Mold Exit Research Articles

The Interrelation between Casting Size, Steel Grade, and Temperature Evolution Along the Mold Length and at the Strand Surface during Continuous Casting of Steel

The present work focuses on the investigation of thermal profiles in copper molds and at the strand surface during continuous casting of different steel grades in an industrial plant. Thermocouples embedded in the mold walls were used to measure temperatures along the mold length. Noncontact pyrometers positioned at different locations along the machine monitored the strand surface temperatures. The experimental results permitted an interrelation between steel grade, mold section (240 × 240 mm, 180 × 180 mm, and 150 × 150 mm), and mold wall, and strand surface temperatures to be established as a function of casting operating conditions. It is shown that the mold outer face (external curved wall) has the highest temperature distribution from the meniscus to the bottom of the mold followed by the inner and side faces, respectively. The deepest meniscus is shown to occur for the 150-mold, and the 180-mold is shown to have the highest temperature profiles along the mold length in all faces examined. Low carbon steels present the highest strand surface temperatures along the machine when compared with those of medium and high carbon steels. When the steel composition is parameterized, it is shown that the 150-mold has the smaller strand surface temperature close to the mold exit when compared with the 180 and 240 molds, and it is shown that this behavior changes after the unbending point.

Mathematical Modeling of Hot Tearing in the Solidification of Continuously Cast Round Billets

Billets produced by continuous casting sometimes show the presence of subsurface cracks that can compromise the quality of the final product. The presence of these cracks is revealed by Baumann prints of billet cross sections in which the chill zone is visible and the short radial cracks are located only where the chill zone thickness is thinner. This experimental finding induces the hypothesis that cracks are formed as a result of the presence of unevenness in the mold heat extraction around the billet perimeter. Cracks start to open in the dendritic front in regions where the shell growth in the mold is slower. The study presented in this article focused on steels with a sulfur content of about 300 ppm. The Baumann prints taken from billet samples of numerous different heats allowed detecting the presence of subsurface cracks and their location nearby visible chill zone thinning areas. To understand the mechanisms of crack formation and to define the possible corrections, a modeling activity has been carried out using the finite element technique on 148-mm diameter billets continuously cast at TenarisDalmine (Dalmine, Italy). The model performs a two-dimensional thermomechanical analysis of the solidification in the mold and within about 4 cm below the mold exit, along which the shell surface is cooled only by radiation to the environment, before the sprays of the first ring impact on the strand. The model includes the contact of the shell with the mold inner surface, which moves according to taper and distortion (this last part is calculated by means of a separate mold model); the steel creep behavior; the calculation of the heat transfer through the gap depending on the local mutual distance between the two surfaces; the effect of the liquid steel fluid dynamics on the solidification growth as a result of the temperature distribution; and the calculation of a hot tearing indicator represented by the porosity fraction caused by mechanical strains applied at the dendrite roots. From the simulation results, it is concluded that subsurface cracks are generated in the space between the mold exit and the first cooling ring; the involved mechanisms of formation also are withdrawn. Nucleation of MnS precipitates of large dimensions is an additional cause of defectiveness in controlled sulfur steels. As a final conclusion of this work, the most important actions to eliminate subsurface cracks are derived.

A note on casting speed of Ohno continuous casting process

The Ohno continuous casting process, known as the OCC process, is a heated mould crystallisation process that permits the generation of single crystal or cast products with a unidirectional structure. In this process, solidification takes place at the mould exit. Thus, the understanding of the process parameters is crucial to the successful application of this technology. The present note is aimed at clarifying the often misunderstood factors influencing the casting speed of the process.

Continuous extruding extending forming of semi-solid A2017 alloy

Continuous extruding/ extending forming process for A20/7 alloy in semisolid state was proposed through installing extending die at the outlet of shearing-cooling-rolling (SCR) machine. A series of experiments to produce flat bar of A 2017 alloy were carried out. The forming process, metal flow behavior in die and microstructure and mechanical property of products were investigated. It is shown that if the pouring temperature of melt was higher, the die was filled with semi-solid slurry with low solid fraction and periodical cracks would occur on the product surface; If its pouring temperature was lower or the preheating temperature of die was lower, semisolid slurry would solidify rapidly and block the die after entering the cavity. The analysis of mass flow trace shows that the semi-solid slurry moves forward layer by layer and fills the die extending caviiy in radiation manner and the velocity of mass flow in the central area of extending cavity and exit of mould is the maximum, and then decreases gradually from the center to both sides of die wall. By increasing the die extending angle, the velocity of mass flow becomes more homogeneous. Under rational process control and die design, the A2017 flat bar with transverse section of 10 × 50 mm and with good surface and fine equiaxed grains can be obtained by continuous extruding/ extending forming process. The product’s tensile strength and elongation are 420.5 MPa and 14.2%, respectively.

