Continuous Casting Vs Cenfrigual Ingot Catsing

Continuous Casting of Steel

Seppo Louhenkilpi , in Treatise on Process Metallurgy: Industrial Processes, 2014

Abstract

Continuous casting process has grown into the biggest casting method for steel, exceeding the conventional ingot casting route in the mid-1980s. Nowadays, the continuous casting ratio has reached the level of 95%. Continuous casting offers not only a high level of productivity and yield but also improved quality. The research and development work in the continuous casting field is continuing intensively because the requirements for steel quality from customers become all the time stricter and the energy efficiency, productivity, and ecological aspects are of increasing importance. One aim of the development has been to construct lower and simpler machines with smaller need for space, low investment costs, and high flexibility in production and maintenance. Today, we have fairly good knowledge of the complex phenomena taking place in continuous casting. Computational simulation and modeling of different phenomena in casting have greatly helped to solve practical problems in industrial casters and to improve process practices and control. Altogether, we still need deeper understanding of the complex solidification phenomena and transformations of microstructure in continuous casting in order to rise to the increasing requirements. This chapter attempts to overview the continuous casting method including machines, solidification phenomena, defects formation, and modeling aspects.

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How Mold Fluxes Work

Ken Mills , in Treatise on Process Metallurgy: Industrial Processes, 2014

1.9.1.1 The Continuous Casting Process

The continuous casting process is shown in Figure 1.9.1. Steel is poured from the ladle into the tundish, which provides a constant head for molten steel to flow from the tundish into the mold via a submerged entry nozzle (SEN). The copper mold is water-cooled and the bottom of the mold is initially sealed by a dummy bar of steel. When steel is poured from the tundish to the mold, it freezes to form a thin, solid steel shell against the cold mold wall. The name, shell, comes from an egg shell since it consists of a thin solid shell containing a liquid. The dummy bar is then removed and the shell is pulled gradually through the mold (where it thickens gradually) and then into a spray chamber. When the shell is thick enough, the slab is bent and later when the steel is completely solidified, the slab is cut off by an oxy-acetylene torch. Two measures are taken to avoid the sticking of the steel shell to the copper mold: (i) the mold is continuously oscillated in vertical direction and (ii) a mold flux is poured into the top of the mold and this forms a liquid slag that which infiltrates between the shell and the mold and provides lubrication to the shell.

Figure 1.9.1. Schematic diagram of the continuous casting process.

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Application of swirling flow in nozzle for CC process

Shinichiro Yokoya , ... Shigeta Hara , in Parallel Computational Fluid Dynamics 2000, 2001

1 INTRODUCTION

In the continuous casting process, it is well known that fluid flow pattern in the mold has a key effect both on the surface and the internal quality of the ingots, because the superheat dissipation induced by the flow pattern has a great influence on the growth of the solidifying shell as well as on the resulting development on the micro-structure. Accordingly, numerous efforts have been expended to control the fluid flow in the mold region. There are many ideas proposed for the controlling using electromagnetic force and some of them have been used in practice until now. Application of the electromagnetic braking and stirring to the fluid in the mold region are typical example. All these electromagnetic installations require quite costly equipment, especially, in the case of mold stirring the electromagnetic field has to penetrate the copper mold.

In this work, we show how to control the outlet flow in the immersion nozzle and the metal flow in the mold region by imparting a swirling motion to the inlet stream of a divergent nozzle.

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Advanced Techniques

Seiichi Koshizuka , ... Takuya Matsunaga , in Moving Particle Semi-implicit Method, 2018

6.8.4 Application to Metal Engineering

In the continuous casting process, molten steel is solidified by water spray. Uniform cooling is favorable to keep the quality of steel. Fig. 6.28 illustrates the geometry of the validation experiment of spray cooling of the steel slab which is supported by the rolls (Yamasaki, 2014; Yamasaki et al., 2015). The spray nozzles are located between the rolls. The rolls are separated to drain water and the flowrate of the drained water of each position is validated by comparing the calculation result and the experimental data. Good agreement is obtained (Yamasaki, 2014; Yamasaki et al., 2015). Fig. 6.29 shows an example of comparison between the MPS calculation and the experiment. We can see that water is accumulated on the lower central roll. This may cause local over-cooling of the steel slab. The mechanism of nonuniform heat transfer on the steel slab is clarified and the manufacturing process is improved.

