Solidifying Steel Shell Research Articles

Formation mechanism and control of a kind of pit-shaped defect in Cf53 rolled products

Aiming at the frequent issue of pit-shaped surface defects in S-bearing and Al-killed steel products and the related poor castability due to submerged entry nozzle (SEN) clogging, the melting and casting process of a popular auto steel Cf53 has been studied based on in-situ investigation and experimental analysis. To clarify the generation of the pit-shaped defects, the local surface chemistry, evolution of steel inclusions throughout the melting process, SEN clogging substances, and the internal relationship among them were systematically analyzed. The results show that this type of defect in the Cf53 rolled products is caused by the large-sized composite inclusions rich in CaS, which originate from the molten steel during the refining process, gradually accumulate on the inner wall of the SEN during casting, and finally lead to SEN clogging. The loose structure of clogging inclusions makes them prone to fall off into the mold and get trapped by solidifying steel shell. The formation mechanism of the inclusions was studied through FactSage software calculation. The results show that the increase in total sulfur and total calcium content, and the decrease in total oxygen content and temperature in steel will lead to CaS inclusion formation, which agrees reasonably well with experimental results. An optimized refining process to reduce calcium addition into molten steel is proposed accordingly, which achieves a remarkably alleviation in casting SEN clogging and an elimination of the pit-shaped defects on the final rolled products as well.

Method to Investigate Mechanical Behavior of Steel Casting near Solidus Temperature

Hot crack formation in continuously cast steel is significantly influenced by the mechanical properties of the solid shell near its solidus temperature. Herein, a new method to study the high‐temperature mechanical behavior of the solidifying steel shell is introduced. In this method, an apparatus is designed utilizing an electric cylinder that is controlled by a servomotor to apply a specified amount of strain to the solidifying steel shell at a controlled strain rate. A special mold configuration is developed to control the dendrite growth in the direction perpendicular to the applied strain and to ensure that the strain is applied in the region of controlled shell growth. Real‐time load, displacement, and temperature data are monitored by a computer‐assisted data acquisition system. The temperature profile of the casting is predicted by MAGMASOFT and compared with experimental data. The Fourier thermal analysis method is applied to calculate a solid fraction and coupled with the temperature profile to determine the solid shell thickness during the test. The maximum strength at different temperatures for a medium‐carbon steel is determined and compared with that from the submerged split‐chill tensile test and hot tensile tests.

Transport of heat and material properties in continuous casting of steel

Liquid steel solidifies within a containment of a continuous casting machine and moves in axial direction as the solidified shell is withdrawn towards the end of the machine. A mechanical model for computing the deformation of the solidifying steel in the roll containment is developed based on the beam theory and on a ferro-static pressure load. The material properties of the steel at high temperatures result from high temperature constitutive equations including creep for the solidified shell between the rolls. The temperature distribution within the solidified shell is computed based on the thermal boundary conditions as in the secondary cooling zone water is spread onto the surface of the steel shell and heat is removed. The solidified steel shell thickness increases along the machine containment up to the point of total solidification. A suitable discretization of the region by the finite beam elements is used to get a suitable mechanical model. The ferro-static pressure acts inside of the solidified shell and causes some bulging of the shell between the roll containment. As there is a slow axial motion of the solidifying shell the numerical modeling of the transport e.g. of heat and of the creep strain, which is computed in each time step, has to be as accurate as possible as a lot of time steps are necessary. Different numerical strategies have been analysed with respect to the accuracy of the computation results.

