Continuous Casting Of Steel Billets Research Articles

Serpentinised olivine used in tundish refractory working layer: A source of blowhole defect in steel billets

In this research, the effect of different types of commercial tundish refractory working layers (gunning mix, prefabricated board (PB), and dry mix) on the formation of blowhole defect in continuous casting steel billets was investigated. To protect the tundish body, three refractory layers are applied on its metallic shell including a safety layer, a permanent layer, and a working layer. Due to the direct contact of molten metal and slag with the tundish working layer, this layer plays an important role in guaranteeing the quality of steel. In the current research, the amount of molten metal hydrogen pick-up due to contact with the abovementioned tundish working layers was measured by a Hydris device. The hydrogen content of all the melts increased after transferring from ladle to tundish, but the rising rate strongly depended on the type of tundish working layer with PB having a minimum amount of hydrogen pick-up. To characterise the working layers, their chemical and mineralogical composition as well as their thermal behaviour up to 1400 °C were evaluated using X-ray fluorescence, X-ray diffraction, and thermogravimetry-differential scanning calorimetry techniques. The results showed that olivine is a main constituent of all the working layers. Also, the amount and quality of olivine (especially the amount of its serpentinization) have a great effect on the formation of blowhole defects in the steel billets.

THE INFLUENCE OF CHEMICAL COMPOSITION, HEATING TEMPERATURE AND COOLING RATE ON THE FORMATION OF STRUCTURAL COMPONENTS OF STEELS

Problem statement. The formation of structural components starts with the hardening of steels. Therefore, it is important to investigate the influence of chemical composition, temperature of additional heating and cooling rate of the melt on the formation of structural components of steels. The purpose of the research – to compare the influence of the chemical composition, heating temperature and cooling rate on the formation of structural components in steels. Methods. To determine the features of the structural state of steels, we use microstructural analysis, X-ray microanalysis, X-ray diffraction method and surface etching of specimens with nital and hot solution of sodium picrate. Results. In the work, a study of steel specimens with different carbon, manganese and silicon content was carried out. We compared the structural state of steel in the cross-section of continuous cast steel billet and steel of a similar chemical composition after additional heating above the liquidus line at 50 °C, 150 °C and cooling at rates of 10–104 °C/s. Scientific novelty. It was found that in the process of manufacturing of continuous cast steel billets, when cooling in a crystallizer at the rate of ~104 ºС/s in the surface zone of the billet of steels with a carbon content of ≥ 0.5 wt.%, manganese ≥ 0.75 wt.% and silicon ≥ 0.45 wt.% the δ-ferrite formation and γ-iron dendrites formation from the melt are completely suppressed. For the first time, the effect of heating temperature above the liquidus line up to 150 °С and cooling rate of 10–103 °С/s on the structure formation of experimental carbon steels was investigated. It was established that additional heating above the liquidus line and cooling at rates of 102–103°C/s almost completely inhibits the formation of ferrite along the grain boundaries, enhances formation of a more uniform structure, an increase in the dispersion of structural components, simultaneous solid-solution and dispersion hardening of steel, and improvement of mechanical properties of steels. In addition, a decrease in the volume fraction of the areas containing liquation elements (manganese and silicon) was observed. The areas were distributed more evenly, which suggests that there could be a decrease in the difference in the content of chemical elements, compared to the condition without additional heating of the melt. Practical value. The use of the obtained results could allow to develop technologies for obtaining steels with a more uniform structure.

Numerical modelling of macrosegregation in three-dimensional continuous casting of steel billets

Macrosegregation presents a considerable defect in the continuous casting of billets and can critically affect the final properties of the product. The numerical modelling can help to predict and better understand the segregation and flow patterns inside the mould. The process is modelled with a physical model described by a set of conservation equations describing the t heat transfer, turbulence, fluid flow, solidification and segregation. A two-equation low-Re k-epsilon model and Abe-Kondoh-Nagano closures are used to close governing equations in this incompressible fluid flow example. The Boussinesq approximation is applied to account for the thermo-solutal buoyancy effects, and the Darcy approximation is applied for the description of the flow through the porous mushy zone. On a microscale, a lever rule solidification model is used to couple liquid fraction, temperature and concentration. The three-dimensional model is solved with the method based on local collocation with multiquadric radial basis functions on seven-nodded subdomains. The aim of this contribution is to explore the three-dimensional macrosegregation patterns of 0.51 wt% carbon steel in the solidified shell of the steel in the mould.

