal oscillation parameters for the two kinds of casting speed has been proven by industrial practice.     

Dissolution of alumina particles in CaO-Al2O3-SiO2-MgO slags

Abstract

The dissolution behaviour of alumina particles in CaO-Al₂O₃-SiO₂-MgO slags was studied by confocal scanning laser microscopy. Three different slag systems were investigated to evaluate the effect of changing the MgO content in a high silica tundish slag, and the effect of decreasing the silica content when dissolving the particles in a ladle slag. A particle of 80 mum diameter dissolved in 50 s in a simulated ladle slag but needed 100 s to dissolve in a high silica slag at the same temperature. When the silica content was decreased, a decrease in the alumina particle dissolution time was noted. During dissolution of alumina particles in a slag containing MgO, the alumina particle reacts with the slag to form an MgAlO₄ layer. No reaction layer was observed during the dissolution of alumina particles in slags that did not contain MgO. This work suggests that the dissolution time of large alumina particles is significant, increasing with particle size and with decreased temperature.     

Viscosity effect of titanium pickup by mould fluxes for stainless steel

Abstract

Mould flux samples from an operating continuous caster showed a steady state increase in titanium oxide content of 3–4% for a titanium stabilised ferritic stainless steel and about 6% for a titanium stabilised austenitic stainless steel. Owing to the reduction of silica the flux basicity increased from 0.8 to 0.9, and from 0.95 to 1.2, for the ferritic and austenitic steels, respectively. Similar changes in composition were found for mould shims, which had been recovered from the mould after casting the ferritic steel. The increased titanium oxide content caused a decrease in the flux viscosity, for the original basicity. However, when the flux basicity was increased from 0.95 to 1.2, the apparent viscosity of the mould flux of the austenitic steel increased markedly, for 10% TiO₂ and temperatures of 1250°C and below. Precipitation of perovskite was found to be responsible for the higher apparent viscosity.     

Water modelling of level fluctuation in thin slab continuous casting mould

Abstract

A water modelling experiment was conducted to study the meniscus instability in a continuous thin slab casting mould using particle image visualisation. The results show that the level fluctuation, circulation centre position and jet impinging depth are unsteady and periodic with a similar period. The probability distributions of the fluctuating meniscus and wave height have been obtained with the highest frequency near the average position. The flow pattern and meniscus profile may be momentarily asymmetrical, and the phase difference of level fluctuation in the two sides of mould centreline is a half period. The average meniscus profile, the highest and lowest meniscus positions are generally symmetrical about the mould centreline. The wave height mainly depends on the jet impinging depth and circulation centre position. The wave height increases as the jet impinging position rises and the circulation centre approaches to the submerged entry nozzle.     

Effect of interphase lift force on fluid flow in air stirred cylindrical vessel

Abstract

In the present paper, based on the two-phase model (Eulerian model), the two-dimensional fluid flow in air stirred water systems is simulated, and the effect of interphase lift force on the fluid flow is specially discussed. In the Eulerian two-phase model, the gas and liquid phases are considered to be two different continuous fluids interacting with each other through the finite interphase areas. The exchange between the phases is represented by source terms in conservation equations. Turbulence is assumed to be a property of the liquid phase. The k–? model is used to describe the behaviour of the liquid phase. The dispersion of phases due to turbulence is represented by introducing a diffusion term into the mass conservation equation. The contribution of bubble movement to the turbulent energy and its dissipation rate are taken into account by adding extra volumetric source terms to the equations of turbulent energy and its dissipation rate. Comparison between the mathematical simulation and experimental data indicates that the interphase lift force has a strong effect on flow behaviour, and considering both drag force and lift force as interphase forces is important to accurately simulate the gas–water two-phase fluid flow in air stirred systems. The interphase lift force makes bubbles move away from the centreline; the gas concentration decreases near the centreline, and increases near the wall. The lift force is smaller than the drag force at the same place, especially far away from the centreline.     

