Ferrostatic Pressure Research Articles

Selection of internal design of crystallizer sleeves for continuous casting of 200 mm billets

The crystallizer is the most critical and most important functional unit of a continuous casting machine (hereinafter referred to as CCM). It is the main technological unit of the CCM, an assembly for removing heat during the solidification of the solidifying metal and the formation of the ingot. The main requirement for the crystallizer is to provide maximum heat removal from the solidifying steel to the cooling water, and to obtain a strong ingot shell with a good surface at the outlet of the crystallizer, which would not be destroyed by the heat of the liquid phase and the ferrostatic pressure. During the reconstruction of the CCM by the contractor company, drawings of crystallizers and crystallizer sleeves with a double‑cone internal geometry for casting 200 mm continuous cast billets were developed. The 200 mm continuous cast billet is a billet for the production of hot‑rolled seamless steel pipes. During the operation of the crystallizer sleeves with a double‑cone internal geometry, an accelerated wear of the protective coating in the lower part of the sleeve was noted, as well as an increased rejection of hot‑rolled pipes due to defects on the outer surface of the pipes. In order to identify the cause of the defect formation, complex metallographic studies of the outer surface of the pipes were carried out. As a result of the metallographic study, surface defects classified as steel mill scale on the outer surface of the pipes were identified. To minimize the “steel mill scale” defect and reduce the wear of the protective coating of the crystallizer sleeves, work was carried out to select the optimal conditions for the crystallization of the continuous cast billet: the use of crystallizer sleeves with a three‑cone internal geometry compared to a double‑cone was investigated; maintaining the temperature difference of the water at the inlet and outlet of the crystallizer within specified limits (ΔT) was tested. It was found that the use of a crystallizer sleeve with a three‑cone internal geometry for casting 200 mm billets and the stable behavior of the ΔT parameter made it possible to ensure the required quality of the surface of seamless pipes, reduce the wear of the protective coating of the crystallizer sleeves and increase the productivity of the CCM.

Fused Filament Fabrication of Thermal-Shock-Resistant Fine-Grained Refractories for Steel-Casting Applications

Three-dimensionally printed fine-grained refractory ceramics ready for use in contact with liquid steel based on developed one-step thermal debindable ceramic filaments that do not require any chemical solvent pre-debinding are investigated. This work exhibits the most favourable debinding and sintering regimes with an excellent form stability and reproducibility of printed products ensured. The structure of the sintered products was examined with computed tomography. The designed inner geometry with micro-porosity introduced during debinding combined with pre-designed printed macro-cavities enabled the outstanding thermal-shock performance of the specimens. The functionality of the sintered refractory products in the form of casting nozzles was preliminarily tested in contact with steel melt using a hot-stage microscope. The structure of the specimen was subsequently examined with laser scanning microscopy and scanning electron microscopy. The mechanical properties of printed samples were studied via mercury intrusion porosimetry, compressive strength testing, and spatial tensile strength testing. According to the results, the cold crushing strength of the 3D-printed specimens in the printing direction was comparable to that of pressed fine-grained alumina specimens (50–60 MPa). The measured porosity was 21.5 vol% with a pore size less than 10 µm, which is suitable for applications in contact with molten steel. In order to show thermal-shock resistance of the 3D-printed casting nozzle, a 100 kg steel-melt flow test was performed in a steel-casting simulator with the nozzle surviving all related thermal shocks as well as the ferrostatic pressure of the melt. The evaluated composition and production route of the filaments can be utilized to produce one-step, thermally debindable, thermal-shock-resistant refractory parts with a complex inner structure that are applicable in an industrial environment.

A Fundamental Investigation of Decarburization Reactions in the Argon–Oxygen Decarburization Converter Using Coupled Computational Fluid Dynamics and Thermodynamics Databases

Metallurgical converters such as the argon–oxygen decarburization (AOD) converter generally utilize gas blowing for the mixing and refinement of liquid steel. Due to the harsh environment of the complex and opaque system, it is common practice to study the stirring of the process through physical and numerical models. Effective mixing in the bath has an important role in refinement such as decarburization and has been vividly studied before. However, high‐temperature chemical reactions that also play a major role are sparsely investigated. With the help of modeling, a computational fluid dynamics model coupled with chemical reactions is developed, allowing the study of both dynamic fluid transport and chemical reactions. Herein, the chemical reactions for a single gas bubble in the AOD are investigated. The study shows that a 60 mm oxygen gas bubble rapidly reacts with the melt and is saturated with carbon in 0.2–0.25 s at low‐pressure levels. The saturation time is affected by the pressure and the composition of the injected gas bubble. The impact of ferrostatic pressure on the reactions is more significant at larger depth differences.

