Mold Exit Research Articles

Large eddy simulation of various EMBr effects on the fluid flow, heat transfer and solidification process in an ultra-high speed thin slab casting mould with multi-port SEN

Effective control of intense turbulence is a crucial challenge for achieving steady production with ultra-high casting speed in thin slab continuous casting. In thin slab casting process, specially designed multi-port submerged entry nozzle (SEN) with four outlets is utilised to ensure an ample supply of molten steel, necessitating the selection and optimisation of suitable Electromagnetic Braking (EMBr) equipment for steel jet control. This study established a comprehensive three-dimensional model of a funnel-type mould, employing a combined experimental-numerical approach to validate and investigate the flow, heat transfer, solidification and electromagnetic behaviour in the mould. The steel grade studied in the simulation is Q235B, and its physical properties were calculated based on its composition with a temperature range of 1450–1826 K. To analyse the influence of high casting speed on the flow and solidification behaviour in the mould, three casting speeds were selected for the study, which were 6, 7 and 8 m/min. The results indicate that the novel Bowl EMBr significantly suppressed the penetration of steel jet and thus enhanced the thickness and uniformity of the solidified shell. As the casting speed increases from 6 to 8 m/min, the solidified shell thickness at the mould exit decreases from 7.91 to 5.94 mm, with the stagnant growth region approaching the mould exit. This highlights the requirement to correspondingly increase EMBr strength under ultra-high casting speed condition to avoid remelting of the shell and the risk of molten steel leakage. The coupled mathematical model established in this study provides guidance for optimising EMBr structures and casting speed under special multi-port SEN conditions, offering recommendations for the rational control of flow, heat transfer and solidification process in the mould.

A hybrid approach towards evaluation of average heat transfer coefficient in twin-roll strip casting mold

Continuous casting stands out as one of the most efficient and productive methods for producing billets, slabs, or thin sheets while conserving energy. Understanding the solidification dynamics and achieving defect-free sheets in continuous casting necessitates the accurate prediction of heat transfer coefficient (HTC) at the mold metal interface. Accurate prediction of HTC helps in selecting process parameters such as superheat temperature and casting velocity, which directly affect the production rate and quality of the cast component. Determining the HTC is a complex process, as it is influenced by various process parameters such as casting size, casting speed, superheat temperature, surface roughness, type of water used as a coolant, nozzle design and wettability. Therefore, the present study proposed a hybrid approach for finding HTC in a twin roll strip casting mold through a solidified shell shape, which is influenced by the factors affecting the heat transfer at the mold-metal interface. This approach integrates the experimental and simulation investigations. The numerical investigation used a three-dimensional coupled fluid flow and heat transfer equations with an assumed range of HTC (100–2000 W/(m2K)). The solidified shell shape for different HTCs was compared with the experimental solidified shape form at the outer wall of the mold using the solidification criteria. Results indicate that the initial HTC range of 100–1000 W/(m2K) does not lead to blockage of the graphite mold outlet. However, as HTC values approach 1000 W/(m2K) to 1400 W/(m2K), the melt obstructs the mold exit based on solidification criteria. The HTC values (1350 ± 150 W/(m2K)) obtained from the proposed methodology for mold, align well with the HTC values (1183 ± 593 W/(m2K)) calculated using lamellar spacing of the Al-Mg2Si composite microstructure. This hybrid approach presents a novel and straightforward method for determining HTC, offering accurate predictions that can enhance the precision of continuous casting models and provide a more nuanced understanding of solidification behavior.

Research on Solid Shell Growth during Continuous Steel Casting.

The continuous steel casting process must simultaneously meet the requirements for production performance, quality and safety against breakouts. Knowing the thickness of the solidified shell, particularly at the exit of the mould, is useful for the casting process control and breakout prevention. Shell thickness is difficult to measure during casting; in practice, it is predicted by indirect methods and models. But after undesired rupture of the shell and leakage of the liquid steel, it is possible to measure the shell thickness directly. This article is focused on the problem of the growth and measurement of the solid shell obtained after the breakout of a round block with a diameter of 410 mm. An original methodology was developed in which a surface mesh of points was created from the individual scanned parts of the block using a 3D laser scanner. Research has shown differences of up to 6 mm between the maximum and minimum shell thickness at the mould exit. A regression function of the average shell thickness on time was found. The results of the real shell growth were further used for the verification of the original numerical model of cooling and solidification of the round block.

