Continuous Casting Of Steel Research Articles

CURRENT PROBLEMS AND PERSPECTIVES OF COMPUTER SIMULATION OF CONTINUOUS STEEL CASTING

Nowadays we can see increase in using of engineering analysis systems in the field of continuous steel casting simulation due to their high accuracy and convergence with industrial experiments results. Such powerful systems as «ANSYS» and «ProCast» allows solving gas-, hydrodynamic and thermal problems, the parallel interaction of which constitutes the essence of most metallurgical processes. Group of authors from the LSTU Chair “Metallurgical technology” have successfully carried out computer experiments in tundish and continuous casting mold processes simulation. The experiments were aimed at first, on studying of further improving in liquid steel flow modifiers (partitions, turbo-stops, thresholds) design, at second, on influence of argon blowing regime on liquid steel flow parameters in the 50-ton tundish workspace, and, at third, on influence of submerged nozzles design on the liquid steel flow in crystallizer, assuming deterministic-dynamic operation mode. The results of calculations are velocity fields of liquid steel flow and flow temperatures fields in tundish and crystallizer, as well as temperature fields in the tundish refractory lining. An equation of primary flow average velocity change on exit from submerged nozzle was also formulated. These preliminary results allow us to assess the velocity changes and direction of the melt flow and formation of volumes with different melt temperatures when using flow modifiers in the ladle, including using of “argon curtain” in the casting chamber. Obtained data on melt flow motion and on location of erosion spots in crystallizing «crust», also on the presence of temperature gradient zones in various regions of crystallizer workspace may be useful to practice engineers engaged in choice of crystallizer submerged nozzle design. Effective control of melt flow in tundish and in crystallizer allows significant improvements in continuous cast slabs and rolled products quality in context of reducing metal products rejection due to defects associated with slag or nonmetallic inclusions presence and due to cracks formed as the result of insufficient thickness of crystallized «crust».

Mechanism of Floater Formation in the Mold during Continuous Casting of Ti-Stabilized Austenitic Stainless Steels

During the continuous casting (CC) of Ti-bearing steel, a steel lump can solidify in the mold (i.e., floater steel) more easily than in the Ti-free steels. This causes severe surface defects or even a breakout. We have examined the mechanisms of floater formation during the CC of 321 stainless steel by analyzing the inclusions in the floater steel and in the 321 steel that was sampled from the mold. Additionally, we calculated the disregistry between the metallic phases and common inclusions. The mineralogy and morphology of the inclusions were examined while using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and X-ray diffraction (XRD). Thermodynamic calculations on the TixOy inclusions at different oxygen potentials were performed while using FactSage 7.2. Using this approach, we determined that ferrite nucleates grow on TiN and MgO inclusions following solidification, which then form micro-aggregates as a result of dynamic collisions and alliances. Analysis of the mold slag from the metallurgy stage indicated that altering the basicity and properties of the mold flux systematically might minimize the reaction between the slag and steel, which would achieve a coordinated control over lubrication and heat transfer.

Comparison of Transverse Uniform and Non-Uniform Secondary Cooling Strategies on Heat Transfer and Solidification Structure of Continuous-Casting Billet

Water flux distribution largely influences the heat transfer and solidification of continuously-cast steel billets. In this paper, a secondary cooling strategy of transverse non-uniform water flux (i.e., higher flux density on billet center), was established and compared with the uniform cooling strategy using mathematical modeling. Specifically, a heat transfer model and a cellular automaton finite element coupling model were established to simulate the continuous casting of C80D steel billet. The water flux was measured using different nozzle configurations to assist the modeling. The mathematical results were validated by comparing the surface temperature and the solidification structure. It is shown that the non-uniform cooling strategy enables the increase of corner temperature and reduction in surface temperature difference, while a higher reheating rate is found on the surface center of the billet. Moreover, the non-uniform cooling strategy can enhance the cooling effect and refine the solidification structure. Accordingly, the liquid pool length is shortened, and the equiaxed crystal density is increased along with the decreased equiaxed crystal ratio. The uniform cooling strategy contributes to reducing internal cracks of billet, and the non-uniform one is beneficial for surface quality and central segregation. For C80D steel, the non-uniform cooling strategy outperforms the uniform one.