Modeling transient slag-layer phenomena in the shell/mold gap in continuous casting of steel

Mold-slag friction and fracture may cause heat-transfer variations in continuous casting, which leads to steel shell temperature and stress variations, resulting in surface cracks. Analytical transient models of liquid slag flow and solid slag stress have been coupled with a finite-difference model of heat transfer in the mold, gap, and steel shell to predict transient shear stress, friction, slip, and fracture of the slag layers. The models are validated by comparing with numerical models and plant measurements of mold friction. Using reported slag-fracture strength and time-temperature-transformation (TTT) diagrams, the models are applied to study the effect of casting speed and mold-powder viscosity properties on slag-layer behavior between the oscillating mold wall and the solidifying steel shell. The study finds that liquid-slag lubrication would produce negligible stresses. A lower mold-slag consumption rate leads to high solid friction and results in solid-slag-layer fracture and movement below a critical value. Crystalline slag tends to fracture near the meniscus and glassy slag tends to fracture near the mold exit. A medium casting speed may be the safest to avoid slag fracture, due to its having the lowest critical lubrication consumption rate. The high measured friction force in operating casters could be due to three sources: an intermittent moving solid slag layer, excessive mold taper, or mold misalignment.

Optimisation of continuous casting process using genetic algorithms: studies of spray and radiation cooling regions

A detailed optimisation study has been carried out of the continuous casting process, starting at the exit of the mould region and continuing up to the end of the radiation cooling zone. Biologically inspired genetic algorithms (GAs) have been used to determine the maximum casting speed and solidified shell thickness at the mould exit. Heat transfer models are proposed for both the spray and radiation cooling regions and the cooling water requirements are optimised using GAs. A number of spray bank configurations, studied earlier by experimental researchers in this field, are analysed separately. The extent of surface reheating below the sprays is predicted using a Newton-Raphson procedure. The results are compared with data existing in the literature and found to be in good agreement.

A numerical analysis of fluid flow, heat transfer and solidification in the bending-type square billet continuous casting process

A numerical modeling system was developed which can simulate the transport phenomena of a bending type square billet continuous casting process. Fluid flow and heat transfer were analyzed with a 3-dimensional finite volume method (FVM) with the aid of an effective heat capacity algorithm for the solidification. For a complex geometry of the bending type billet caster, a body-fitted-coordinate (BFC) system was employed. The bent structure of the caster allows a recirculating flow to develop in the upper and outer-radius region and the main stream to shift toward inner radius. This causes the thinner solid shell in the inner radius region than in the outer one. Besides standard operation conditions, we have analyzed the results when casting speed, caster shape, and tundish superheat changes. Lower casting speed makes the solid shell thicker by reducing heat flux from the mold. In the vertical caster, solid shell thickness are more uniform than that in the bending-type in entire region. When superheat increases by 5°C, solid shell thickness at the mold exit becomes thinner by 1 mm.

A Simple Estimation Method for Shell Thickness at the Mold Exit in the Continuous Casting of Steel.

A Simple Estimation Method for Shell Thickness at the Mold Exit in the Continuous Casting of Steel.

Mathematical modeling of copper and brass upcasting

A study has been performed to establish basic knowledge in heat transfer and solidification for copper and brass upcasting. The study combined pilot scale measurements, mathematical modeling, and metallographic examination of the cast rod samples. The pilot scale measurements involved temperature measurements with several thermocouples inserted in the copper jacket of the mold. Temperature measurements of the surface of the cast rod were carried out as well. A three-dimensional (3-D) mathematical model of the copper mold and graphite die was constructed to characterize the heat flux profiles quantitatively from the measured mold temperature data. The heat flux was observed to have a maximum value near the first contact point between the copper mold and the graphite die and to decrease rapidly with increasing distance up to the mold. The calculated heat flux profiles were used as boundary conditions for another mathematical model, which calculated temperature profiles in the cast rod. A model for estimation of material data and microstructure was used for simulating the thermophysical data needed in the calculations and to predict certain microstructural properties in the cast rods. The calculated surface temperatures of the cast rod at the mold exit agreed well with the measured temperature values. Also, the calculated microstructural properties, such as secondary dendrite arm spacing, phase distribution, and microsegregation of zinc, were in good agreement with the measured ones.