Numerical investigation on role of vertical electromagnetic brake system in reducing remelting effect and improving thermal characteristics in thin slab continuous casting

The direct impact of high temperature liquid metal on narrow face of the continuous casting mould results in remelting in the jet impact region on narrow face which causes an adverse effect on solidified shell growth. In this work, a numerical study has been conducted to analyse the turbulent flow behaviour of the molten steel, temperature distribution inside the mould region, and the growth of solidified shell within the mould when V-EMBr (Vertical-Electromagnetic Brake) system is applied. The V-EMBr system significantly changed the extent of double-roll flow of liquid metal inside the mould by imposing a strong electromagnetic force to the liquid metal throughout the EMBr region especially at high magnetic fields. Meniscus velocities have been lowered with V-EMBr close to the magnetic pole region, which inhibits foreign inclusions within the molten pool near the narrow face regions. The alterations in fluid flow patterns inside the mould changed the thermal and hence solidification behaviour in the mould region, leading to favorable conditions for increasing productivity and controlling cast quality. The reduction in non-uniform shell growth and improvement in shell thickness obtained with the application of V-EMBr may help in reducing surface defects, and providing stronger shell at the mould exit.

Effect of Mold Electromagnetic Stirring on Metallurgical Behavior in Ultrahigh Speed Continuous Casting Billet Mold

A three‐dimensional model of fluid flow, heat transfer, solidification, and inclusion motion in billet mold at ultrahigh casting speed under electromagnetic field is developed. The low Reynolds number k–ε model coupled with electromagnetic model and Lagrangian discrete phase model is used to investigate the flow field, temperature field, solidification, and inclusions transport under different current intensities. The results show that mold electromagnetic stirring (M‐EMS) significantly alters the flow pattern of molten steel. The impact depth of the molten steel decreases as the stirring current intensity increases, and the horizontal swirl intensities of the molten steel increase with the stirring current intensity. As the current intensity increases from 500 to 700 A, the impact depth decreases from 0.637 to 0.575 m and the maximum tangential velocity increases from 0.477 to 0.898 m s−1. When the stirring current is intense, the flow of molten steel near the mold exit is reversed, and the molten steel flows asymmetrically and unsteadily. The M‐EMS effectively improves the transverse heat transfer of the molten steel in the mold, contributing to the dissipation of the molten steel superheat and the growth of the solidified shell. In addition, the removal ratio of the inclusions is improved significantly.

Thermo-mechanical behaviour analysis of slab corner during special steel continuous casting

ABSTRACT The corner crack is an important issue during special steel continuous casting. To comprehensively investigate the heat transfer, stress, and shrinkage behaviours of slab corner, a full-scale thermo-mechanical model of a slab and its mould was developed, the inverse algorithm was applied to the model. The model was validated by comparing the calculated data with the measured data in the plant trials. The results showed that the thermo-mechanical behaviours of the slab corner showed a similar tendency but were not the same, which reflected the necessity of building a full-scale model. There exists ‘hot spots’ at the slab off-corner region, and its position moved away from the corner along the casting direction, which affects the temperature and stress distributions. The maximum stress value reached approximately 16 MPa at the mould exit. The model provides an in-depth understanding of slab corner during special steel continuous casting.

Effect of initial solidification characteristics on longitudinal crack formation in thin slab caster

ABSTRACT Longitudinal surface crack has been one of the serious problems of thin slab casting (TSC). The present study was undertaken to comprehend the characteristics of longitudinal facial crack (LFC) and minimise the same in hot rolled low carbon steel sheets produced through thin slab casting and rolling (TSCR). Macro-structural characteristics of solidified shell has been investigated and correlated with plant observations. The centre and edges of funnel region showed maximum propensity of LFCs. TSC solidification is initiated with fine columnar dendrites which gradually coarsened towards the slab centre. This has been attributed to the characteristics of liquid steel flow instability in the upper mould region. Meniscus flow instability may lead to non-uniform heat transfer besides mould flux entrapment in the solidifying shell. Examination revealed that the mould hot faces ware covered with brass containing voids and cracks. Brass formation was found to be the most prominent around the meniscus of the wide hot face, which gradually faded out towards the mould exit. Tramp elements (zinc and lead) in liquid steel were found to be responsible for harmful brass formation during casting. The present findings were implemented in plant operation to minimise its concentration.