Process simulation of powder injection moulding: identification of significant parameters during mould filling phase

Abstract

Simulating and optimising the powder injection process are complex problems since a number of linked material, geometry and process variables have to be considered. In addition, it is very difficult to identify critical parameters for designing binder systems, feedstocks, parts, moulds and processing conditions owing to the fact that multiple objective functions have to be considered. Towards the goal of identifying the level of significance of various material, process and geometry parameters during powder injection moulding, a systematic procedure for sensitivity analysis has been successfully developed for the mould filling phase of the PIM process. In this sensitivity analysis, all input parameters were defined for the mould filling simulation and all output parameters for optimum design of part, mould and processing conditions and dimensionless sensitivity values for all input and output parameters were calculated, which allow parameters with different units to be compared quantitatively. The sensitivity analysis procedure developed will be an invaluable tool for both the design engineer in the PIM industry who has to determine the critical input parameters for given design targets, as well as for the production engineer who has to optimise and monitor the production stage.     

Flow pattern and alloy dissolution during tapping of steel furnaces

Abstract

Alloying of steel during tapping from BOFs and EAFs has been studied by computational fluid dynamics in two-dimensional axisymmetric models of two ladles. The flow patterns and particle trajectories have been computed for six different levels of steel in the tapping ladle, five different alloy sizes, two alloy injection points, and three types of bulk alloy (FeMn, SiMn, and FeSi75). Based on the fluid dynamics in the ladle and a definition of good alloying practice, conclusions with regard to alloy sizing and timing of alloy addition have been established. The computational results support findings in plant tests, which show the benefit of using small sized alloys. Furthermore, a method that allows us to estimate the optimum feeding rate for alloy additions during steelplant operation has been developed. Results from full scale tests in steelplants are shown.     

Mould lubrication and friction behaviour with hydraulic oscillators in slab continuous casting

Abstract

It is very important to obtain reliable lubrication from casting powder both at the meniscus and in the gap between strand and mould as it affects slab surface quality and caster productivity. With knowledge of mould friction, a quantitative insight into the behaviour of powder during caster operation is possible. In the present research, the friction was studied based on a slab continuous caster equipped with hydraulic oscillators. The effects of mould oscillation and the abrupt change of casting speed on mould friction force were evaluated, and the characteristics of lubrication behaviour in a casting sequence were investigated. In particular, a comparison between the mould friction force between sinusoidal oscillation mode and non-sinusoidal oscillation mode was made. Finally, the characteristics of friction before a breakout are discussed. The experimental and analytical results may contribute to the development of mould friction online measurement and more clearly learn the lubrication behaviour in different conditions.     

Investigation of strand surface defects using mould instrumentation and modelling

Abstract

The surfaces of continuously cast steel blooms exhibit a variety of surface features and defects, which were investigated to reveal the interactions at the meniscus between the steel shell and interfacial flux layers that caused them. One such defect formed at periodic intervals along the surface of first and second blooms in a sequence. It was characterised by gradually deepening oscillation marks, followed immediately by longitudinal striations or 'glaciation marks'. In severe cases, deep depressions were clearly visible within the glaciated region. These defects were investigated through plant trials and both physical and mathematical modelling. The defects were found to exhibit a characteristic temperature history: temperature troughs that move down the mould at the casting speed. These defects may be monitored in much the same way as sticker breakouts, thereby allowing existing thermocouple based breakout detection systems to be modified to include a quality alarm. This study attributes these defects to high amplitude, low frequency, mould level fluctuations. A mechanism is proposed which ascribes the generation of these defects to the interaction of the meniscus with the slag rim at peaks in the mould level cycle. Installing an improved mould level control system eliminated the defects.     

