High Speed Continuous Casting Research Articles

Design and heat transfer numerical analysis of high-speed continuous casting mold water slot system

The water slot structure can reduce the temperature of the mold and increase the heat transfer between the mold and cooling water. It has significant impacts on high-speed continuous casting. Therefore, a three-dimensional model of heat transfer from mold to cooling water was used to analyze the influence of water slot size on the cooling capacity of a single water slot. Then, the fluid flow, heat transfer, and solidification three-dimensional models were used to analyze the temperature distribution and cooling effect of three kinds of water slot molds and non-water slot molds. Finally, the water slot mold was designed to achieve high-speed continuous casting. The results show that to make full use of the cooling water in the water slot, the width and height of a single water slot should be less than 8 mm. Compared with the conventional mold without the water slot structure, the temperature of the mold with the water slot structure significantly decreases. The final design of the mold water slot system is as follows: the water slot width and depth are 8 mm, and the number of water slots on one side of the mold is 9. The water slots have a symmetrical distribution, and the symmetrical plane is the middle plane of the mold. The high-speed continuous casting experiment was carried out, and the casting speed can reach 4–4.5 m/min. The water slot mold in this study can be applied in the high-speed continuous casting process.

The Effect of Mold Structure and Cooling Parameters on Heat Transfer during Billet High-Speed Continuous Casting.

Mold structure and cooling parameters are significant factors that affect the heat transfer capacity of high-speed continuous casting molds of billets. Therefore, a three-dimensional fluid flow and heat transfer model of a 160 mm × 160 mm billet mold was established, and its accuracy was verified. Thereby, the characteristics of heat transfer and influences of mold structure and cooling parameters on heat transfer in the high-speed continuous casting billet mold region were revealed. It was found that extending the effective length of a mold is the most valuable method to improve its heat transfer capability and achieve high-speed continuous casting. The total heat and the shell thickness at a mold outlet increased by 19% and 9.21% on average with every 100 mm extension. Enlarging the fillet radius could enhance the uniformity of heat transfer in the mold. Considering the loss of material, the optimal fillet radius of the mold was determined to be R = 10 mm.

Analysis of Uneven Wear Mechanism of Narrow-Face Copper Wall of Funnel Mold

In thin-slab continuous casting, due to the influence of the special shape of the funnel mold, cracks at the corner of the slab shell are more likely to occur than those in conventional slab shells, and a serious wear phenomenon also appears on the narrow face of the copper wall of the mold. Aimed at the corner cracks of thin slabs and the wear phenomenon of the copper wall, a new 3D stress analysis model in a funnel mold has been developed to simulate the stress-–train behavior of the slab shell under high-speed continuous casting. The results show that at the position 600 mm below the meniscus, the gap begins to appear at the corner of the slab; the maximum value of the first principal stress appears at the corner. The shell is squeezed by the copper wall during the downward movement in the funnel mold, and the slab shell in the funnel area moves towards the narrow face. The displacement causes the deformation of the slab shell to extend to the corners, the deformed shell is pressed against the corner of the copper wall. A new type of copper wall was designed for production, and it was found that the cracks at the corner of the slab shell were greatly reduced.

THE PRACTICE AND APPLICATIONS OF PHYSICAL MODELLING IN FLOW DYNAMICS RESEARCH IN CONTINUOUS CASTING

The importance of physical modeling as one of the effective methods of scientific research is noted. The practical features of modeling the hydrodynamics of flows in continuous casting of steel are considered, concerning the scale, design and material of equipment models that are part of continuous casting machines. Attention is paid to the choice of modeling fluids, methods of visualization and control of their flow rates. Methods and results of modeling studies are presented, devoted to the development of technical means for shielding a steel jet from the surrounding atmosphere in order to provide protection against secondary oxidation of the metal supplied to the tundish; regulation of the direction of movement of the layers of the melt from the place of entry into the receiving part and to the outlet channels of the tundish to increase the duration of the steel casting in it and remove the non-metallic inclusions; decrease in the degree of dynamic impact of metal flows on the lining of the tundish bottom; reducing the duration of the destabilization of the velocity field of metal flows in the upper part of the mold, which occurs during the replacement of submersible nozzles. The obtained data of modeling studies served as the basis for creation of new types of equipment that allow the implementation of technological operations, the implementation of which will increase the technical and economic efficiency of modern high-speed continuous casting machines.