Thermal Mapping of Metal Casting Mold Using High-Resolution Distributed Fiber-Optic Sensors

This article reports a technique to embed optical fiber into a copper mold plate for generating high-density thermal maps of the mold during the process of metal casting. The temperature measurements were based on acquiring and interpreting Rayleigh backscattering (RBS) signals from embedded fiber, using the interrogation technique of optical frequency domain reflectometry (OFDR). The instrumented mold plate was used to perform a cast-iron dip test and a steel dip test in a 200 lb induction furnace. The maximum temperatures recorded by the embedded fiber-optic sensors were 469 °C and 388 °C in the cast-iron and steel dip tests, respectively. The closely spaced and rapidly fluctuating temperature features that were imparted to the mold wall during solidification were successfully mapped with a high spatial resolution (0.65 mm) and a fast measurement rate (25 Hz) using a commercial OFDR interrogator (LUNA ODiSI 6108). Moreover, the thickness of the solidified steel shell was measured, and a thickness map of the shell was generated. A good correlation was observed between the thickness of the solidified shell and the temperature of the mold, as regions with higher and lower temperatures in the thermal profile of the mold corresponded to thicker and thinner areas on the shell, respectively. The dip testing experiments demonstrate that RBS-based fiber-optic sensing is a feasible and effective method for generating information-rich thermal maps of caster molds. The information obtained from thermal maps can be useful for improving the quality of the metal and productivity of the metal casting process.

Thermal Analysis of Solidifying Steel Shell in Continuous Casting Process

A finite element simulation of the steel shell formation in continuous casting has been developed. The current research is focused on the solidification of molten steel during the initial stages of the mould cooling. The model allows predicting the temperature field throughout the process: temperature gradient, solidification front, cooling rates. In stationary state, the prediction of shell thickness reasonably agrees with analytical models and experimental observations. The simulation tool is used to study the alteration of the mould thermal field in case of sticking defects encountered in industrial practice. A numerical analysis increases the understanding of the phenomena.

Effect of Nozzle Port Angle on Transient Flow and Surface Slag Behavior During Continuous Steel-Slab Casting

Undesirable flow variations can cause severe instabilities at the interface between liquid mold flux and molten steel across the mold top-region during continuous steel casting, resulting in surface defects in the final products. A three-dimensional Large Eddy Simulation (LES) model using the volume of fluid method for the slag and molten steel phases is validated with plant measurements, and applied to gain new insights into the effects of nozzle port angle on transient flow, top slag/steel interface movement, and slag behavior during continuous slab casting under nominally steady conditions. Upward-angled ports produce a single-roll flow pattern with lower surface velocity, due to rapid momentum dissipation of the spreading jet. However, strong jet wobbling from the port leads to greater interface variations. Severe level drops allow easy entrapment of liquid flux by the solidifying steel shell at the meniscus. Sudden level rises may also be detrimental, leading to overflow of the solidified meniscus region. Downward-angled ports produce a classic double-roll pattern with less jet turbulence and a more stable interface everywhere except near the narrow faces. Finally, the flow patterns, surface velocity, and level predicted from the validated LES model are compared with steady-state standard k-e model predictions.

Engulfment Behavior of Inclusions in High-Carbon Steel: Theoretical and Experimental Investigation

Previous studies on inclusions behavior at the front of the solidifying steel shell have mainly focused on low-carbon steels. However, with the increasing applications of high-carbon steel in recent years because of its superior properties, it is crucial to understand this behavior in high-carbon steel. Most of the high-carbon steels are deoxidized by silicon, calcium treated, and contain higher sulfur percentage. Also, higher carbon content has a determining influence on the viscosity and surface tension, which will affect the inclusion behavior. In this study, we have investigated the engulfment behaviors of inclusions in front of the solidifying interface in high-carbon steels using concentric solidification method. The critical velocity of the growing shell, at which the particle is engulfed in the solidifying shell, instead of being pushed by this shell, was determined. The inclusion identified in this study is a bi-component form of CaO-SiO2-based oxide and CaS. It was revealed that engulfment behavior is strongly affected by convection of liquid steel that originates from carbon push out in high-carbon steels. This study provides new crucial information to produce high-carbon steel with fewer inclusions, which opens new application pathways for this emerging grade of steel.