Simulación termofluidodinámica en un molde de colada continua de acero

In the present study, a 3D multiphysics mathematical model was solved using the finite volumes method to predict the phenomena of fluid flow and heat transfer in a continuous casting mold of 20CrMnTi steel billets. The results showed the high cooling capacity of the system for the formation of a progressive and uniform solid layer that begins very close to the meniscus and reaches a thickness of 10 % of the section of the casting at the exit of the mold. At the exit of the SEN, the liquid steel underwent a reflux with a depth of 0.45 m measured from the meniscus. Some of the rising reflux reached the meniscus and descended, infiltrating the walls of the mold. Velocities less than 0.2 m/s, in the upper zone of the mold, and great penetration of the steel jet, in the center of the mold, were observed. It was concluded, based on what is described in the literature, the following: a) the thickness of the solidified crust, at the exit of the mold, is sufficient to avoid breakage in the piece due to the ferrostatic pressure exerted by the liquid steel, c) The fluid dynamic conditions that occur in the upper zone of the mold can be counterproductive for the transfer of heat in the meniscus and the dissipation of overheating in the steel.

Formation Mechanism of a Wide-Face Longitudinal Off-Corner Depression During Thick Slab Continuous Casting

In practical continuous casting, consecutive longitudinal depressions often occur in the off-corner areas of slab wide faces, which may cause surface cracks in the depression areas and subsurface cracks beneath the depressions. To investigate the formation mechanism of the longitudinal depression, a 3D transient thermomechanical-coupled model was developed in the present work with consideration of the heat transfer in steel strand, thermal shrinkage of solidifying shell, ferrostatic pressure, contact with mold and guiding rolls, etc. Based on the model, evolution of the longitudinal off-corner depression and the associated thermal, distortion, and strain behaviors of steel slab were studied partly or fully throughout the continuous casting process. The results showed that the shell shrinkage in mold leads to an obvious shell-mold interfacial gap in the corner and off-corner areas. At the mold exit, the continuous expansion of corner gap at the wide-face side results in a thin spot forming at approximately 50 mm off the corner. When the shell moves out of the lateral strand guide rolls, narrow face of the shell bulges outward and rotates the corner, during which the thin spot in the wide-face off-corner area bends inward and shapes a longitudinal depression. As the shell moves down, the longitudinal depression deepens and widens in zones 2 to 4, and becomes stable in the subsequent cooling zones. When the shell moves through the soft reduction segments, depth and width of the longitudinal depression decrease as the marginal areas of the depressions are flattened. The model in this work is supposed to reveal an insight into the formation of consecutive longitudinal off-corner depressions at the wide face of slab during continuous casting.

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.

Considerations Regarding the Evolution of the Steel Thermal State in the Continuous Casting Plant

In continuous casting it is very important that the cooling system is correctly dimensioned. Improper cooling conditions can reduce the quality of the finished product and cause perforations of the solidified crust. In order to be able to minimize the occurrence of defects, it is preferable that, under the conditions of a given cooling system, the temperature distribution in the steel wire be known. It is also necessary to know the thickness of the solidified crust, especially immediately below the crystallizer, because a thin crust at this level can be perforated due to the ferrostatic pressure of the steel. The knowledge of the thermal state of the steel in the continuous casting installation is necessary for the transposition on mathematical bases of the technological process and the elaboration of some models of simulation of the solidification, which would allow to optimize the operation of the afferent installations.By considering the convection motion can be explained the influence of electromagnetic agitation and deformation of the steel wire on the incompletely solidified area on the quality and structure of cast steels; convective currents reduce overheating, which leads to a decrease in columnar crystallization in favour of axial crystallization. By running the simulation program were determined the variation of the thermal flux at different levels in the tundish, as well as the evolution of the temperature at the surface of the continuously cast steel thread.

Experiments Regarding Thermal and Mechanical Deformations of Continuous Cast Steel Wire

During the casting process, the crust of the steel wire is permanently subjected to thermo mechanical stresses (temperature variations, pulling, bending and straightening forces, ferrostatic pressure), which, when exceeding the deformation capacity of the crust, can lead to cracks.Therefore, it is necessary to know what kind of stresses act on the crust and how the steel behaves at high temperatures at certain process parameters (extraction speed, cooling intensity, etc.).