Optimization of the rhomboidity of continuously cast billets using linear regression and genetic programming: A real industrial study

During the continuous casting of steel billets, several geometrical, inner and surface defects can occur due to the thermomechanical behavior during solidification. One of them is rhombic distortion (i.e. rhomboidity), which can lead to the occurrence of off-corner cracks and twisting of cast billets during further plastic deformation (i.e. rolling). Based on data of 2088 cast batches (64 different hypoeutectoid steel grades), 109,514 billets, produced from January 2022 to September 2022 in Štore Steel Ltd. (Slovenia), chemical composition (content of C, Si, Mn, S, Cr, Mo, Ni and V), casting parameters (average casting temperature, average difference between input and output cooling water, melt level, average cooling water flow and pressure in the first and second zone of secondary cooling) the linear regression and genetic programming were used in order to predict rhomboidity of continuously cast billets. The rhomboidity, in our case defined as relative diagonal difference, was determined using in-house developed computer vision system for measuring of rhomboidity. Based on the modelling results 9 batches (419 billets) of 42CrMos4 were cast in September 2022 with a 10 % higher water pressure in the first zone of secondary cooling (from 2.41 bar to 2.67 bar). The rhomboidity of continuously cast billets improved by 18.18 % (from 1.43 % to 1.21).

The Effect of Temperature, Cooling Rate and Casting Speed on Quality of Continuous Cast Steel Billets

In continuous casting, the cooling rate, casting speed, and molten metal temperature significantly affect the quality of cast steel billets. Appropriate casting parameters can minimize quality problems such as surface, subsurface and interior crack, rhomboidity, oscillation mark depth, and central porosity. This research determines the relationship between defects and three significant factors temperature, cooling rate, and casting speed. The work has been performed on the two-strand continuous casting machine to investigate the billet (BS-4449 steel grade) cross-section of 150 x 150 cm2. The defects analysis through macro examination at different tundish temperatures (1510 °C to 1560 °C), varying cooling rates, and casting speeds (0.9 to 1.6 m/min). The present study provides detailed insight into the three parameters mentioned earlier, which directly affect the quality of cast steel billets.

Особливості формування неоднорідних структур у вуглецевих сталях

Obtaining a homogeneous structure and uniform-phase distribution is critical to a high set of mechanical and operational properties of rolled metal. However, in practice it is not always possible to create metal products with the specified characteristics. In order to determine the morphological features of the structure of rolled carbon steel, a comparative study of carbon steel samples with a carbon content of 0.49 % C and 0.2 % C selected from hot-rolled billets was carried out. The billets of each group were produced under the conditions of the same enterprise, with close temperature-time modes of deformation processing. The main difference was in manufacturing processes of the output continuous cast steel billets. This research shows that with identical normalized chemical composition of steel and the same thermomechanical treatment, the formation of the morphological structure features of hot-rolled steel occurs in a different way. Therefore, we can assume that the liquation, the diffusive mobility of elements is particularly influenced by the content of impurity elements and gases in steel, which leads to a different type of structures in the finished rolled metal. At the same time, these differences are observed in carbon steels with different carbon content. A sample of non-vacuumed OC grade axle steel (0.49 % C) from converter steelmaking has a more homogeneous structure without local areas of pearlite or ferrite accumulation. It was shown that the formation of ferrite rim in the microsegregation areas occurs not only in manganous sulfides, but also arises on the background of the smallest oxide inclusions. There is significant structural heterogeneity in the samples of electric steel, despite the lower sulfur content and gassiness of steel; at the same time, a dense perlite layer is formed around the sulfides. There is also a difference in steel grade 20 (0.2 % C) of different manufacturing processes. The structure is more homogeneous in qualitatively deoxidized vacuum degassed steel; no local areas with different dimensional characteristics were detected. The size of the structural elements is much larger and the structure has mostly large sections of the Widmanstatten ferrite. Since a large number of non-metallic inclusions and gassiness of steel is not a positive factor for providing a high set of properties of metal products, the modes of thermomechanical treatment used today require adjustments depending on the characteristics of steel melting. Keywords: microstructural heterogeneity, ferritic-pearlitic banding, mechanical properties, manganous sulfides.