Improvement of Castability and Surface Quality of Continuously Cast TWIP Slabs by Molten Mold Flux Feeding Technology

An innovative continuous casting process named POCAST (POSCO’s advanced CASting Technology) was developed based on molten mold flux feeding technology to improve both the productivity and the surface quality of cast slabs. In this process, molten mold flux is fed into the casting mold to enhance the thermal insulation of the meniscus and, hence, the lubrication between the solidifying steel shell and the copper mold. Enhancement of both the castability and the surface quality of high-aluminum advanced high-strength steel (AHSS) slabs is one of the most important advantages when the new process has been applied into the commercial continuous casting process. A trial cast of TWIP steel has been carried out using a 10-ton scale pilot caster and 100-ton scale and 250-ton scale commercial casters. The amount of mold flux consumption was more than 0.2 kg/m2 in the new process, which is much larger than that in the conventional powder casting. Trial TWIP castings at both the pilot and the plant caster showed stable mold performances such as mold heat transfer. Also, cast slabs showed periodic/sound oscillation marks and little defects. The successful casting of TWIP steel has been attributed to the following characteristics of POCAST: dilution of the reactant by increasing the slag pool depth, enlargement of channel for slag film infiltration at meniscus by elimination of the slag bear, and decrease of apparent viscosity of the mold slag at meniscus by increasing the slag temperature.

A Novel Technology to Develop a Nickel-Enriched Layer on Slab Surface by Utilizing NiO-Containing Synthetic Powder

Cu, as one of the typical tramp elements, is known to cause hot shortness during reheating of slabs followed by hot rolling of sheet products. In order to prevent such harmful aspects, a new idea is proposed by using synthetic powders containing NiO in the mold flux during continuous casting of the slab. During the casting, NiO is reduced and absorbed on initial solidified steel shell, and a Ni-rich layer is developed near the surface region of the slab. According to the proposed idea, it is expected that both the Cu solubility and the melting temperature of Cu-segregated region would increase considerably by virtue of Ni-rich layer, which is believed to play an important role to prevent the Cu hot shortness. A series of laboratory-scale experiments were carried out in order to confirm the reduction and the absorption of Ni into the steel matrix. It was observed by SEM–EDS and FE-EPMA that a Ni-enriched layer, as thick as a few hundred μm, formed near the surface of the slab. Also, a number of laboratory-scale heat treatment tests under oxidizing atmosphere showed that the samples with the Ni-enriched layer had a decreased Cu enrichment at the interface between scale and steel, compared to a case without Ni-rich layer. A pilot-plant-scale steel slab (medium carbon steel containing 0.3 wt pct Cu) was obtained in a continuous casting process with the NiO-containing mold flux, and a Ni-enriched layer was also observed. It was concluded that the use of NiO in the mold flux is a promising new approach for suppressing the hot shortness of Cu-containing steel, without an expensive addition of Ni to the whole steel matrix.

Controlling shear thinning property of lime silica based mold flux system with borate additive at 1623 K

We have studied the shear thinning property of mold flux with borate additive using a rotational type rheometer at various shear rates. Mold flux at 1623K is currently used in the steel industry as a lubricant to reduce the shear force exerted on a solidifying steel shell in an oscillating mold. In an actual casting process, the enhancement of shear thinning property might be beneficial for a casting process. Therefore, the present study is designed to control the shear thinning property of mold flux at 1623K by changing a borate concentration. For the first step of the present study, Raman spectroscopy is used to identify structural change of mold flux with increasing a borate concentration. The second step is to conduct viscosity measurement by changing shear rates with a rotational rheometer. The degree of shear thinning has been successfully estimated by the flow behavior index of Oswald–de Waele's power law. In doing so, we successfully achieved the strong shear thinning property of mold flux and discovered the rationale behind this. In order to maximize the shear thinning property of lime silica based glass forming liquid, it is essential to understand the degree of polymerization (DP) and stiffness of entire mold flux structure.