Online Modelling of Heat Transfer, Solidification and Microstructure in Continuous Casting of Steel

Advanced numerical simulation models for continuous casting of steel were developed in Finland by Casim Consulting and Aalto University in cooperation with the University of Oulu and the steel industry. The aim was to develop models that are scientifically rigorous, but also computationally fast enough to be used in online applications. The models developed are a transient three-dimensional heat transfer model, CastManager, and a solidification and microstructure model, IDS. The computing time of these models are short, and they are integrated together in one online concept. This concept is installed in the automation systems of four slab casters in Finland. Testing and validation work is in progress. The system simulates the important heat transfer, solidification and microstructural phenomena in continuous casting online. The future aim is that this information will be used for online quality control and for optimizing the process conditions to avoid formation of defects. Many quality indices have already been developed. A steady state version of the CastManager tool has also been developed, called Tempsimu.

Dendrite growth direction measurements: understanding the solute advancement in continuous casting of steel

Maintaining competitiveness in steel manufacturing requires improving process efficiency and production volume whilst enhancing product quality and performance. This is particularly challenging for producing value-added advanced steel grades such as advanced high strength steels and electrical steels. These grades due to higher weight percentage of alloying elements cause difficulties in various stages of upstream and downstream processing, and this includes continuous casting, wherein high solute levels are critical towards macro-segregation. Interface growth direction in systems with more than one component is dictated by the solute profile ahead of the moving solidification front. Understanding the profile of growth direction with casting process parameters during the progress of casting will provide an important perspective towards reducing the macro-segregation in the cast product. In the present study, two steel slab samples from conventional slab caster under the influence of electromagnetic brake (EMBR) at Tata Steel in IJmuiden (The Netherlands) have been investigated for dendrite deflection measurements. The samples showed a transition zone where a change in the deflection behavior occurs. Also, the magnitude of the deflection angle decreases away from the slab surface. Correlating these experimental data with modeled fluid flow profile will help in improving the understanding of the dynamic nature of the solute advancement so that the casting parameters can be optimized to improve product quality.

Optimization of Thermal Performance of CCM Mold

A physical simulation technique was used to study the processes of heat transfer between solidifying metal and CCM mold during continuous casting of steel. Variation trends have been established with respect to the amount of heat transferred from liquid metal to solidified crust, from billet surface to the mold tube, and from the mold tube to cooling water. Practical recommendations were suggested concerning an increase in heat transfer efficiency for the described stages of heat transfer from solidifying metal to cooling water.

Development of Manufacturing Technology for High-Sulfur Steel with a High Aluminum Content in the Volga Pipe Plant Electric-Melting Workshop

Continuous casting of steel with a high sulfur content is difficult due to clogging of the submerged nozzle. Physicochemical calculations and studies of a submerged nozzle inner cavity showed that clogging occurs due to formation of calcium sulfide and a low slag assimilation capacity. With serial melting of steel there is entry of slag into the tundish. Development of a slag forming mixture with whose application in the steel-pouring ladle there is formation of slag of this composition, which enters the tundish without worsening slag assimilation properties. Use of this development makes it possible to reduce submerged nozzle clogging and recycling of melts considerably due to stopping casting.

Research Status and Control Measures of Transverse Cracks for Micro-alloyed Continuous Casting Billet

The transverse cracks of the micro-alloyed steel continuous casting billet seriously damage the quality of the continuous casting billet and cause great losses to the enterprise. The influence of the hot ductility, microstructure, and micro-alloying elements on transverse crack was discussed, which has been researched in-depth at home and abroad. Through a great deal of studies, various control measures of transverse crack are proposed, such as secondary cooling technology, chamfering mold, refining the surface structure of the casting slab, and double phase transformation process. However, the transverse cracks have not been fundamentally solved. Therefore, the formation mechanism and control measures of the transverse crack are summarized for providing with the idea of eliminating transverse crack.

Effect of final electromagnetic stirring on solidification microstructure of GCr15 bearing steel in simulated continuous casting

The effect of final electromagnetic stirring on the solidification structure, element segregation, and carbides of GCr15 bearing steel was investigated. The experiment of continuous casting of the steel was carried out by using a continuous casting physical simulation apparatus with the final electromagnetic stirring. The liquid core profile was revealed by the pour-out method during the solidification process. Eight ingots were produced under different electromagnetic stirring parameters for comparison. The results show that, at the final stage of solidification, intense electromagnetic stirring exacerbates the central carbon segregation, resulting in more serious precipitation of the primary carbides. It is also found that the carbides in the segregation area are dominantly M3C type, together with a small amount of M7C3-type and M3C2-type carbides. In this physical simulation model, the optimal stirring parameters to obtain a fine and homogeneous structure are the liquid pool width of 15 mm, current intensity of 150 A, and stirring frequency of 5 Hz.