Study on Parameter Optimization of Diversion Wall in an Eight-Strand Tundish during Continuous Casting of Billet with High Casting Speed

With the increasing demand for high-efficient continuous casting, parameter optimization during high-speed continuous casting is critical. To clarify the changes in flow characteristics in a multistrand tundish and the optimization principles for the diversion wall, a numerical investigation of an eight-strand tundish during continuous casting of billet was carried out in this paper. The simulation results were validated with the physical results of a 1:3 water model experiment. The results show that, for a tundish with the same flow control device, the average residence time and the maximum residence time difference of liquid steel in different strands are significantly reduced with higher casting speed. At different casting speeds, the effect of the hole diameter and deflection angle of diversion wall on the average residence time and the dead region proportion is very minor, while that on the maximum residence time difference of liquid steel in different strands is significant. For a given tundish, to improve the flow uniformity among multiple strands, parameter optimization of diversion wall should be optimized when the casting speed increases. When the casting speed is 4.4 m/min, the hole diameter of the diversion wall is 80 mm, and the deflection angle of the diversion wall is 74°, the flow field parameters of liquid steel in the eight-strand tundish are good, especially flow uniformity among multiple strands.

Analysis of the Kinetics of Devitrification and Crystallization of a Melilite Mold Powder Slag for Medium Carbon Steel Billet Casting

High-speed continuous casting of steel billets entails considerable turbulence in the liquid core, which results in significant slag entrapment unless high viscosity slags are used. It has been suggested that such slags remain glassy or crystallize slightly under the prevailing mold cooling, temperature and residence time conditions. Slags whose composition lead to formation of melilite minerals (i.e., to formation of calcium sodium magnesium aluminum iron disilicates), as the main crystalline phase, possess viscosities > 0.5 Pa/s at 1573 K (1300 °C), are low or do not contain F, and incorporate transition metal oxides (e.g., FeO, MnO and TiO) to absorb infrared radiation and soften steel shell-to-mold heat transfer. In this work, a commercial melilite mold powder, for casting medium carbon steels round billets, is selected to carry out detailed analysis of the kinetics of precipitation of crystalline phases from glassy—devitrification—and super-cooled liquid slags—crystallization—under non-isothermal and isothermal conditions. High-temperature confocal laser scanning microscopy is used in both cases and differential scanning calorimetry just in non-isothermal ones. Assessment of amorphous and crystalline phases in treated samples is done by quantitative X-ray powder diffraction analysis. It is found that even after prolonged treatment (» 36,000 seconds) at 1248 K (975 °C) approximately 13 wt pct of the slag remains amorphous. Additionally, the results indicate that nucleation of crystalline phases in super-cooled liquid and glassy slags occurs on the surface. Thus, it is found that crystallization kinetics is strongly influenced by the topography of the surface with which the super-cooled liquid is in contact, as well as, by shearing actions imposed on the liquid slag surface, which contribute by developing nucleation sites. Devitrification tends to be stronger on surfaces contacting foreign walls and over cracks. Moreover, it is found that predictions of the kinetic model, developed for estimating time–temperature–transformation diagrams from non-isothermal differential scanning calorimetry data, portray reasonably well experimental results of crystallization, as well as devitrification of consolidated samples.