Improvement of three - dimensional bulge deformation model for continuous casting slab

An improved model was proposed to calculate the bugle deformation for fully considering the continuous casting production process by using finite element analysis. In this model, the temperature and the solidified shell thickness were gradually changing in the direction of continuous casting and the dynamic contacts between the slab and rolls were also established. Moreover, the ferrostatic pressure of molten steel between the solidified shells was fully considered for avoiding applying simple uniform pressure on the shell. The bulge deformation of the improved model had been compared with that of the traditional model. The results indicate that the bulging deformation on the wide side of the improved model is approximately greater than that of the traditional model and there is a big difference in the narrow side bulge deformation between the traditional model and the improved model. Therefore, the improved model should be adopted for accurately analyzing the deformation of the slab.

Optimization of the Gap Parameters of Supporting Rollers of Curvilinear Continuous Casting Machine

A study was performed to determine the influence of technological and design parameters of continuous casting of steel on reducing the thickness of the billet along the technological line of a curvilinear continuous casting machine (CCM) designed by Uralmashzavod OJSC. The effect of carbon content and casting speed on linear shrinkage through the billet’s thickness was investigated. It was established that in all cases there was a discrepancy between the thickness of the billet and the settings used for CCM rollers. This discrepancy can lead to defects in the billet caused by the distortion of its shape (owing to the blow-up under the action of ferrostatic pressure) and to an increase in friction and, accordingly, pulling forces. On the basis of the obtained results, new settings for the rollers on the hard and hydraulic sections were proposed. The use of recommended parameters considerably reduces the discrepancy between the thickness of the billet and the gap of the rollers, which will improve product quality.

Effect of Submerged‐Entry Nozzle (SEN) Design on Fluid Flow and Heat Transfer in a Thin‐Slab Steel Caster

Occurrence of breakouts (BO) during casting of narrow slabs is observed to be influenced by the submerged‐entry nozzle (SEN) design in one of ArcelorMittal's thin‐slab casters. To understand the root cause for the BO, three‐dimensional computational fluid dynamics (CFD) models are developed to simulate steady‐state and transient molten steel flow and heat transfer in the SEN and in the CSP caster for three four‐port SEN designs (type‐ A, B, and C), among which exist two major design differences, including the presence of a top insert at SEN tube entrance and height of the flow divider at SEN bottom. CFD models are validated by water model measurements of SEN port opening velocities. Both water model experiments and numerical simulation results suggest that SEN design with a taller bottom flow divider increases both mean and variations of the upper port flow rates and large‐scale asymmetric flows in the CSP mold region. Transient heat transfer simulation results further show that this unstable biased flow in the mold increases “hot spots” near the shell around the funnel‐to‐flat transition region, which could be responsible for local steel re‐melting and potential breakout under certain critical ferrostatic pressure.

Integrated System of Thermal/Dimensional Analysis for Quality Control of Gray and Ductile Iron Castings Solidification

The main objective of the present work is to introduce a specific experimental instrument and technique for simultaneously evaluating cooling curves and expansion/contraction of cast metals during solidification, adapted for commercial foundry use. The recorded data are processed using specialized software, which conveniently displays both cooling and contraction/expansion curves and their specific parameter values. Experiments compared hypoeutectic gray (GI) and ductile (DI) irons with white (WI) irons. Three important moments were found on the expansion/contraction-cooling curves: the start of eutectic freezing, the point of maximum expansion and the end of solidification. All of the tested irons have similar values for initial expansion up to the start of eutectic freezing (0.44%) due to the ferrostatic pressure, silica sand mold expansion, mold movement, etc. The maximum expansion, reached between the temperature of eutectic recalescence and the end of solidification, depends on the carbides/graphite ratio and graphite morphology: WI-0.465%, GI-0.552%, DI-1.032%, as averages. Graphitic expansion, absent for WI, increased to 0.109% (GI) and up to 0.596% (DI). For both GI and DI, the expansion at the end of solidification is only 6% lower compared to the maximum level, while for WI it decreased more than 50%. Specifically, the acceleration rate of the graphitic expansion up to the maximum level (Kgr1) is different compared with its deceleration to the end of solidification (Kgr2), and also are different for irons being tested, GI versus DI. Nodular graphite led to the (Kgr1) factor being 2.5 times higher, compared to GI, whereas only a slight difference was observed between GI and DI for the Kgr2 factor. Higher graphitic expansion in DI led to higher shrinkage sensitivity, compared to GI, as measured in furan resin mold test castings.