Figure 6.28. Geometry of validation calculation for spray water cooling of steel slab in continuous casting process (unit mm) (Yamasaki, 2014). (A) Perspective view of the model. (B) Three orthographic views of the model.

Source: Courtesy: Nippon Steel & Sumitomo Metal Co.

Figure 6.29. Calculation and experimental results of spray water cooling, view from the slab side (Yamasaki, 2014). (A) Calculated result (20   L/min/nozzle). (B) Experimental result (20   L/min/nozzle).

Source: Courtesy: Nippon Steel & Sumitomo Metal Co.

A casting process of aluminum was analyzed using the MPS method by Regmi et al. (2015), where semisolid physical properties were considered. Analysis of a welding process was carried out by Saso et al. (2016), where phase change between liquid and solid, surface tension, and Marangoni force were considered.

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Manufacturing methods

Donald B. Richardson , ... (Section 16.5), in Mechanical Engineer's Reference Book (Twelfth Edition), 1994

16.5.3.5 Semi-continuous casting

Semi-continuous casting processes ('vacuum arc remelting' and 'electroslag remelting') are employed to produce very high quality stock for mechanical working. In both, one or more cast or forged billets are made into one electrode (where three parallel electrodes are employed three-phase current may be used). The electrode(s) are fed downwards and as they melt, the metal melted by the arc (or by the heat generated by the resistance of a bath of slag) is transferred downwards to the other electrode which is located in a cylindrical water-cooled copper vessel.

In vacuum arc remelting (see Figure 16.119) the arc is contained in a vacuum and the consumable electrode is fed downwards through a seal which retains the vacuum. In electroslag remelting (see Figure 16.120) a bath of fused slag is retained above the other electrode in the copper vessel. In both cases, after the start continuous casting and cooling and feeding conditions exist in the bath of metal which is formed, and climbs continuously up inside the vessel. The resulting billet which is fed and rapidly cooled during solidification is the best stock that can be produced for mechanical working.

Figure 16.119. Vacuum arc remelting furnace

Figure 16.120. Electroslag remelting furnace

Electroslag remelting refines the metal by dissolving such non-metallics as alumina in the flux. Vacuum arc remelting breaks and disperses non-metallic inclusions but does not remove them completely. It does, however, completely remove gaseous impurities such as hydrogen and oxygen from steel.

In general, electroslag remelting is used for steel and nickel and cobalt alloys, vacuum arc remelting for refractory metals, titanium, zirconium, tantalum and molybdenum, but it is also used for steel and nickel alloys.

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Continuous Casting: Complex Models

B.G. Thomas , in Encyclopedia of Materials: Science and Technology, 2001

2 Solidification and Heat Transfer

In the continuous casting process ( Fig. 1), molten metal is delivered from the bottom of a transfer vessel (the tundish) into a mold cavity. Here, the water-cooled walls of the mold extract heat to solidify a shell that contains the liquid pool. The shell is withdrawn from the bottom of mold at a "casting speed" that matches the inflow of metal, so that the process ideally operates at steady state. Below the mold, water sprays extract heat from the surface, and the strand core eventually becomes fully solid when it reaches the metallurgical length.

Figure 1. Schematic of continuous casting processes.

Heat flow/solidification models are used for basic design and troubleshooting of this process. These models solve the transient heat conduction equation

(1) $\rho \left(\frac{\partial h}{\partial t}+\mathbf{v}\cdot \nabla H\right)=\nabla \cdot (k_{\text{eff}}\nabla T)+Q$

Temperature, T, depends on the temperature-dependent material properties of effective thermal conductivity, k _eff, and density, ρ, the velocity field, v, various heat sources, Q, and the boundary conditions. Latent heat evolution and heat capacity are incorporated into the constitutive equation that must also be supplied to relate temperature, T, with enthalpy, H.

Axial heat conduction can be ignored in models of steel continuous casting because it is small relative to axial advection, as indicated by the small Peclet number ((casting speed×shell thickness)/thermal diffusivity). Thus, Lagrangian models of a horizontal slice through the strand have been employed with great success for steel. Aluminum continuous casting has a short metallurgical length, owing to its high thermal conductivity and slow casting speed, so this assumption cannot be made. (See Modeling: Scaling Analysis for details on how decisions like these are made.)