Modeling of the as-cast structure and macrosegregation in the continuous casting of a steel billet: Effect of M-EMS

Mold electromagnetic stirring (M-EMS) has been introduced into the continuous casting of steel billets to promote the formation of a central equiaxed zone; however, the formation mechanism of the equiaxed crystals and the effect of M-EMS on crystal transport are not fully understood. Currently, a three-phase volume average model was used to study the solidification in a billet continuous casting (195 mm × 195 mm). The modeling results showed that the main function of M-EMS in this type of billet casting is to promote superheat dissipation in the mold region, leaving the liquid core out of the mold region undercooled. Although both, heterogeneous nucleation and crystal fragmentation, are considered to be the origins of equiaxed crystals, M-EMS appeared to impact crystal fragmentation more effectively. A small portion of equiaxed crystals could be brought by the M-EMS induced swirling flow into the superheated zone (upper mold region) and remelted; most equiaxed crystals settled in the lower undercooled zone, where they continued to grow and form a central equiaxed zone. These simultaneous phenomena represent an important species/energy transport mechanism, influencing the as-cast structure and macrosegregation. Negative segregation occurred in the central equiaxed zone, positive segregation occurred at the border of the columnar zone, and a trail of negative segregation occurred in the subsurface region of the billet. Finally, parameter studies were performed, and it was found that the shielding effect of the copper mold, electrical isolation at the strand-mold interface, and relatively high electrical conductivity of the strand shell affect the M-EMS efficiency.

RANS versus Scale Resolved Approach for Modeling Turbulent Flow in Continuous Casting of Steel

Numerical modeling is the approach used most often for studying and optimizing the molten steel flow in a continuous casting mold. The selection of the physical model might very much influence such studies. Hence, it is paramount to choose a proper model. In this work, the numerical results of four turbulence models are compared to the experimental results of the water model of continuous casting of steel billets using a single SEN port in a downward vertical orientation. Experimental results were obtained with a 2D PIV (Particle Image Velocimetry) system with measurements taken at various cut planes. Only hydrodynamic effects without solidification are considered. The turbulence is modeled using the RANS (Realizable k-ε, SST k-ω), hybrid RANS/Scale Resolved (SAS), and Scale Resolved approach (LES). The models are numerically solved by the finite volume method, with volume of fluid treatment at the free interface. The geometry, boundary conditions, and material properties were entirely consistent with those of the water model experimental study. Thus, the study allowed a detailed comparison and validation of the turbulence models used. The numerical predictions are compared to experimental data using contours of velocity and velocity plots. The agreement is assessed by comparing the lateral dispersion of the liquid jet in a streamwise direction for the core flow and the secondary flow behavior where recirculation zones form. The comparison of the simulations shows that while all four models capture general flow features (e.g., mean velocities in the temporal and spatial domain), only the LES model predicts finer turbulent structures and captures temporal flow fluctuations to the extent observed in the experiment, while SAS bridges the gap between RANS and LES.

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.

Prediction of Central Carbon Segregation in Continuous Casting Billet Using A Regularized Extreme Learning Machine Model

Central carbon segregation is a typical internal defect of continuous cast steel billets. Real-time and accurate carbon segregation prediction is of great significance for lean control of the production quality in continuous casting processes. In this paper, a data-driven regularized extreme learning machine (R-ELM) model is proposed for the prediction of carbon segregation index (CSI). To improve model performance, outliers in industrial data were eliminated by means of boxplot tool. Besides, Pearson correlation combined with grey relational analysis (GRA) was conducted to avoid multicollinearity and redundancy in input variables. The new model shows potential to evaluate online quality of steel billets. When predictive errors were within ±0.03 and ±0.025, the prediction accuracy of the R-ELM model was 94% and 89%, respectively, which was higher than that of the multiple linear regression (MLR) model and ELM model. Moreover, the effects of several key continuous casting process parameters on CSI were investigated based on the predictions of the R-ELM model via response surface analysis. The conclusions are consistent with the metallurgical mechanism, and the predictive values of the R-ELM model agree well with experimental values, which further verifies the correctness and generalization ability of the R-ELM model.