Analysis of the Equilibrium State of Flux Powders in the Mould During Continuous Casting of Steel

Abstract Mould fluxes in the continuous casting of steel perform a protective function against oxidizing effect of the atmosphere on liquid metal, reduce friction between the solidifying steel shell and the walls of the mould and regulate the transfer of heat to the mould walls. Their chemical composition is crucial as it impacts the determination of the characteristic temperatures. The paper presents calculation results of the equilibrium composition of the selected mould powders in thermodynamic conditions similar to the actual conditions during continuous casting of steel slabs based on FactSage. The calculations were verified by high temperature tests of mould flux powders.

Transient Thermo-fluid Model of Meniscus Behavior and Slag Consumption in Steel Continuous Casting

The behavior of the slag layer between the oscillating mold wall, the slag rim, the slag/liquid steel interface, and the solidifying steel shell, is of immense importance for the surface quality of continuous-cast steel. A computational model of the meniscus region has been developed, that includes transient heat transfer, multi-phase fluid flow, solidification of the slag, and movement of the mold during an oscillation cycle. First, the model is applied to a lab experiment done with a “mold simulator” to verify the transient temperature-field predictions. Next, the model is verified by matching with available literature and plant measurements of slag consumption. A reasonable agreement has been observed for both temperature and flow-field. The predictions show that transient temperature behavior depends on the location of the thermocouple during the oscillation relative to the meniscus. During an oscillation cycle, heat transfer variations in a laboratory frame of reference are more severe than experienced by the moving mold thermocouples, and the local heat transfer rate is increased greatly when steel overflows the meniscus. Finally, the model is applied to conduct a parametric study on the effect of casting speed, stroke, frequency, and modification ratio on slag consumption. Slag consumption per unit area increases with increase of stroke and modification ratio, and decreases with increase of casting speed while the relation with frequency is not straightforward. The match between model predictions and literature trends suggests that this methodology can be used for further investigations.

Crystallization Characteristics of CaO-Al2O3-Based Mold Flux and Their Effects on In-Mold Performance during High-Aluminum TRIP Steels Continuous Casting

Crystallization behaviors of the newly developed lime-alumina-based mold fluxes for high-aluminum transformation induced plasticity (TRIP) steels casting were experimentally studied, and compared with those of lime-silica-based mold fluxes. The effects of mold flux crystallization characteristics on heat transfer and lubrication performance in casting high-Al TRIP steels were also evaluated. The results show that the crystallization temperatures of lime-alumina-based mold fluxes are much lower than those of lime-silica-based mold fluxes. Increasing B2O3 addition suppresses the crystallization of lime-alumina-based mold fluxes, while Na2O exhibits an opposite effect. In continuous cooling of lime-alumina-based mold fluxes with high B2O3 contents and a CaO/Al2O3 ratio of 3.3, faceted cuspidine precipitates first, followed by needle-like CaO·B2O3 or 9CaO·3B2O3·CaF2. In lime-alumina-based mold flux with low B2O3 content (5.4 mass pct) and a CaO/Al2O3 ratio of 1.2, the formation of fine CaF2 takes place first, followed by blocky interconnected CaO·2Al2O3 as the dominant crystalline phase, and rod-like 2CaO·B2O3 precipitates at lower temperature during continuous cooling of the mold flux. In B2O3-free mold flux, blocky interconnected 3CaO·Al2O3 precipitates after CaF2 and 3CaO·2SiO2 formation, and takes up almost the whole crystalline fraction. The casting trials show that the mold heat transfer rate significantly decreases near the meniscus during the continuous casting using lime-alumina-mold fluxes with higher crystallinity, which brings a great reduction of surface depressions on cast slabs. However, excessive crystallinity of mold flux causes poor lubrication between mold and solidifying steel shell, which induces various defects such as drag marks on cast slab. Among the studied mold fluxes, lime-alumina-based mold fluxes with higher B2O3 contents and a CaO/Al2O3 ratio of 3.3 show comparatively improved performance.