Heat transfer in the mold region is controlled mainly by heat conduction across the interface between the surface of the solidifying shell and the mold. The greatest difficulty in accurate heat flow modeling is determination of the heat transfer across this gap, q _gap, which varies with time and position depending on the thickness, d _gap, and the properties of the gas or lubricating flux layers that fill it, such as k _gap.

(2) $q_{\text{gap}}=\left(h_{\text{rad}}+\frac{k_{\text{gap}}}{d_{\text{gap}}}\right)(T_{0_{\text{shell}}}-T_{0_{\text{mold}}})$

Where metal shrinkage is not matched by taper of the mold walls, an air gap can form, especially in the corners. This greatly reduces the heat flow. Advanced models simulate the mold, interface, and shell, and use shrinkage models to predict the size of this gap, d _gap. In steel slab casting operations with mold flux, such models feature a detailed treatment of the interface, including heat, mass, and momentum balances on the flux in the gap and the effect of shell surface imperfections (oscillation marks) on heat flow and flux consumption (Thomas et al. 1998). The coupled effect of flow in the molten metal on delivering superheat to the inside of the shell, and, thereby, retarding solidification, is also modeled. Mold heat flow models can be used to identify deviations from normal operation and thus predict quality problems, such as impending breakouts or surface depressions, in time to take corrective action.

Heat flow models which extend below the mold are needed for basic machine design to ensure that the last support roll and torch cutter are positioned beyond the metallurgical length for the highest casting speed. Below the mold, surface temperature of the strand is maintained by air mist and water spray cooling, while the interior solidifies. Online open-loop dynamic cooling models can be employed to control the spray flow rates in order to ensure uniform surface cooling even during transients, such as the temporary drop in casting speed during a nozzle change. A heat flow model can also be used to troubleshoot defects, e.g., the location of a misaligned support roll that may be generating internal hot-tear cracks can be identified by matching the position of the start of the crack beneath the strand surface with the location of solidification front down the caster calculated with a calibrated model.

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23rd European Symposium on Computer Aided Process Engineering

Yun Ye , ... Christodoulos A. Floudas , in Computer Aided Chemical Engineering, 2013

1 Introduction

Scheduling of steelmaking-continuous casting (SCC) processes is of major importance in iron and steel operations since the SCC process is often a bottleneck in iron and steel production [1]. Li et al. [2] developed a novel and effective unit-specific event based continuous-time formulation for this process and extended the rolling-horizon approach [3–4] to decompose the entire MILP problem. The computational results show that the extended rolling horizon approach reduced the computational time and generated the same or better feasible solution than that without using the rolling horizon approach. Their model assumes that all the parameters are deterministic in nature.

However, several uncertainties such as demand fluctuation, and processing time uncertainty frequently happen during the real operations. In the presence of these uncertainties, the nominal schedule may often be suboptimal or even become infeasible. In general, two approaches can be used to address those uncertainties: reactive scheduling and preventative scheduling [5]. While reactive scheduling is a process to revise the generated schedule from nominal parameters when a disruption has occurred during the actual execution of the schedule, preventive scheduling seeks to accommodate future uncertainty at the scheduling stage. For detailed reviews on planning and scheduling under uncertainty the reader is directed to Li and Ierapetritou [6] and Verderame et al. [5]

In steelmaking continuous casting process, reactive scheduling is often used for handling different types of disruptions such as machine breakdowns, rush orders, and order cancellations [ 7–9]. Yu et al. [10] used a fuzzy programming approach to address uncertain processing time in the steelmaking and continuous casting production process. The uncertain processing time was denoted by triangular fuzzy number.

In this paper, we first employ the robust optimization framework from Lin et al. [11], Janak et al. [12] and Li et al. [13–14] to develop a deterministic robust counterpart optimization model for demand uncertainty during steelmaking continuous casting operations. The robust solution from the robust optimization framework is guaranteed to be feasible for the varying demand parameters. While the robust optimization framework aims at finding a single schedule that is immune to all possible uncertainty realizations within an uncertainty set, a two/multi-stage stochastic programming method provides flexibility of implementing different operational decisions after the realization of uncertainty. A scenario based two-stage stochastic programming framework was also studied for the scheduling of steelmaking and continuous operations under demand uncertainty. To make the resulting stochastic programming problem computationally tractable, a novel scenario reduction method [15] has been applied to reduce the huge number of scenarios to a small set of representative realizations. The selected scenarios are incorporated into the stochastic programming model, which is further reformulated into its deterministic equivalent model and solved.