Features of determining the melting temperature of slag-forming mixtures for continuous casting of steel

A method for determining the melting temperature of the slag-forming mixture on a high-temperature microscope is described and its accuracy indicators are given. The main parameters affecting the results of determining the melting temperature of slag mixtures using the DSC method, and rational conditions for conducting experiments aimed at determining the liquefaction, melting and spreading temperatures of slag are given. Ill. 3. Ref. 27. Tab. 1.

Influence of differential roll rotation speed on evolution of internal porosity in continuous casting bloom during heavy reduction

Heavy reduction (HR), during which a large reduction deformation is implemented around the strand solidification end during continuous casting, is a novel technology that could effectively improve the internal quality of large section continuous casting steel. In the present work, a 3 dimensional (3D) thermal-mechanical coupled model with two pairs of rolls was developed. By setting different rotation speed of the front and the reduction rolls, the effect of differential roll rotation speed on the evolution of internal porosity in continuous casting bloom during HR was investigated. Initial temperature distribution of the 3D thermal-mechanical coupled model was determined by the predicted heat transfer results of a 2 dimensional (2D) heat transfer model. An Arrhenius-type constitutive equation, derived based on the measured true stress-strain curves under different temperatures and strain rates, was adopted in the 3D thermal-mechanical coupled model to describe the metal flow behavior of the casting strand during HR. Accuracy of the 3D thermal-mechanical coupled model was verified by the feedback pressure of the hydraulic cylinders during plant trial of HR. The simulation results indicate that a relatively larger rotation speed of the front rolls (Vf) than that of the reduction rolls (Vr) could effectively suppress the porosity elongation along the casting direction and the bloom width direction, and benefit the decrease of porosity dimension along the bloom thickness direction. As a result, the effect of HR on improving the bloom internal porosity could be effectively promoted. With HR amount of the reduction roll (Rr) decreased or the small reduction amount of the front rolls (rf) increased, the improvement effect of differential roll rotation speed on HR becomes more significant. With roll rotation speed ratio of Rv (Rv = Vf / Vr) increased from 1.0 to 2.0, efficiency of differential roll rotation speed on promoting the process effect of HR continuously decreased.

Investigation on the Slag-Steel Reaction of Mold Fluxes Used for Casting Al-TRIP Steel

The reaction between [Al] in molten steel and (SiO2) in the liquid slag layer was one of the restrictive factors in the quality control for high Al-TRIP steel continuous casting. In this work, the composition and property variations of two slags during a slag-steel reaction were analyzed. Accordingly, the crystalline morphologies of slag were discussed and the solid layer lubrication performance was evaluated by Jackson α factors. In addition, a simple kinetics equilibrium model was established to analyze the factors which affected SiO2 consumption. The results reflected that slag-steel reacted rapidly in the first 20 minutes, resulting in the variation of viscosity and the melting temperature of slags. The slag-steel reaction also affected the crystal morphology significantly. Slag was precipitated as crystals with a higher melting temperature, a higher Jackson α factor, and a rougher boundary with the consumption of SiO2 and the generation of Al2O3. In other words, although generated Al2O3 acted as a network modifier to decrease the viscosity of the liquid slag layer adjacent slab shell, the consumption of SiO2 led to the deterioration of the lubrication performance in the solid slag layer adjacent copper, which was detrimental to the quality control for high Al-TRIP steel. Finally, a kinetics equilibrium model indicated that it is possible to reduce a slag-steel reaction by adjusting factors, such as the diffusion coefficient k, cSiO2, ρf and Lf, during the actual continuous casting process.