Initiation of Surface Cracks on Beam Blank in the Mold during Continuous Casting

Surface cracking seriously affects the quality of beam blanks, a relatively new blank in the continuous casting in China. In order to study the mechanism of the initiation and propagation of surface cracks, this study established a 2D micro-segmented model of the solidification process for a beam blank in the mold, with a user subroutine DFLUX written in Fortran. Using a contact algorithm, the stress in the shell of the beam was analyzed considering the mechanical properties of the material (Q235B), thermal stress, surface friction force and ferrostatic pressure. The results showed that at the center of the web, surface longitudinal cracks were most likely to initiate at a height of 180 mm from the meniscus; at the fillet, surface longitudinal cracks were most likely to initiate at a height of 200 mm from the meniscus. Moreover, the casting speed showed a greater effect on surface crack initiation than the pouring temperature did. This study reveals the cause of longitudinal crack initiation, and the most likely positions of cracks on the strand. Thus, it is instructive for controlling surface cracks in production.

СХЕМА НАПРЯЖЕННО-ДЕФОРМИРОВАННОГО СОСТОЯНИЯ НЕПРЕРЫВНО-ЛИТОГО СЛЯБА В ТЕХНОЛОГИЧЕСКОЙ ЛИНИИ МНЛЗ

The aim of the work is to develop a model of the deformation-stressed state of a continuous-cast billet, taking into account its behavior in the continuous casting machine line. A new physical model of the ingot crust bending process in the inter-roller gap has been proposed. The model is based on the regularities of the change in the deformed state of the surface of the crust under the action of ferrostatic pressure. It is established that the crust undergoes a cyclic alternating deformation in the process of moving it across the inter-roller gap. It is shown that the surface of the crust comes into contact with the surface of the roller before approaching the inter-roller gap. This causes the appearance of an anomalous area with a constant curvature of the crust at the final stage of its buckling. The second anomalous deformation zone of the ingot crust is the junction zone of the previous crust and the subsequent inter-roller gap, where the crust instantly changes its curvature. In combination with the flat scheme of the stress-strain state, a physical model is presented, which makes it possible to identify in each of the inter-roller gaps the zone of maximum values of the tensile strain rate. Identifying the zone of maximum values of the tensile strain indicator and comparing it with the maximum allowable one allows determining and correcting the length of the inter-roller gaps of the continuous casting machine.

Finite element simulation of bulge deformation for slab continuous casting

This paper aims to provide an investigation of the bulge deformation rules by using finite element analysis software, ABAQUS/Standard. A three-dimensional thermo-mechanical coupling model, considering the dynamic contact between the slab and rolls, has been established to study the temperature and bulge deformation distributions of the slab over the casting process. The simulated results of temperature profile were compared with the measured temperatures during the continuous casting process, the results of which showed a high agreement. Moreover, the bulge deformation rules of the slab were investigated systematically, especially at the thickness direction of the cross section which is the bulge deformation of the slab on the narrow side. A wave motion was found in the bulge deformation on the wide side of the slab cross section due to the joint effect of the ferrostatic pressure and the roller supporting force. The bulge deformation on the narrow side without the constraint from the roller support was cumulative, which was greater than the recovery count and compensated the bulge deformation on the wide side. In addition, the influences of various casting process parameters on bulge deformation were studied systematically in this paper. The results showed that bulge deformation on the narrow side and the wide side both increased with an increase in casting speeds and roll pitch. The method of fixed-gap and variable-diameter was suggested to be applied in the manufacture process in order to reduce the bulge deformation.