Role of B2O3 on structure and shear-thinning property in CaO–SiO2–Na2O-based mold fluxes

Abstract Most traditional mold fluxes are Newtonian fluids, and their constant viscosity has certain limitations in continuous casting. A new non-Newtonian fluid mold flux with shear-thinning behavior, i.e., a mold flux with a relatively high viscosity at lower shear rates and a relatively low viscosity at higher shear rates, is required to satisfy the mold-flux performance requirements for high-speed continuous casting. The addition of a certain amount of B2O3 to a CaO–SiO2–Na2O-based mold flux can result in a shear-thinning property. To obtain an improved understanding of the mechanism of this characteristic, a molecular-dynamics simulation method was used to study the microstructural changes of the mold flux. Structural changes of mold-flux samples were analyzed and verified by Raman spectroscopy. The results of the two methods were almost the same, both resulted from the addition of B2O3 and changed the microstructure and degree of polymerization of the mold flux, which resulted in the shear-thinning property of the mold flux. This non-Newtonian fluid mold flux was used in square-billet casting tests, and the quality of the slab was improved effectively.

Evaluation of Solidified Shell Thickness by Thermocouple in Mold

High speed continuous casting technology has been developed to improve productivity. The method of real-time calculation of the solidified shell thickness by using multiple thermocouples embedded in the mold copper plates was tested at JFE Steel Kurashiki No.4CCM, and the possibility of detecting various types of breakouts in high-speed casting was demonstrated. The main results are summarized as follows.

Shear-thinning behavior of the CaO–SiO2–CaF2–Si3N4 system mold flux and its practical application

Satisfying the mold-flux performance requirements for high-speed continuous casting necessitates the development of a new non-Newtonian-fluid mold flux with shear-thinning behavior, i.e., a mold flux whose viscosity is relatively high under lower shear rates and relatively low under higher shear rates. In this work, a mold flux that exhibits shear-thinning behavior was developed by adding different amounts of Si3N4 to the CaO–SiO2–CaF2 mold flux. The shear-thinning behavior was investigated using a rotational viscometer. In addition, the microstructure of the newly prepared slags was studied by high-temperature Raman spectroscopy and X-ray photoelectron spectroscopy. The results showed that the mechanism of shear-thinning was attributable to a temporary viscosity loss caused by the one-way shear stress, whereas the corresponding magnitude of shear-thinning was closely related to the degree of polymerization (DP). Finally, the non-Newtonian fluid mold flux was used for laboratory casting tests, which revealed that the mold flux could reduce slag entrapment and positively affect the continuous casting optimization.

Recovery mechanism of the sticking-type breakout during continuous casting of steel

ABSTRACTSticking-type breakout is one of key factors which restrict the development of high-speed or wide-heavy plate continuous casting technology. In order to better heal and control the sticking-type breakout, this paper presents a comprehensive study for its recovery mechanism. First, according to force characteristics of solidified shell at mold exit, calculation method of safe shell thickness is proposed. Then, considering sticker propagation behaviour and safe shell thickness, a more practical recovery model of sticking-type breakout is established and verified by actual examples. The result shows the proposed recovery strategies avoid casting speed to be decreased too low or cast stop, thus save unnecessary operating time, and are better than original strategies. Furthermore, influencing factors of recovery behaviour are analyzed. The result shows hot spot location, operating casting speed, and mold oscillation frequency have a significant effect on minimum speed for the recovery and oscillation stoke has little influence on it.

Effect of flow control mold on flow field during high-speed continuous casting

An experimental mold was built to study the flow in a flow control mold under high speed continuous casting. The effect of the magnetic field on the flow was investigated using mercury. The results show that the magnetic field can not only dampen the flow of liquid metal but also change its direction, and then redistribute the flow in the mold. When maintaining a constant distance between magnets, the fluctuation of the free surface is dampened because of the increasing magnetic flux density. The flow at the free surface is improved, and the penetration depth of the downward stream is reduced. The decrease in the distance between magnets promotes the brake effect and the flow is dampened in the upper eddy.

Understanding mould powders for high-speed casting

The development and application of mould powder for high-speed continuous casting of steel is described. For thin slab casting, the main requirements are proper powder melting, undisturbed slag infiltration, adequate strand lubrication and the control of mould heat transfer. For increased casting speeds i.e. up to 8 m/min, slag infiltration and in particular the control of mould heat transfer via crystallisation of the slag film becomes even more important. It was found that a low powder consumption and hence a thin slag film is no restriction for an undisturbed thin slab casting process. Given a stable casting process and machine condition, the mould powder properties are not as critical as widely assumed.