Numerical Simulation of the Fluid Flow, Heat Transfer, and Solidification in Ultrahigh Speed Continuous Casting Billet Mold

Ultrahigh‐speed casting is the most remarkable feature of billet high‐efficiency continuous casting. The transient fluid flow, heat transfer, and solidification behavior under different casting speeds, submerged entry nozzle (SEN) parameters, and mold electromagnetic stirring (M‐EMS) are investigated using a 3D transient mathematical model. The results show that a typical roll flow pattern is generated for all casting speeds, while a third small recirculation zone developed near the solidified shell at the casting speed of 6.0 m min−1. When the casting speed increases, the impact depth of the molten steel increases from 0.748 to 0.844 m, the velocity and level fluctuation at the steel/slag interface increases, the high‐temperature zone moves downward, and the shell thickness is reduced from 16.2 to 11.1 mm. As the SEN inner diameter and immersion depth increases, the impact depth increases, the velocity and level fluctuation at the steel/slag interface decreases, and the shell thickness at the mold exit is ≈11.0 mm. When the SEN immersion depth and inner diameter are 120 and 40 mm, respectively, the flow and temperature fields in the mold are the most appropriate. In addition, the M‐EMS have a great effect on the fluid flow and heat transfer behavior.

Heat transfer behavior in ultrahigh-speed continuous casting mold

AbstractThe growth rate of the shell thickness in the funnel mold for ultrahigh-speed continuous casting of steel has a significant effect on the shell surface quality. By using the node temperature inheritance algorithm, a three-dimensional transient thermal conductivity model of the liquid steel solidification inside the mold was established, and the effects of casting temperature and speed on the heat transfer behavior of the thin slab were investigated. The results show that with an increase in the casting speed from 4.0 to 6.0 m·min−1, the maximum thickness of the shell at mold exit was reduced from 26 to 12.8 mm, and the maximum temperature of the slab surface at mold exit increased from 1,210 to 1,305°C. However, the increase of casting temperature from 1,550 to 1,560°C had little effect on the surface temperature and thickness of the slab shell.

Fluid flow, solidification and solute transport in billet continuous casting with different stirrer positions

ABSTRACT A 3D mathematical model of 180 mm × 220 mm billet continuous casting process has been established, which includes electromagnetic field, flow field, solidification and solute transport, and the effect of stirrer position on multiple physical fields distribution is investigated. The results show that, when the stirrer centre positions are located at Y = 515 mm, Y = 615 mm and Y = 715 mm, the growth of the solid shell near the stirrer centre slows down. And the solid shell is remelted and thinned at the mould exit for the stirrer centre position placed at Y = 815 mm. As the stirrer centre position is lowered, the carbon concentration at the mould top reduces gradually. Furthermore, when the stirrer centre position is moved from Y = 515 mm to Y = 815 mm, the carbon concentration on the surface of billet reduces from 0.00211 to 0.00197.

Effect of mold corner structures on the fluid flow, heat transfer and inclusion motion in slab continuous casting molds

Mold corner structures will influence the multiphase flow, heat transfer and inclusion motion behaviors in the mold, which have significant impacts on the quality of strands. Herein, a three-dimensional fluid flow and heat transfer mold model was employed to investigate the effect of four different corner structures (right-angle, big-chamfered, multi-chamfered and fillet) on the multiphase flow, heat transfer and inclusion motion behaviors in the mold. The Volume of Fluid model was used to describe the molten steel and slag, while the Discrete Phase Model was employed to track the motion of non-metallic inclusions. The dynamic mesh was applied to simulate the mold oscillation. Results show that the chamfered structures can obviously increase the flow velocity around the strand corner, especially for the big-chamfered mold. The chamfered molds can remarkably improve the corner temperature of strands at the mold exit, and the fillet mold is the most effective. The temperature on the narrow and chamfered faces of the copper plate in the chamfered molds is lower than that in the right-angle mold. The maximum amplitude of temperature fluctuation of the meniscus reaches 4.6 K. The shell thickness around the strand corner in the chamfered molds is obviously thinner than that in the right-angle mold. In addition, the chamfered structures will increase the number of inclusion particles around the strand corner.

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.

Effect of M-EMS current intensity on the subsurface segregation and internal solidification structure for bloom casting of 42CrMo steel

ABSTRACT As-cast inherited defects are usually observed especially in quenched and tempered (QT) steels, such as 42CrMo due to macro-segregation. A three-dimensional numerical model has been presented to reveal the heat mass transportation and solidification behaviour of the molten steel in a 250 mm × 280 mm continuously cast bloom with and without M-EMS application. The reliability of the coupled model was proved by comparing the measured data of magnetic flux density with the calculated one along the centreline. The research indicates that the phenomena of a decrease in the solidification rate and an increase in molten steel washing velocity are observed as the current intensity increases from 0 to 450 A, which leads to a decrease of the minimum segregation degree by 0.039 and an increase of the width of the harmful negative segregation band by 3.6 mm. The washing effect that increases with the current intensity can reinforce the heat transfer between molten steel and solidification front and stimulate the formation of the equiaxed crystal. Accordingly, the average, local temperature gradient, and the superheat degree at the centre of mould exit decrease by 0.12, 2.62 K·mm−1, and 2.9 K, respectively, together with an increase of the equiaxed crystal ratio by about 9.44%. It suggests that the M-EMS may lead to the more serious subsurface negative segregation but can improve the equiaxed crystal ratio of bloom castings, which influences the heat treatment performance and flaw detection pass rate of subsequent rolled products.