Simulation of low carbon steel solidification and mold flux crystallization in continuous casting using a multi-mold simulator

An inverted water-cooled multi-mold continuous casting simulator was used to investigate initial solidification of low-carbon steels and crystallization of mold flux. Embedded mold thermocouples showed characteristic temperature profiles dependent on parameters including casting speed, oscillation frequency, and stroke. Higher maximum temperatures for thermocouples at higher casting speeds, higher frequencies, and lower stroke lengths were observed. The surface of the as-cast steel strips showed oscillation marks similar to those of industrially cast slabs and higher casting speeds resulted in shallower oscillation marks. The measured pitch agreed well with the theoretical pitch suggesting the multi-mold simulator to be a cost-effective alternative to pursue fundamental studies on initial solidification in the mold. Analysis of the mold flux taken between the copper mold and solidified steel shell showed highly dendritic uni-directional crystallization occurring within the flux film suggesting that the heat transfer direction is dominantly horizontal towards the water-cooled copper mold. In addition, the solidified flux located at the upper to lower part of the mold suggested morphological differences in the size and shape of the crystalline phases indicating that crystallization ratio can increase depending upon the retention in the mold and subsequently decrease radiative heat transfer as the flux traverses down the mold.

Investigation of Viscosity's Effects on Continuous Casting of Steel Mold Powders Containing B<sub>2</sub>O<sub>3</sub>, Li<sub>2</sub>O, TiO<sub>2</sub>, Fe<sub>2</sub>O<sub>3</sub>, ZnO and Na<sub>2</sub>O

Mold powders are mainly formed of such oxides as SiO2, CaO, Al2O3, Na2O along with F and C. One of the main constitutional composes of mold powders is F, which is utilized in chemical composition of mold powders due to its features of controlling viscosity and producing desired lubrication between solidified steel shell and mold. However, the emission of F through such compositions as HF (g) causes health and environmental problems. The purpose of this research is to decrease, and subsequently substitute, F in chemical composition of a lubricating high speed powder, which is used in continuous casting of steel industry. A combination of Fe2O3 and TiO2 compositions to substitute F in chemical composition of a fluorine-free mold powder was used to meet the purposed of this research. In addition, nine powder samples (A-I) with laboratory scale were prepared. In order to evaluate the samples’ viscosity behavior, groove viscometer test and image analysis software was used and to evaluate the crystalline behavior of samples C and G, XRD and SEM analyses were conducted. The results of these analyses demonstrated that the fluorine-free sample, due to its viscosity resemblance in comparison with molten reference powder and through creation of such crystalline phases as Perovskite (CaTiO3) and Fayalite (Fe2SiO4), will result in viscosity control of the mold powder and therefore optimizing continuous casting conditions. Finally, it is possible to derive that this sample may be an appropriate substitution for the reference powder being utilized in steel continuous casting industry.

Influence of Different Oxides on the Viscosity of Fluorine-Free Mold Fluxes

Mold fluxes play an important role in the steel continuous casting process. Mainly, they must prove adequate lubrication between the solidified steel shell and the copper mold. Thus, the knowledge of mold flux viscosity with the temperature is a relevant property for industry applications in order to prevent operation problems and product defects. On the other hand, for environmental reasons, the replacement of fluorine by non-pollutants compounds, is one of the objectives in the design of new compositions for these slags. In this paper, the viscosity is calculated in the range: 1250-1400 ¼C applying two models based on their chemical composition and another one using the differential thermal analysis technique. The results obtained with this model showed a good correlation with respect to other traditional models using the chemical composition of the mold flux to estimate the viscosity. The aim is to observe the effect of different oxides on the viscosity of mold fluxes. One of the mold powders was designed with a chemical composition similar to a commercial one (containing 10 wt% of fluorine), while the others powders were fluorine-free (containing boron and lithium). It should be noted that a fluorine-free powder with a viscosity similar to the powder with fluorine was produced replacing 10 wt% fluorine by 4 wt% Li2O and 6 wt% B2O3.