Development of a Mold Cracking Simulator: The Study of Breakout and Crack Formation in Continuous Casting Mold

Based on the mold simulator technology, a mold-cracking simulator has been successfully developed to study the process of breakout and the shell surface crack formation during the initial solidification of molten steel inside the continuous casting mold. First, a spheroidal protrusion was installed on the mold hot surface to mimic the abnormal force that generated by mold wall deformation, and then the external force was applied to the initial solidified shell, to facilitate the formation of breakout and shell surface cracks. Second, the responding temperature and heat flux across mold hot surface were recovered by an inverse heat conduction problem. The experimental results indicated that the mold breakout occurs around the shell tip by the combined efforts from external horizontal force, ferrostatic pressure, and thermal stresses during positive strip time. The breakout tends to introduce the peak of the responding temperature and heat flux across the mold hot surface. The vertical propagation velocity of the rupture point in the solidification shell has been calculated as 0.42 m/s in this study, which is in good agreement with industrial slabs. The paper also suggested that surface transverse crack formation is related to the segregation of sulfur during the initial solidification of molten steel.

Mathematical Modelling of Stress-Strain State of Multilayer Shell Molds

The analysis of the trends for development of technological processes to form shell molds shows that the accuracy and purity of the surface of shaped castings are one of the main criteria for the level of development of the science and technology of foundry engineering. The equations of continuum mechanics and thermal conductivity and the numerical method were used to build a mathematical model of SSS of multilayer shell mold to determine the temperature fields in a shell mold and solidifying casting, thickness of solidified layer of the casting, stress, strain, and displacement of shell layers against each other, as well as ferrostatic pressure on the metal form. The calculation results show that at metal casting, the highest temperature differences occur between the first and second layers of a shell mold, and the maximum values of the stresses and strains are typical for the first layer of a shell mold. The mechanism of the effect of temperature difference between the layers of the shell mold on fracture toughness of the shells and its critical value are determined, above which either a decomposition of a shell mold occur, or through-thickness cracking occur, which lead to the destruction of a shell mold. The ignition process of shell molds in the control filler has no significant influence on the formation of cracks, since the temperature difference between the shell layers does not reach critical values.

Generation Mechanism of Unsteady Bulging in Continuous Casting-2 -FEM Simulation for Generation Mechanism of Unsteady Bulging-

In the continuous casting of steel, unsteady bulging contributes to degradation of slab quality. It has been reported that unsteady bulging is promoted by uneven solidification in the mold, but the effect of uneven solidification on unsteady bulging has not been clarified. In this study, a Finite Element Method (FEM) simulation was conducted. Shell deformation was calculated by an elasto-plastic analysis assuming that the slab moves between the rolls, considering time dependency. The bulging value and mold level fluctuation, which change corresponding to the solidified shell thickness, ferrostatic pressure and roll pitch, were obtained.

連続鋳造における非定常バルジング発生機構−2 −非定常バルジング発生機構についてのFEMシミュレーション−

In the continuous casting of steel, unsteady bulging contribute to degradation of the slab quality. It has been reported that unsteady bulging is promoted by uneven solidified in the mold, but the effect of uneven solidified on unsteady bulging had not been clarified. In this study, a FEM (finite element model) simulation was constructed. Shell deformation was calculated by an elasto-plastic analysis assuming that the slab moves between the rolls, considering time dependency. The bulging value and mold level fluctuation, which change corresponding to the solidified shell thickness, ferrostatic pressure and roll pitch, were obtained. In the simulation results, the shell is deformed by ferrostatic pressure. The bulging shell pushes out under the rolls in the thickness direction, and unsteady bulging causes. While the shell is passing through the same pitch rolls, unsteady bulging becomes larger. When the solidified shell is uneven, stress concentrates on the thinner portions. The stress concentration accelerated the unsteady bulging even at the same average shell thickness. Based on this result, an operational index for suppressing unsteady bulging by reducing unevenness of the solidified shell is proposed.

Analysis of solidification behaviour of low alloy steel ingot casting – simulation and experimental validation

Solidification in a small experimental steel ingot casting was studied using finite element based simulation. Using the model, the phenomenon associated with fluid flow, temperature distribution, mushy zone formation, thermal gradient ahead of solidification front, local solidification time at various instances of solidification was examined. The heat transfer was found significant till a critical thickness of the solidified ingot. Air gap analysis during the solidification showed that, in spite of high ferrostatic pressure of liquid metal there is notable air gap in the ingot bottom. The model predicted the final piping shrinkage and some small zones of axial porosity formation. The experimental ingot showed a good match on piping shrinkage and porosity obtained from simulation. The microstructure formation in the experimental ingot could be correlated with simulation results. The approximate regime of columnar to equiaxed transition was estimated form the simulation and was matched with that obtained in the actual experimental ingot. The microstructures of the ingot at typical zones were examined in the ingot and correlated to local solidification time.