高速水膜による鋳型直下強冷却法

高速水膜による鋳型直下強冷却法

Modeling the thin-slab continuous-casting mold

A three-dimensional mathematical model has been developed to compute the thermomechanical state in the mold of thin-slab continuous casters. The thin-slab mold differs from those used in conventional slab casters in that the upper portion of the broad side walls defines a funnel-shaped chamber which allows the nozzle to be submerged into the liquid metal. The chamber converges with distance down the mold, reducing to the rectangular cross section of the finished casting near the mold exit. The new mold, along with casting speeds up to 6 m/min, allows slabs to be cast 50–60 mm thick, compared with 150 to 350 mm in conventional continuous slab casting. However, the mold shape and high casting speed lead to higher mold temperatures and shorter mold life than are found in conventional slab casters. In this article, we develop mathematical models of the process to determine the role of various process parameters in determining the mold life. Finite-element analysis is used to determine the temperatures in the mold and cast slab, and these data are then used in an elastic-viscoplastic analysis to investigate the deformation of the mold wall in service. Cyclic inelastic strains up to 1.75 Pct are found in a region below the meniscus along the funnel edge. These large strains result from the combination of locally high temperatures coupled with geometric restraint of the mold. The deformation leads to short mold life because of thermal fatigue cracking of the mold. The computed locations and time to failure of the mold in fatigue agree very well with observations of the appearance of mold surface cracks in an operating caster. The models are also used to develop an improved mold design.

Effects of Process Variables on Cast Surface Quality of Alloy Rod Produced by the Horizontal Ohno Continuous Casting (OCC) Process

Sn–Pb alloys in the compositional range 2.5%Pb–22.5%Pb have been continuously cast by the OCC process in order to investigate the causes of cast surface defects occurring in the OCC system. Hot tears often occurred, owing to friction with the mold surface, at lower lead contents such as 2.5%Pb and 7.5%Pb additions. At higher lead contents, however, the tendency for hot tearing diminished because of an increase in the liquid layer which eliminates friction at the strand surface. The liquid layer at the surface can be increased not only by increasing solute content but also by moving the liquid-solid zone more outside the mold. The thickness of this liquid layer and the mold surface conditions are prime factors influencing cast surface quality. The cast rod surface became rough by decreasing the mold exit temperature, the casting speed, or the mold-cooler distance. The roughness on the surface was caused by the protrusions of tin dendrite arms in the eutectic region. The cause of the protrusions can be attributed to solidification shrinkage arising from an insufficient supply of liquid to the strand surface. Thus, an appropriate metal head must be chosen in order to obtain cast products with a high surface finish. Résumé Des alliages Sn–Pb dont la composition en plomb varie entre 2.5–22.5% ont été coulés en continu par le procédé horizontal Ohno (OCC) afin de déterminer les causes des défauts de surface apparaissant dans le système OCC. La formation de larmes à chaud est fréquente pour de faibles teneurs en Pb (2.5–7.5%), principalement à cause de la friction avec la surface du moule. A plus haute teneur en plomb, cette tendance s'estompe car l'augmentation de la fraction liquide élimine la friction à la surface de la tige. La couche liquide à la surface peut être augmentée non seulement par l'augmentation de la concentration en soluté mais aussi en déplacant la zone liquide–solide un peu plus à l'extérieur du moule. L'épaisseur de cette couche liquide et les conditions de surface du moule sont les facteure principaux qui influencent la qualité de surface de la coulée. La surface de la tige devient plus rugueuse lorsqu'on abaisse la tempérapture de sortie du moule, la vitesse de coulée, ou la distance moule-refroidisseur. La rugosité de surface est causée par la protrusion des bras dendritiques d'étain dans la région eutectique. La cause des protrusions peut être attribuée au retrait de solidification découlant d'un manque de liquide à la surface de la tige. Donc, un front de métal approprié doit être choisi pour assurer un produit coulé ayant un fini de surface de grande qualité.

Experimental Study of the Horizontal Ohno Continuous Casting System

Pure tin rod of 8.5 mm diamétér has been cast using the horizontal Ohno continuous casting (OCC) system. Thermal profile measurements have been made during casting in order to obtain an understanding of the phenomena which occur within the OCC mould. The influence of interactive casting variables on the temperature gradient in the liquid ahead of the solid-liquid interface, the superheat within the mould, and the solid-liquid interface position have been investigated. It was found that the superheat of the melt within the mould increased linearly with decreasing casting speed and decreasing mould-eooler distance. It was also found that the temperature gradient is sensitive to the mould exit temperature whereas it remains independent of the casting speed within the range investigated. Heat transfer coefficients have been evaluated and conditions established for the production of single crystals. Résumé Une tige d'étain pure de 8.5 mm de diamètre a été coulée au moyen du systéme de Coulée Continue Horizontale Ohno (OCC). Des mesures de profil thermique ont été enregistrées durant la coulée afin de comprendre les phénomènes qui interviennent à l'intérieur du moule OCC. L'influence des variables interactives de coulée sur le gradient thermique dans le liquide en avant de l'interface liquide-solide, sur la surchauffe dans lemoule ainsi que sur la position de l'interface, a été étudiee. Il apparait que la surchauffe du liquide a l'interieur du moule augmente linéairement avec une diminution de la vitesse de coulee et de la distance moule-refroidisseur. Le gradient thermique est également sensible à la température de sortie du moule bien qu'il soit indépendant de la vitesse de coulée pour l'intervalle de température étudié. Ces coefficients d'échange de chaleur ont été évalués et des conditions de production de monocristaux ont été établies.