Measuring Mechanical Behavior of Steel During Solidification: Modeling the SSCC Test

The submerged split chill contraction (SSCC) test can measure forces in a solidifying steel shell under controlled conditions that match those of commercial casting processes. A computational model of this test is developed and applied to increase understanding of the thermal-mechanical behavior during the initial solidification of steel. Determining the stress profile is difficult because of the complicated geometry of the experimental apparatus and the nonuniform temperature and strength across the shell. The two-dimensional axisymmetric elastic-viscoplastic finite-element model of the SSCC test features different mechanical properties and constitutive equations for delta-ferrite and austenite that are functions of both the temperature and the strain rate. The model successfully matches measurements of (1) temperature history, (2) shell thickness, (3) solidification force, and (4) failure location. In addition, the model reveals the stress and strain profiles through the shell and explains what the experiment is actually measuring. In addition to the strength of the shell, the measured force is governed by the strength of the junction between the upper and lower test pieces and depends on friction at the shell–cylinder interface. The SSCC test and the validated model together are a powerful analysis tool for mechanical behavior, hot tear crack formation, and other phenomena in solidification processes such as continuous casting.

Multiphysics Model of Metal Solidification on the Continuum Level

Separate three-dimensional (3-D) models of thermomechanical behavior of the solidifying shell, turbulent fluid flow in the liquid pool, and thermal distortion of the mold are combined to create an accurate multiphysics model of metal solidification at the continuum level. The new system is applied to simulate continuous casting of steel in a commercial beam-blank caster with complex geometry. A transient coupled elastic-viscoplastic model [1] computes temperature and stress in a transverse slice through the mushy and solid regions of the solidifying metal. This Lagrangian model features an efficient numerical procedure to integrate the constitutive equations of the delta-ferrite and austenite phases of solidifying steel shell using a fixed-grid finite-element approach. The Navier-Stokes equations are solved in the liquid pool using the standard K–ϵ turbulent flow model with standard wall laws at the mushy zone edges that define the domain boundaries. The superheat delivered to the shell is incorporated into the thermalmechanical model of the shell using the enhanced latent heat method [2]. Temperature and thermal distortion modeling of the complete complex-shaped mold includes the tapered copper plates, water cooling slots, backing plates, and nonlinear contact between the different components. Heat transfer across the interfacial gaps between the shell and the mold is fully coupled with the stress model to include the effect of shell shrinkage and gap formation on lowering the heat flux. The model is validated by comparison with analytical solutions of benchmark problems of conduction with phase change [3], and thermal stress in an unconstrained solidifying plate [4]. Finally, results from the complete system compare favorably with plant measurements of shell thickness.

Compensator design for the mould level of a thin slab caster to suppress periodic fluctuations

The design of a parameter-dependent controller to reject periodic mould level fluctuations at a thin slab caster is presented. These fluctuations can be related to bulging of the solidified steel shell between the supporting rolls. They deteriorate the steel quality and prohibit high speed casting. A state of the art controller has been designed to suppress these casting speed dependent fluctuations and is based on a H∞-optimal controller design technique. The controller has been implemented at caster of the Corus Direct Sheet Plant in IJmuiden. Measurement results show a significant reduction of these periodic mould level fluctuations.

Effect of Sen Design on Surface Fluctuation and Solidifying Shell in Slab Mold and Its Optimization

Turbulent flow and heat transfer coupled with solidification in slab continuous casting mold was studied by numerical simulation method. Volume of fluid (VOF) model is used to solve steel-air two-phase flow problem and enthalpy-porosity scheme is introduced to solve the fluid flow problem involving solidification. Contributions of various nozzle port angles and port widths and heights on the free surface fluctuation and the thickness of solidifying shell in slab mold were particularly investigated, based on which the structure of submerged entry nozzle was optimized. Flow inside the common nozzle port cannot fill the entire outlet area, having a recirculation in the upper portion of the port, which is enlarged for the nozzle port with both larger height and width. Results show that the flow in mold cavity is mainly controlled by the nozzle port angle. The increase of the angle of upper face of the port to shape a roughly streamlined inner-wall improves the effective area fraction of the nozzle, resulting in less jet impingement, weaker free surface turbulence and thicker solidifying steel shell.