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The fracture of liquids

John Campbell , in The Mechanisms of Metallurgical Failure, 2020

1.6.1 Ingot casting

Although much steel is now cast by the continuous casting process, smaller batches for special purposes are produced as ingots. Thus, much of our "special steels" are ingot cast. Ingots vary from a few kg to about 500,000  kg, but the casting techniques are much the same for all.

Fig. 1.59 shows various casting techniques used for ingots. It does not take much imagination to realize that direct top pouring of liquid metal into an ingot mold (Fig. 1.59A) is probably not good for the metal. In fact, after extraction of the ingot from its mold, an inspection of the ingot often reveals laps, cracks, and bifilms clearly visible to the unaided eye from a distance of up to 10   m. Frankly, the structure is awful. The problems of cracks in the faces and sides of ingots, in ingot corners, and vertical cracks up the flutes of fluted ingots (designed with crenelated sides to avoid cracking!) and cracks opening up during subsequent working of the ingot as face cracks, edge cracking, and crocodile cracking, are only to be expected. The failure by cracking of final products made from such ingots are also to be expected. It is relevant to ask, "Could it be worse?" "How can the metallurgical industry have lived with such a catalog of problems for so long?"

Figure 1.59. Ingot casting by (A) top pouring; (B) uphill teeming; (C) contact pouring.

The massively destructive turbulence of top pouring also applies to vacuum casting, where inside the vacuum chamber it is not easy to arrange the delivery of the metal in any other way than by tilting the pot of molten metal and emptying its contents into the ingot mold by top pouring. In my view this is the Achilles heel of the vacuum induction melting (VIM) process. It is conventionally expected that the vacuum will avoid oxidation problems, allowing cracks and laps to heal. In general, this does not happen. The vacuum in metal melting and casting operations can never be particularly good because of the natural and prolific outgassing of the refractories of the melting unit. As a consequence of pouring in the relatively poor vacuum, the surface of the falling liquid is covered with an oxide film, which naturally becomes entrained as bifilms. In fact, the VIM ingot is probably the wolf in sheep's clothing of the clean steel scene: its bifilms will be so much thinner than air-cast metal, giving the appearance of a good quality product.

Bottom gating (sometimes called "uphill teeming," Fig. 1.59B) is often selected as an improved technique for the casting of ingots. I suspect the real reason is mainly the apparently improved surface finish of the ingot, which may have benefits in reducing some cracking during subsequent working.

In reality, however, the interior of the bottom gated ingot is probably almost as bad as that by top pouring as a result of the huge volume of air which is entrained by the conical trumpet-shaped intake at the entrance to the filling system channels. The cone, with the central jet of liquid, acts as a venturi air pump with awesome efficiency. The consequent mix of air and metal is at least 50/50 by volume, and often probably nearer 80% air and 20% metal. Worse still, the air bubbles will generate long bubble trails of oxide double films. It could hardly be worse. On looking down into the mold as it fills, the rising metal appears to boil. Despite the claims and remonstrations of die-hard traditionalists, it is clearly not possible to make good steel by such an inappropriate process.

The air pump action appears to be by far the most important reason for inclusions in uphill teemed ingot steels. The attempts to deal with this problem by providing an argon shroud are negligibly effective; the shroud gas typically contains 10 or 20% air.

The only effective technique to avoid air entrainment is to exclude air entirely. This is achieved by eliminating the trumpet and eliminating the gap between the ladle nozzle and the filling system intake. The ladle nozzle and filling intake are brought into contact (usually with a compressible seal of ceramic fiber blanket). The technique is simply known as "contact pouring" (Fig. 1.59C). It is totally effective. It makes excellent cast products. It is recommended to be used to the exclusion of all top pouring ingot casting techniques.

The details of ensuring that the filling system channels are not crushed by the weight of the ladle, or the ladle nozzle forced upwards into the ladle, together with the accuracy of a few millimeters required for alignment, are all solvable. Because the bottom of ladles are always rather different (the ladle feet, among other features, get in the way!) every casting operation in which the technique has been introduced has been forced to find a different engineering solution. With regard to accurate positioning, in this digital age, and with laser positioning a mature technology, such challenges should represent minimal difficulty.