A New Fabrication Method for the Casting of Cored Materials

Using a heated double-channel mould, cored rods with pure tin as a clad material and Sn-Bi, Sn-Pb, and Sn-Zn eutectic alloys as core materials have been continuously cast. With precise control of casting variables, cored rods were successfully produced with a unidirectional structure for both core and clad materials. The effects of casting speed and mould exit temperature on cast structure were examined in an attempt to obtain an understanding of this new casting process. It was found that the dissolution of clad material occurs during casting, and the dissolution process is rapid and sensitive to the process variables. Excess dissolution of clad material leads to metal breakout. In order to reduce the dissolution, lower mould exit temperatures with faster casting speeds should be employed.

Mold behavior and its influence on quality in the continuous casting of steel slabs: Part II. Mold heat transfer, mold flux behavior, formation of oscillation marks, longitudinal off-corner depressions, and subsurface cracks

Axial heat-flux profiles have been determined quantitatively from temperature measurements conducted on a slab mold under routine operating conditions. As in earlier studies, the heat flux was observed to have a maximum value at the meniscus and to decline with increasing distance down the mold. The mold heat flux increased with increasing casting speed and was greater with a mold powder having lower viscosity and melting point being applied as lubricant. The heat extraction was largest while casting 0.29 pet carbon steel and least for a 0.09 pet carbon grade; reducing the depth of the submerged entry nozzle increased the heat flux slightly in the upper region of the mold. Most significant was the higher heat flux observed at the meniscus of the outside-radius face, attributable to the locally greater copper plate thickness compared to that of the opposite broad face. All of the measurements can be explained straightforwardly by heat flow in the vicinity of the meniscus and the resulting behavior of the so-called slag rim adjacent to the mold wall. It is postulated that the difference in copper plate thickness between the two broad faces at the meniscus causes the slag rim to be smaller on the outside-radius face which gives rise to shallower oscillation marks, as observed, higher heat transfer, and a slightly thicker solid shell. The dissimilar behavior has implications for quality because the inside-radius shell, experiencing reduced heat extraction, cools and shrinks less than the outside-radius shell. Thus, for a given end-plate taper, the narrow face of the slab adjacent to the inside radius can push against the end plate, accelerating copper wear, and, owing to squeezing of the broad face, cause an off-corner depression and subsurface crack toward the mold exit. If this is correct, maintenance of the same copper plate thickness at the meniscus is fundamental to preventing such an occurrence. Moreover, adjustment of the heat extraction at the meniscus should be achievable by changing copper plate thickness, mold coating thickness/conductivity, cooling water velocity, cooling channel configuration, and mold flux composition for a given steel grade.

An experimental study on process variables in crystal growth by ohno continuous casting

Crystal growth, by the recently developed Ohno continuous casting (O.C.C) process, was studied using pure tin. The effects of four process variables on crystal growth were studied: these were the mold exit temperature, the casting speed, the melt temperature, and the cooling condition. Small single crystals of 6.4 mm diameter were produced both by repeated necking and by using single crystal seeds. Excellent surface quality was obtained in these small-diameter crystals.

Design of continuous casting machines based on a heat-flow analysis: state-of-the-art review

AbstractThe design of the mould and spray sections of a continuous casting machine for steel has been examined in detail with the aid of a heat-flow mathematical model. The thermal requirements that the design must meet have been established using Experimental data obtained from commercial continuous-casting machines. For the mould, the requirement is the solidification of a sufficiently thick shell; for the sprays, the requirements are the minimization of surface reheating and the maintenance of a reasonably high solidification rate. Correlations are developed which relate the working mould length, casting speed and shell thickness at the mould exit. Design curves are also presented of spray length, spray heat transfer. coefficient and water flux distributions for two sizes of billets. The method by which the theoretical spray water flux distribution can be related to practical spray parameters — spray pressure, nozzle type and arrangement — is also discussed. Resume On a etudie en detail, a l'aide d'un ...