A novel method for determining the three dimensional variation of non-linear thermal resistance at the mold-strand interface in billet continuous casting process

A three-dimensional numerical model has been developed to study the turbulent flow of the molten steel, heat transfer from the solidifying strand to the cooling water and the subsequent solidification within the mold of an industrial billet continuous casting process. Particular attention is paid to calculate the unknown non-linear thermal resistance at the mold-strand interface, based upon a physical formulation. The model results are validated with the data available in the literature, as well as from plant experimental data. Recirculation flows are observed near the mold walls, causing remelting of solidified shell. The results obtained from the simulations show that one of the major reasons for higher value of solidified shell thickness at the corner region than that at the off-corner region is the nature of recirculatory flows. The thermal resistance is found to vary along the casting direction as well as across the mold face. The plant data of solid shell thickness of the strand at mold exit shows that the current model involving physical formulation of thermal resistance at the mold-strand interface, provides a better estimation of the thermal behavior, compared to previous models involving empirical formulation for the thermal resistance or heat flux profiles at the interface.

Optimization of cooling conditions to avoid surface cracks in Direct Chill casting of Cu-Fe-P alloy

The industrial production of Cu-Fe-P billet Direct Chill (DC) casting exhibits often difficulties related to the generation of surface cracks on the cast piece. The demand for always higher productivity leads to increased casting speed with significant cooling requirements. This in turn affects the thermal gradient within the billet leading often to stress concentration responsible for surface cracking. In order to deal with this issue while keeping productivity at high levels lowering the industrial cost, a 2D thermo-mechanical Finite Element (FE) model was developed, which was applied and validated based on industrial conditions. The model targets to predict the material failure during DC casting. Based on the proposed approach the billet’s temperature profile and its stress state during secondary cooling are calculated. The simulation results revealed that strong cooling using water jets cannot be applied immediately at the exit of the mold at any casting speed. However, as the distance of the secondary cooling unit from the mold exit increases, the stress concentration drops significantly, so that the jet nozzles can be applied from 750 to 2500 mm away from the mold, depending on the casting speed, without material failure. On the contrary, milder water spraying applying a lower local heat transfer coefficient, but on the same time covering broader impact surface, can result in a more efficient billet cooling. Thus, due to milder cooling the cast-piece material is less prone to failure and cooling can be applied immediately at the mold exit. Furthermore, a sequence of numerous spray arrays can further increase the cooling capacity. Concluding, it is shown that a validated thermo-mechanical 2D FE model can provide deeper insights used industrially as a setting tool, which optimizes the casting conditions in order to further improve productivity and the quality characteristics of the as-cast billet.

Numerical simulations of solidification and hot tearing for continuous casting of duplex stainless steel

Hot tearing is one of the major defects in continuous casting of steels, which severely limits the productivity of steelmaking processes. To further understand the defect, the problem of hot tearing in duplex stainless steel produced by a vertical continuous caster was investigated. A three-dimensional heat transfer and elastic–plastic model was developed based on the realistic roller layout in continuous slab casting, using ProCAST software. According to the hot tearing indicator criterion, the influence of operating parameters on the hot tearing susceptibility was evaluated. The results show that the surface temperature distribution is not sensitive to the superheat. The center of wide surface shell at the mold exit is the thinnest, and the thickness is about 10.52 mm at the superheat temperature of 40 °C. The hot tearing mainly concentrates on the slab solidification front and near the narrow face. However, corner cracks are prone to appear near the corner. With the increase in casting speed and the decrease in the cooling intensity in the secondary cooling zone, the solidification end point is rushed, which leads to the position of hot tearing lowering accordingly.