The remaining issues with contact pouring of bottom-gated ingots includes the use of pre-formed refractory tubing which is normally used to construct the pouring channels. These are made from fired clay, akin to the making of pottery. They are therefore often referred to as a "pot running system." The tubing has a parallel bore, quite different to the tapering shape of the falling metal as it is accelerated by gravity. Thus, the pot channels contain much redundant air which can become mixed with and degrade the quality of the falling steel. The correct shaping of filling system channels for castings is now understood with considerable sophistication (Campbell, 2018). It seems probable that a filling system correctly molded to shape in sand would give a superior quality ingot (sand inclusions are never observed if the channels are correctly shaped) and would be significantly more economic and environmentally friendly. The filling system design is more fully described in Section 1.5.5.

A contact pour ingot may be 10 to 100 times cleaner than a standard ingot, but this may on occasions not be good enough for some special products. In the quest for perfection therefore, the remaining impairment of a contact poured ingot resides in the priming process. The air already in the filling system channels mixes with the initial fall of metal, so that a clutch of bubbles and oxidized metal is released into the ingot mold during the first few seconds of the pour. A design of filling system in which this first metal is discarded, and bubbles diverted by the use of a "filter" (actually a ceramic block permeated by pores usually around 1   mm diameter) into a centrifugal trap, is a relatively low cost sophistication which may be sometimes worthwhile (Campbell 2018).

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Preventing clogging in a continuous casting process

BO SUNDMAN , in The SGTE Casebook (Second Edition), 2008

Publisher Summary

This chapter discusses the prevention of clogging in a continuous casting process. The problem presented in the chapter was caused by an attempt to modify an alloy produced by a continuous casting process. The process worked well for stainless steel with 20 wt% Cr and the manufacturer wanted to use the same process for steel with 25 wt% Cr. However, the manufacturer then obtained problems with clogging by solid oxide formation, which prevented the flow of liquid steel. The oxide formed at the outlet was found to consist mainly of Cr ₂O₃. The manufacturer faced an expensive and time-consuming experimental scheme in order to find out how to prevent the formation of the Cr₂O₃. As an alternative route, the Thermo-Calc thermodynamic databank was tried to be used in order to simulate the process on the computer in order to find a remedy. Such a simulation can be made in less than a day if the necessary thermodynamic data is available. The problem arises because the partial pressure of oxygen or equivalently the oxygen activity in the liquid steel is high enough to precipitate Cr₂O₃ at the higher chromium content. The stability of Cr₂O₃ is determined by the product of the oxygen and chromium activities are raised to their respective powers. Therefore, the solution must be to decrease the oxygen activity in the liquid steel.

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Sheet Steel: Low Carbon

R.C. Hudd , in Encyclopedia of Materials: Science and Technology, 2001

1 General Processing Considerations

Almost all sheet steel is now made using a continuous casting process to produce slabs 200–250mm thick. After cooling these slabs are usually reheated to temperatures up to 1250°C for hot rolling. The first stage of hot rolling, called roughing, reduces the thickness usually into the range 30–45mm and the second stage, called finishing, reduces the thickness to the final hot-rolled gauge required, often in the range 1–2mm up to 5–12mm. The steel leaves the last stand of the hot mill at a finishing temperature which is determined by metallurgical requirements. Water cooling is then used along a runout table to achieve the required temperature for coiling. Over the last few years thin-slab casting processes have been introduced in a few steel plants which enable intermediate gauges, usually obtained by roughing, to be cast directly.

Cold rolling is used for gauges usually in the range 0.4–3mm using cold reductions of 50–80%. During cold rolling the steel becomes hard and loses its ductility; therefore an annealing process is used to restore ductility and to develop almost the final properties. In the batch process several tight coils are stacked on a furnace base and heated and cooled under a protective atmosphere, the complete process usually taking several days. In the continuous process each coil is uncoiled and then recoiled after passing through a furnace, the complete process taking several minutes. After annealing, cold-rolled steels may be given a light cold reduction, called temper rolling, primarily to remove the yield point but also to improve flatness and develop the required surface roughness and texture. Cold-rolled and annealed steel would normally have a better formability, gauge control, flatness, and surface than a hot-rolled steel but the former would have a higher cost due to the additional processing.

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