Behaviour of inclusions in RH vacuum degasser

Abstract

Vacuum degassing processes are now an integral part of secondary steelmaking operations and besides the more obvious benefits of steel chemistry control to precise specifications, they also reduce the number of non-metallic inclusions. Care has to be taken, however, that the change in steel composition through loss of C, O, Mn, etc. does not result in a deleterious change in the composition of the remaining inclusions. Attempts in the past to determine the effect of vacuum on inclusion compositions have been through the use of thermodynamic models, following the inclusion engineering approach. The calculated inclusion compositions do not, however, compare well with the inclusions as analysed in samples taken from the liquid steel after the degassing operation. Clearly, it is important to take into account time dependent effects during degassing and this has been achieved by the development of a combined fluid flow-thermodynamic model. Using the computational fluid dynamics model, CFX, to establish the temperature, flow and species contours in a two-dimensional steel ladle under the influence of natural convection, the results are transferred as start conditions in a three-dimensional RH degasser model. A body force is then applied to simulate the argon bubbles that are injected into the up-leg of the degasser and changes in flow, temperature and species concentrations are calculated. Allowing for additions made during the process, the composition of the top slag and any local inclusions within the steel is predicted. The influence on top slag and inclusion chemistry of any glaze on the snorkels of the degasser is also taken into account.     

Design of corners of mould in square billet casting

Abstract

In continuous casting, heat flow optimisation in the mould is key to improving the quality of the product and production savings. The heat flow influences, and is influenced by, several phenomena of a mechanical or metallurgical nature, so its optimisation should include these. In particular, shrinkage of the strand and solid phase formation are among the most influencing factors affecting the cooling of the solidifying product. The present paper describes a model implemented by a software tool that can carry out simulation of shell formation of carbon steel within the mould, for rectangular shapes. The first aim of the software is to simulate formation of the solid shell in the strand and the deformations to which this solid is subjected. Deformations are a result of both thermal shrinkage, related to phase changes, and stresses caused by metallostatic pressure or the mould-shell interaction. The output of the model consists of temperature maps of the strand, maps of formation of the shell and the ideal mould contour.     

Form of fluorine in Na2O-NaF-SiO2 slags determined by infrared spectroscopy

Abstract

The form of fluorine in Na₂O-NaF-SiO₂ slags has been investigated using Fourier transform infrared spectroscopy. Additions of NaF shifted absorption peaks for stretching modes of Si O bonds to higher wavenumbers, where the basicity of (mol-%Na₂O)/ (mol-%SiO₂ ) was kept constant, and the shift became larger with an increase in the concentration of NaF. This shift can be explained by the change from Si-O to Si-F bonds, with reference to harmonic oscillation theory of diatomic molecules. It is likely that added NaF depolymerises the silicate network to form Si-F bonds in Na₂O-NaF-SiO₂ slags.     

Slag entraining vortexing funnel formation during ladle teeming: similarity criteria and scale-up relationships

Abstract

There are several motivations for minimising slag entrainment during the teeming of steelmaking ladles. Cleaner steel, improved yield, and higher productivity are all at stake. As one of several identifiable contributors to slag entrainment, vortexing has received considerable attention in the past decade and a half. What is commonly referred to as 'vortexing' in fact comprises two distinct phenomena, namely, vortexing funnels and non-vortexing funnels, each controlled by entirely different sets of variables. Dimensionless correlations describing the two phenomena were determined, and validated, using separate sets of dimensional analyses and appropriately designed scale model experiments. The importance of these findings to the teeming of steel is discussed. Performance results of a patented 'vortex buster' device, developed on the basis of the understanding gained from these studies, and validated in water models as well as in a 12 ton tundish, are also presented.     

Thermal behaviour of moulds with different water channels and their influence on quality in continuous casting of beam blanks

Abstract

Two-dimensional finite element heat transfer models have been developed to predict temperature distribution in beam blank moulds with large and small hole water cooling channels. The effects of water channel design and grinding thickness on transverse temperature profile in meniscus region were analysed in detail. The effects of both moulds during plant trials are also compared. The results show that the peak temperature is found in the fillet area of the large hole mould and is 20°C higher than that of the small hole mould. With increasing grinding thickness, peak temperature in both moulds decreases linearly, and when the grinding thickness reaches 9 mm, the peak temperature of the small hole mould exceeds that of the large hole mould. The transverse temperature uniformity of the hole mould is superior to that of the large hole mould. It is also found that longer mould life, better strand surface quality and more homogeneous surface microstructure are obtained when using the small hole mould.