Growth of solidified shell in bloom continuous casting mold of hypo-peritectic steel based on a FeS tracer method

Solidification behavior in the mold region plays an important role in production efficiency and steel quality. To investigate shell growth within a mold, the sulfur prints of the entire shell thickness profile from the meniscus to 100 mm below the mold were obtained by adding FeS tracer into molten steel during bloom continuous casting of hypo-peritectic steel. The law of shell thickness evolution along mold height and circumference was analyzed. The results show that there are three weak regions of solidification, which are in the mold upper part, in the mold lower part, and just below mold exit, possibly resulting from periodic fluctuation of air gap between the shell and the mold, the impingement of melt jets on the solidification front, and the decreasing cooling intensity, respectively. Initial solidification point along casting direction appears at approximately 35 mm below the meniscus. Overall, the solidified shell thickness in the inner side of the mold is a little larger than that in the outer side, and the former and the latter reach 25.5 and 24.3 mm at the mold exit, respectively. The non-uniform shell growth in the inner side of the bloom is provided, while shell thicknesses in the narrow face and the outer side follow relatively regular growth. Out of the mold, the thinnest shells on the transverse section exist in the regions of 60–90 mm and 40–70 mm from the corners of the inner and outer sides, respectively, i.e., the off-corners.

Effect of the Gap Between Copper Mold and Solidified Shell on the Fluid Flow in the Continuous Casting Strand with Mold Electromagnetic Stirring

Considering the gap between the copper mold and the solidified shell (mold‐bloom gap), the electromagnetic field, fluid flow, solidification, removal of inclusions, and slag entrainment in the bloom continuous casting process with mold electromagnetic stirring (M‐EMS) are numerically investigated. As the mold‐bloom gap is considered, the electromagnetic force increases with the distance from the meniscus and reaches a maximum value near the stirrer center. With the distance further increasing, the electromagnetic force decreases continuously. With the mold‐bloom gap ignored, there are two peaks of the electromagnetic force along the distance below the meniscus. One is 40 mm above the stirrer center and the other is near the mold exit. When the mold‐bloom gap is considered, the velocity magnitude is higher and the temperature field on meniscus is more uniform. The net slag entrainment rate is 0.0144 kg s−1 and is higher than that ignoring the mold‐bloom gap. Inclusions are more removed with considering the mold‐bloom gap.

Initial Transfer Behavior and Solidification Structure Evolution in a Large Continuously Cast Bloom with a Combination of Nozzle Injection Mode and M-EMS

A three-dimensional numerical model combining electromagnetic field, fluid flow, heat transfer, and solidification has been established to study the effect of nozzle injection mode and mold electromagnetic stirring (M-EMS) on the internal quality of a continuously cast bloom. The model is validated by measured data of the magnetic flux density along the stirrer center line. According to the simulation and experimental results, M-EMS can introduce a horizontal swirling flow ahead of the solidification front, promoting the superheat dissipation of molten steel and columnar to equiaxed transition (CET). As the stirring current increases from 0 to 800 A, the superheat at the mold exit in the bloom center decreases by 1.9 K for the single-port nozzle case and 3.8 K for the five-port nozzle case. The resulting increase in the equiaxed crystal ratio is about 5.65% and 4.06%, respectively. In comparison, the injection mode shows a more significant influence on the heat transfer and solidification structure in the bloom under the present casting conditions. The superheat at the mold exit in the bloom center decreases by 5.1–7.7 K as the injection mode changes from a single-port nozzle to a five-port nozzle, and the increase in the equiaxed crystal ratio ranges between 14.8% and 17%. It is found that the flow velocity of the molten steel in front of the solidification interface for the five-port nozzle is higher than that for the single-port nozzle regardless of the M-EMS power. The washing effect here reinforces both the heat exchange through the solidification interface and the dendrite re-melting or fragmenting, stimulating the formation of an equiaxed crystal at the bloom center. In addition, it is observed that both the central shrinkage and carbon segregation have decreased with the five-port nozzle plus M-EMS. This suggests that the combined application of a five-port nozzle and M-EMS can effectively improve the internal quality of large bloom castings.

Influence of mould curvature on the metallurgical performances of beam blank continuous casting

ABSTRACT A 3-D finite volume mathematical model was established by customizing pull velocity field function of solidified shell, to simulate the coupled fluid flow, heat transfer, and solidification of beam blank continuous casting, taking into consideration of the effects of mould curvature on the metallurgical performances. When the central impact stream reaches to a certain depth, part of it circulates to scour the solidified shell, causing the remelting of shell at fillet and narrow face centre from about 370 mm down the meniscus to mould exit. The differences of strand surface temperature and shell thickness between inside and outside radius caused by the mould curvature are obvious only at the fillet of the mould exit. The fillet of outside radius having the highest temperature but the thinnest solidified shell is prone to arising quality problems. The metallurgical performances under different casting speed and superheat indicated that the strand surface temperature arises and shell thickness decreases integrally at the mould exit as the casting speed increases. The superheat has the same influence trend as the casting speed does, but the sites it affects are concentrated in the fillet and narrow face centre.