Fluid Flow Control in a Billet Tundish during Steel Filling Operations

The startup of casting sequences in continuous casting of steel using three different turbulence inhibitors were modeled and simulated through the multiphase volume of fluid model (VOF) in a four-strand tundish. In the actual caster, one of the inhibitors released the liquid steel with a superheat high enough to avoid freezing problems in the outer strands. A second inhibitor improved the flow, yet it yielded steel freezing in these strands. A two-phase air–water system was used to model the liquid steel–air system and the interfaces were tracked by a donor–acceptor principle applied in the computational mesh. These activities led to the design of a third inhibitor. Experimental outcomes and the mathematical simulations agreed remarkably well regarding the velocity of the stream front in the tundish floor and the mass of steel reaching the outer strands. A larger steel mass and a faster stream front helped to completely prevent the steel from freezing in the outer strands. Finally, flow fields during the filling of the tundish using two of these inhibitors were simulated and the results explain the different performances observed experimentally.

Calculation Accuracy and Efficiency of a Transient Model for Submerged Entry Nozzle Clogging

A transient numerical model was proposed and validated by the current authors for nozzle clogging (Barati et al. in Powder Technol 329:181-98, 2018). The model can reproduce the experiment in pilot scale satisfactorily. In the present article, the main objective is to validate the model for application in industry process continuous casting of steel, referring to the model accuracy and calculation efficiency. The results have shown that for the complex geometry of submerged entry nozzle (SEN), where it is difficult to create hexahedron mesh in the entire domain, a mixed mesh type is recommended, i.e., the wedge mesh for regions adjacent to SEN walls and the tetrahedron mesh for inner regions. Another challenge to the calculation of real SEN clogging is the huge number of particles involved in the industry process. An artificial factor, the N-factor, has to be introduced to reduce the calculation cost. A dimensionless number (α) is defined to limit the N-factor and ensure the modeling accuracy. Simulation of a test case has indicated that by an appropriate N-factor (1000, corresponding to α = 6 × 10−5), the calculation time would be reduced significantly to a reasonable time.

Numerical Modeling of Diffusion-Based Peritectic Solidification in Iron Carbon System and Experimental Validation

Continuous casting of high-strength steels is challenging owing to peritectic phase transformation during solidification. This transformation is reported to be either diffusion controlled or “massive” like. The experimental evidence suggests that constant thermal gradients lead to diffusion-controlled phenomena, whereas the concentric solidification technique induces massive transformation. Diffusion-controlled peritectic solidification is more desirable during continuous casting to ensure a suitable cast quality compared with massive transformation. Accordingly, the authors demonstrate a general one-dimensional numerical modeling of the solidification process in steel by incorporating a diffusion-controlled peritectic phase transformation. The model is dynamically linked with the FactSage thermodynamic database through ChemAppV 7.1.4 library for input of accurate thermodynamic data. The modeling details are presented for a binary Fe-C system, and the results are compared with the experimental data available in the literature. The growth and dissolution of phases are accurately predicted as a function of composition and cooling rate.

Characterization, Mechanism and Control Measures of V Segregation in Continuous Casting Billet of C-Mn Steel

To eliminate V segregation, which significantly deteriorates the homogeneity of continuous casting steel billets, V segregation in the C-Mn steel billet was observed by microscope, SEM and EDS. The formation mechanism and control measures for V segregation were proposed. Results show that many slim V segregation strips are distributed on an inverted conical surface and that most segregation strips start from the vertex of the inverted conical surface. A series of inverted conical surfaces containing V segregation strips periodically appears along the longitudinal direction of the billet. The microstructures in the V segregation strip are fine granular structures, while the microstructures outside the V segregation strip are coarse Widmanstatten. The V segregation strip also contains many inclusions and porosities. The formation mechanism for V segregation, which is illuminated by a physical simulation of Al-5wt pctCu alloy, is attributed to the fissure in the equiaxed dendritic network torn by solidification contraction. By controlling the percentage and distribution of equiaxed grains, the V segregation in the billet of C-Mn steel is completely eliminated.