In-depth study of mold heat transfer for the high speed continuous casting process

Mold heat transfer during the commercial high speed continuous casting up to 7 m/min was investigated in order to clarify the influence of various operating conditions such as casting speed, mold flux, mold thickness, thickness and height of mold coated layer and so on. A simple, but practical formula of heat flux has been derived in terms of those operating conditions by analyzing the heat flux data obtained in CEM® (Compact Endless Casting and Rolling Mill) caster based on simplified one dimensional heat transfer model. Especially, impact of mold parameters such as mold thickness, mold coated layer thickness and its height on the heat flux can be linearly expressed in the empirical formula derived. Heat flux ratio (HR), the ratio of the narrow face heat flux to the wide face one, is one of the important indicators to evaluate whether the solidified shell is evenly robust or not. Averaged HR in CEM® caster is around 0.87, which varies according to the caster specifications and operating conditions. It is suggested that the mold taper should be adjusted to maintain the HR as close to 0.87 as possible.

Study of Molten Steel Flow and Inclusions Motion Behavior in the Solidification Processes for High Speed Continuous Casting Slab by Numerical Simulation

A three-dimensional numerical simulation of transient fluid flow field during the solidification process for high speed continuous casting slab has been developed to predict the inclusions, captured at the solidification front with different casting speed and the distribution of the inclusions at the different location in the inner surface layer for high speed process. The motion behavior of over 50 μm non-metallic inclusions has been simulated and visually displayed. The results show that the casting speed has no significant influence on the inclusion size distribution and the total number and average diameter of the captured inclusions do not increase with the increase in the casing speed.

Novel mold breakout prediction and control technology in slab continuous casting

To realize smooth and high speed continuous casting, the urgent problem is to control the sticking-type breakout efficiently. In addition to improving casting process conditions of inducing stickers, the key is to develop active and accurate breakout prediction and healing control technology. In this paper, a novel logic judgment model for predicting stickers is presented, which is designed for more rows of thermocouples (TCs) in high density, and uses temperature change rate and vertical and horizontal detections to well recognize sticker propagation. Simulation results indicate that new model can accurately and timely detect all stickers, and has a good robustness. After sticking alarm, dynamic control strategies of casting speed according to the location of alarmed thermocouples are proposed for breakout prevention, by investigation on formation mechanism and recovery behavior of sticking-type breakout. The proposed model and control strategy have been validated through its application on a steel plant. The results show that the detection ratio for stickers and frequency of false alarm are 100% and 0.15% times/heat respectively, and the healing rate of the stickers has reached 100%. The case in which breakout still occurs after alarm has been eliminated. The present technology has achieved better performance in prediction and control of the sticking-type breakout and satisfies the requirement for field application.

Application of High Speed Continuous Casting on Low Carbon Conventional Slab in SGJT

Based on the achievement of high speed continuous casting in SGJT, fluid flow, heat transfer, and lubrication behavior were analyzed elaborately. The results indicated that well-bottom SEN with the angle of −20° together with FC mold could reduce the level fluctuation effectively. When FC mold was applied at 1.9 m min−1, there was an increase of average heat flux about 0.06–0.1 MW m−2. During heat transfer analysis, the empirical equation of heat flux was proposed and solidification crater end was validated by pin shooting method. Furthermore, a kind of mold powder employed in high speed casting was described and a new criterion of the consumption rate for high speed casting was put forward.