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.

RESEARCH OF THE BEHAVIOR OF MACROSTRUCTURE DEFECTS OF THE PRE-DEFORMED CONTINUOUS CAST BILLETS DURING ROLLING

The most significant trend in recent decades in the field of continuous casting of steel billets is a partial transfer of the process of deformation of the exposure to the field of complete solidification (rolling mill) in the area of two-phase (solid-liquid)state (technological line of CCM). However, the implementation of such two-stage deformation of the continuous cast ingot leads to the need for changes to the methodology of physical simulation of the behavior of defects (surface and bulk) in the course of the subsequent rolling, first and foremost, in terms of the correct choice of their geometric shape and spatial orientation. The paper presents the results of a study of the influence of spatial orientation of the surface defects and continuity of the macrostructure of the metal axial region deformed in the line of the continuous casting machine using layered physical models. An experimental study performed in the conditions of deformation of reduced billets by two schemes with rolling simulation scale1:5: in smooth rolls, simulating the process of groоvelles rolling as applied to the first two stands of the crimp group of medium-section mill 300; in the first and second pairs of rectangular grooves of breakdown stand of the mill 500/370 of PJSC “Donetsk Metal Rolling Plant”. According to the multiplicity of the problem, the universal design of physical models was developed to simulate the spatial location of both surface and internal defects. Studies have shown that in case of rolling of physical models with extract ratio over 2.0 and an angle of misalignment defects imitators close to 60° their complete “healing” is possible. In turn, decrease in the angle to 30° contributes to greater extraction of the defect-simulator and only a small decrease in their width. In the case of total misalignment of defect-simulators (angle 90°) there is a broadening of the defects and their deposition to the initial length after tilting by 90°. The obtained experimental data allowed understanding of the mechanisms of “healing” defects integrity of metal depending on magnitude of the total extracts, the angle of misalignment of the longitudinal axis of the defect with the rolling direction and the distance of their occurrence from the longitudinal-transverse plane of symmetry.

A pseudo-transient heat transfer simulation of a continuous casting process, employing the element-free Galerkin method

The present work has been conducted in order to develop a novel solution for the heat transfer problem during the continuous casting of steel billets and blooms, by using the element-free Galerkin method under a pseudo-transient moving cross-section slice approach. The transport laws, non-linear aspects and boundary conditions of the initial value problem have been specified. A detailed explanation concerning the characteristics inherent to the application of the element-free Galerkin method to this problem has also been provided. The feasibility and suitability of this novel approach has been verified by comparison with the numerical techniques proposed and the results reported by other researchers as well as an analytical solution of a simple 1-D alloy solidification problem. The results have revealed that this technique could be used successfully in the pseudo transient moving cross-section solution of the heat transfer problem involved in the continuous casting of blooms and square billets.

Development of Criteria Specifying the Conditions of Interaction between the Scale with the Rolls of Continuous Cast Machine and Continuous Cast Billets

The study of surface condition of the rolls of continuous casting machine (CC-machine) of the secondary cooling zone is undertaken. It was stated that their working surfaces include the segments with stuck and peeled-off scale. The surface profilograms of the rolls with pickup scale are obtained. The domelike, tapered and trapezoidal form of fragments of the scale with a height 3-4 mm and tilt angles between the side faces of 43-88 are the most typical forms.Based on the elements of the plasticity theory, the problem of determination of stress-strain behavior of continuous casting steel billets is solved in the process of indentation of the scale fragments into it. The mathematical relation between geometrical parameters of scale fragments and the thickness of crystallized jacket is found that allows determining a stress strain state criteria in accordance with that the scale pickup on the roll surface is carried out or its indentation into internal layers of CC steel billets with its following holding. It was stated that the indentation of good-size scale fragments in relatively thin crystallized skin creates comfortable facilities for pick-up of scale on the rolls, and the indentation of small-sized scale fragments in the thick crystallized jacket especially with large tilt angles between the side surfaces of fragments makes possible for their hold-up at the surface of concast steel billets in the imbedded condition. It was determined the criteria which contains two necessary conditions whereby the scale sticking on the rolls of CC machine is carried out or the indentation in continuous-casting machine followed by holding in it.