Effects of CeO2 on Viscosity, Structure, and Crystallization of Mold Fluxes for Casting Rare Earths Alloyed Steels

The CaO-Al2O3-based mold fluxes are proposed for the Ce-bearing heavy rail steel continuous casting because of the low reactivity. Effects of CeO2 on the melting temperature, the viscous property, and the crystallization behavior of the CaO-Al2O3-Li2O-B2O3 mold fluxes were studied using hemisphere melting point method, rotating cylinder method, and X-ray diffraction (XRD) in the present work. The microstructure of the mold fluxes was analyzed by Fourier-transform infrared spectroscopy (FTIR). The results revealed that the addition of CeO2 would increase the melting temperature, but decrease the viscosity at each temperature due to its influence on increasing the depolymerization of the mold fluxes at high temperature. The precipitation of CaO was restrained and CaCeAlO4 generated with increasing CeO2 content since the crystal phases were affected by the microstructure of the melts. The change of the crystalline phases in mold fluxes influences the break temperature and the viscosity of the mold fluxes below the break temperature. These results obtained can provide guidelines for designing new mold fluxes for casting rare earth alloy heavy rail steels.

Wetting and Erosion of ZrO2-Graphite Refractory by CaO-SiO2 and CaO-Al2O3-Based Mold Slags for Submerged Entry Nozzle

To moderate the erosion in a slag-line zone and extend the service life of a submerged entry nozzle during continuous casting of high-Mn and high-Al steel, the current study developed a low-reaction CaO-Al2O3-based slag with lower SiO2 and higher Al2O3 contents than that of conventional CaO-SiO2-based slag. The static sessile drop technique and dynamic rotating cylinder method were utilized to investigate the erosion behavior of ZrO2-graphite refractory by the above slags. The main results indicated that silicate-based slag could wet the ZrO2-graphite substrate completely, while the aluminate-based slag spread rapidly in the first 20 minutes, and then a slag drop with an unchanged profile formed on the substrate. The equilibrium contact angles with the substrate at 1623 K were 0 and 79 deg for the CaO-SiO2 and CaO-Al2O3 based slags, respectively. The comparison of the influences of the rotating speed and the temperature revealed that the decrease of erosion rate could be realized by the developed slag. An examination of ZrO2-saturation limit also indicated that the low-reaction slag could dissolve much more ZrO2 oxidation, accompanied by a less-negative effect on the viscosity of the molten slag.

FORMATION OF TEMPERATURE FIELDS AND THERMAL STRESSES ARISING DURING SOLIDIFICATION OF CYLINDRICAL CONTINUOUSLY CAST STEEL BILLETS

The article presents investigation results of the effect of inhomogeneity of boundary conditions on the intensity of metal cooling in the process of continuous casting of cylindrical billets from corrosionresistant steels. It is assumed that the boundary conditions are nonuniform along the billet perimeter. In the longitudinal direction, the cooling intensity is assumed to be constant within the cooled sector of the billet. During the research it was believed that there are flows of thermal energy between the cooling sectors. A comparative analysis of temperature gradients and resulting thermal stresses in the solidified billet at different cooling intensities realized in the secondary cooling zone was carried out The values of thermal stresses are compared with the maximum permissible for each grade of steel in order to find those cooling conditions in which the thermal stresses do not exceed the permissible values. Based on the results obtained, conclusions are drawn about the effect of cooling intensity on the occurrence of external and internal defects in the resulting cylindrical continuous cast billets. The authors have also made the conclusions about the effect of inhomogeneity of the boundary conditions on the formation of temperature fields in a solidified cylindrical continuously cast billet. The results of the conducted studies are presented in a graphic form and their detailed analysis is carried out. To calculate the temperature fields in the solidifying billet, a specially developed mathematical model was used, based on the equation of nonstationary heat conductivity. For the calculation of thermal stresses, known mathematical formulas have been used that allow calculating the values of thermal stresses arising between cooling zones in the solidifying billet during the continuous casting of steel. The obtained data are of high practical importance, since they can be used to develop rational cooling regimes, in which excess permissible thermal stresses will not be observed. This, as a consequence, will reduce the number of internal and external defects arising in the solidifying continuously cast billet.

Reasons for Breakage of Welding Rod and Wire

Reasons for breakage of welding rod and wire at drawing are analyzed. It is shown that the basic reason for breakage is microphysical tree-like segregation which is caused by irrational addition of boron-containing material at after-furnace treatment, low effectiveness of steel continuous casting process, unproven technology of metal electromagnetic agitation in a mold at continuous casting. Peculiarities of alloyed elements liquation, both in welding rod and wire are studied. It is shown that non-uniformity of boron, manganese and silicon distributions in the longitudinal and transversal welding rod cross sections will become the main reason for increasing wire breakage during further drawing.