Experimentally-aided Simulation of Directional Solidification of Steel

Dendritic microstructures are most dominant patterns in solidified alloys. The microstructural features of these structures control the segregation profiles of solute elements in the interdendritic regions, thus determining the mechanical properties of cast structures. In this study, a 2D model of solid/liquid interface instability in a low carbon steel was introduced, using the multi-phase-field software code MICRESS® combined with an in-situ study of solidification in a laser-scanning confocal microscope. The use of a moving-frame boundary condition and a linear temperature gradient within the simulation allows further optimization of the solidification studies in the laser-scanning confocal microscope. By analysing the shape of the delta-ferrite grain boundary at the solid/liquid interface, in-situ and at temperature, it was possible to experimentally determine the Gibbs-Thomson coefficient and the solid/liquid interfacial energy of the alloy. The interface mobility of the solid/liquid interface was calibrated in the model so as to reproduce the experimentally measured interface velocity at the onset of interface instability. The proposed model was used to describe the morphological transitions from planar to cellular to dendritic modes during solidification and solute segregation under a variety of processing conditions such as cooling rate and temperature gradient. The importance of this approach is that the verified model has been used to extend the prediction of microstructural development to cooling rates well beyond what can be achieved experimentally and into the regime pertinent to high-speed continuous casting. Significant microstructural differences that arise as a result of varying processing conditions are discussed.

Optimization of Oscillation Model for Slab Continuous Casting Mould Based on Mould Friction Measurements in Plant Trial

Lubrication and friction between the mould and strand are strongly influenced by mould oscillation, and play an important role in slab quality and operating safety during continuous casting processes. Investigation of mould oscillation is therefore essential for getting a better online control of the mould processes. A feasible approach for the development and optimization of mould oscillation was put forward, which combined online measurement of mould friction, design of negative oscillating parameters and evaluation for powder consumption. Three different control models including sinusoidal and non-sinusoidal oscillation for mould oscillations were developed to investigate and evaluate the effects of oscillation on mould friction and powder lubrication. For the purpose of investigating mould friction between mould and strand, online measurement was carried out on a slab continuous caster equipped with a hydraulic oscillator. Also the comparison of the mould friction in sinusoidal and non-sinusoidal mould oscillation was made for subsequent analysis. The industrial experiment result shows that the combination of inverse control model and non-sinusoidal oscillation mode will contribute to the proper powder consumption, leading to a suitable effect of friction force on strand surface, especially for high speed continuous casting. The proposed method provides reliable basis for guiding and optimizing mould oscillation among control models, sinusoidal oscillation and non-sinusoidal oscillation.

Properties of High Basicity Mold Fluxes for Peritectic Steel Slab Casting

In high speed continuous casting of peritectic steel slabs, mold fluxes with high basicity are required for less surface defect product. However, the basicity of remaining liquid slag film tends to decrease in casting process because of the crystallization of 3CaO · 2SiO2 · CaF2. Thus, a way is put forward to improve mold fluxes properties by raising the original basicity. In order to confirm the possibility of this method, the effect of rising original basicity on the properties of mold fluxes is discussed. Properties of high fluorine based mold fluxes with different basicities and contents of CaF2, Na2O, and MgO were measured, respectively. Then, properties of higher basicity mold fluxes were discussed and compared with traditional ones. The results show that increasing the basicity index can improve the melting and flow property of mold fluxes. With the increasing basicity, crystallization rate of mold fluxes increases obviously and crystallization temperature tends to decrease when the basicity exceeds 1. 35. The method presented before is proved as a potential way to resolve the contradiction between horizontal heat transfer controlling and solidified shell lubricating for peritectic steel slab casting. But further study on improving the flow property of liquid slag is needed. This work can be used to guide mold fluxes design for high speed continuous casting of peritectic steel slabs.

Effects of Li<sub>2</sub>O on Properties of Low Fluoride Content Mould Flux for High Speed Continuous Casting

In order to reduce fluorine pollution, partial fluoride could be replaced by Li2O in mould flux. The effects of Li2O on properties of low fluoride content mould flux for high speed continuous casting were studied. The properties included melting temperature, viscosity, fluidity, surface tension and Al2O3 inclusion adsorption. The results showed that with the increase of Li2O content, melting temperature and viscosity of mould flux decreased obviously, fluidity increased, surface tension changed little, and Al2O3 inclusion adsorption of mould flux increased. In order to play an effective role, the appropriate Li2O content should be less than 4 per cent. The crystallization state of mould flux changed after slag adsorbed Al2O3. There were some small cuspidine and wollastonite crystals in the sample.