Cooling of a mold at preforming cylindrical continuous cast steel billets

The article provides the results of structural calculation of cooling of a nickel mold of continuous casting machine for cylindrical billets, superheated to 180°C with water at a density of the supplied heat flow of 1.5–3.3 MW/m2. The novelty of the work has been confirmed by a patent for an invention.

Design and thermodynamic analysis of a flash power system driven by process heat of continuous casting grade steel billet

Hot charging of continuous cast steel billets is usually considered an effective method for recovering heat. However, for certain grades of steel, the hot charging process may affect the steel surface quality. To overcome this issue, the billet is commonly cooled down before rolling, resulting in substantial heat loss. Thus, the recovery of useful waste heat without affecting steel quality is an important research topic. In this study, the design of a flash power system driven by the process heat of continuous casting grade steel billet is proposed. The influences of shunted water temperature and flash pressure on exergy recovery rate are studied via the thermodynamic methods. Moreover, a case study based on actual data is analyzed to verify the thermodynamic results and the energy recovery of the proposed system. The results indicate that the system exergy recovery rate increases with increasing shunted water temperature. Keeping the shunted temperature unchanged the exergy recovery rate firstly increases and later decreases with increasing flash pressure. At a fixed shunt temperature, an optimum flash pressure exists. At the maximum exergy recovery rate the net output power of the studied plant is 6361 kW, a value that greatly reduces the need for purchased electricity.

Erratum to: Detection of Non-metallic Inclusions in Centrifugal Continuous Casting Steel Billets

Erratum to: Detection of Non-metallic Inclusions in Centrifugal Continuous Casting Steel Billets

Detection of Non-metallic Inclusions in Centrifugal Continuous Casting Steel Billets

In the current study, automated particle analysis was employed to detect non-metallic inclusions in steel during a centrifugal continuous casting process of a high-strength low alloy steel. The morphology, composition, size, area fraction, amount, and spatial distribution of inclusions in steel were obtained. Etching experiment was performed to reveal the dendrite structure of the billet and to discuss the effect of centrifugal force on the distribution of oxide inclusions in the final solidified steel by comparing the solidification velocity with the critical velocity reported in literature. It was found that the amount of inclusions was highest in samples from the tundish (~250 per mm2), followed by samples from the mold (~200 per mm2), and lowest in billet samples (~86 per mm2). In all samples, over 90 pct of the inclusions were smaller than 2μm. In steel billets, the content of oxides, dual-phase oxide–sulfides, and sulfides in inclusions were found to be 10, 30, and 60 pct, respectively. The dual-phase inclusions were oxides with sulfides precipitated on the outer surface. Oxide inclusions consisted of high Al2O3 and high MnO which were solid at the molten steel temperature, implying that the calcium treatment was insufficient. Small oxide inclusions very uniformly distributed on the cross section of the billet, while there were more sulfide inclusions showing a banded structure at the outside 25 mm layer of the billet. The calculated solidification velocity was higher than the upper limit at which inclusions were entrapped by the solidifying front, revealing that for oxide inclusions smaller than 8μm in this study, the centrifugal force had little influence on its final distribution in billets. Instead, oxide inclusions were rapidly entrapped by solidifying front.

Distribution Pattern of Nonmetallic Inclusions on a Cross Section of Continuous-Cast Steel Billets For Rails

Distribution Pattern of Nonmetallic Inclusions on a Cross Section of Continuous-Cast Steel Billets For Rails

Controller design of a steel billet mould oscillation system with iterative feedback tuning

In the continuous casting of steel billets, mould oscillation is a key process. The mould driven by a hydraulic cylinder is a big inertia system. Its trajectory is required to track a specific oscillation rule, but the tracking performance will be influenced by the molten steel weight. In this paper, dynamic pressure feedback (DPF) is proposed as compensation for the PID controller, to eliminate the adverse effect caused by the variation of the molten steel weight. The iterative feedback tuning (IFT) method is applied to tune the PID controller parameters and the DPF coefficient. In contrast to common IFT tuning structure, this paper designs a new scheme to estimate the cost function gradient with respect to the controller parameters. Numerical simulations and experimental results show the effectiveness of the DPF compensation strategy and the proposed parameter tuning method.