Role of Mould in Continuous Casting of Steel
Role of Mould in Continuous Casting of Steel
Continuous casting (CC) transforms liquid steel into solid on a continuous basis and includes a variety of important commercial processes. Continuous casting is distinguished from other solidification processes by its steady state nature, relative to an outside observer. The liquid steel solidifies against the mould walls while it is simultaneously withdrawn from the bottom of the mould at a rate which maintains the solid / liquid interface at a constant position with time.
Continuous casting for steel solidification processing is being used commercially since 1950s. The process has become increasingly popular because of its low operating cost. The advances in casting technology have led to interesting developments in the field of mould design in terms of better productivity and product quality. Manufacturing of inside tapered curved moulds for bow type casting machines started in the years 1963 to 1965. Chromium (Cr) plating for inner walls of moulds for better abrasion resistance is available since 1965. Multiple tapered and parabolic taper mould geometry for better mould wall and solidifying steel shell contacts has been introduced in 1980s. In 2010, a special type of mould tube with waves at the inner wall gas been designed for billet casting for reducing the shape defects like rhomboidity of cast products. Apart from these, continuous efforts have been made for homogenizing of the temperature profile inside the mould for getting better results related to thermal stresses in the cast product.
Moulds play an important role in the process of continuous casting of liquid steel. They are the heart of the continuous casting process. In the process of continuous casting, liquid steel is poured from the tundish into the casting mould through the submerged entry nozzle (SEN) immersed in the liquid steel. The moulds are water cooled. Solidification of liquid begins in the mould by indirect cooling. The cooling process in the mould is known as primary cooling process.
The mould is a mechanical component through which the liquid steel flows. It is designed for solidifying a thin shell of steel which is continuously withdrawn away up to a complete through thickness solidification. Different cross sections can be adopted (square, rectangular, round, or shapes) as per the final geometry of the product. The main function of the mould is to provide an intensive cooling of the liquid steel to achieve a robust shell of metal. A precise control of the shape is needed for matching the shell contraction and for ensuring the product geometry. High dimensional stability at any operational regime is hence a need of primary relevance.
Once the liquid steel refining process is completed, the liquid steel contained in the ladle is normally sent to a continuous casting machine. The liquid steel flows under gravity from the ladle to a tundish and then from the tundish into a bottom-less water-cooled copper (Cu) mould. The main purpose of the tundish is to act as a buffer between ladle changes so that the process is continuous. Once in the mould, the liquid steel freezes against the water-cooled walls of the mould to form a solid shell. The mould is oscillated vertically in order to discourage sticking of the shell to the mould walls. Drive rolls lower in the machine continuously withdraw the shell from the mould at a rate or ‘casting speed’ which matches the flow of incoming liquid steel, so the process ideally runs in steady state. The liquid steel flow rate is controlled by restricting the opening in the nozzle as per the signal fed back from a level sensor in the mould. Fig 1 shows schematic of mould region of continuous casting process for steel slabs.
Fig 1 Schematic of mould region of continuous casting process for steel slabs
The main function of the mould is to produce and stabilize a solid shell resistant enough to contrast the metallic pressure of the liquid core and, hence, contain the liquid phase at the entry of the secondary spray cooling zone. If the mould system does not work properly, a break-out can take place and the hot liquid steel core can burst open, pouring liquid steel onto the machine and causing a very dangerous situation.
The surface of the Cu mould coming in contact with the hot liquid steel is frequently plated with Cr or Ni (nickel) for providing a harder working surface and for avoiding Cu pick-up on the surface of the cast strand, which can facilitate the development of the surface cracks on the cast product. Maintaining a reliable, crack-free mould within close dimensional tolerances is also crucial for safety and productivity.
During continuous casting process the liquid steel flows from tundish to the mould through open type metering nozzles or SEN. The mould tubes are made of Cu and surrounded by a stainless-steel jacket and through the gap cooling water flows. In the process, either de-mineralized water or soft water is used for cooling for ensuring smooth heat transfer without scale formation. Initial solidification of liquid steel takes place inside the mould.
In the mould, a thin shell of steel next to the mould walls solidifies before the middle section which is called a strand. The strand leaves the base of the mould into a spray chamber. The bulk of liquid steel within the walls of the strand is still liquid. The strand is immediately supported by closely-spaced water-cooled rollers which support the walls of the strand against the ferro-static pressure of the still solidifying liquid steel within the strand. For increasing the rate of solidification, the strand is sprayed with large quantities of water as it passes through the spray chamber. This is the secondary cooling process. Final solidification of the strand can take place after the strand has left the spray chamber.
The most critical part of the process is the initial solidification at the meniscus, found at the junction where the top of the shell meets the mould, and the liquid surface. This is where the surface of the final product is created, and defects such as surface cracks can form, if issues such as level fluctuations occur. For avoiding this, a thin film of lubricating oil or the casting powder (lubricating flux) is added to the steel meniscus, which flows into the gap between the mould and the shell for preventing the steels direct contact with the mould, which can potentially endanger and damage the mould itself. In addition to lubricating the contact, the casting powder layer protects the liquid steel from air, provides thermal insulation, and absorbs inclusions. The solidifying strand is withdrawn from the bottom of the mould at a rate called the casting speed, which matches the rate at which new liquid steel solidifies.
Heat transfer in the mould is the core of the continuous casting process and its quantitative analysis has been pioneered by Keith Brimacombe. With several different processes presently competing, it is appropriate to apply modeling for investigating the theoretical limits of continuous casting speed and productivity. The heat transfer rate during solidification process drops with time so the shell thickness at mould exit drops with the increasing casting speed.
When the steel solidifies in a mould, there is an initial area of perfect contact, where the strand shell is still thin with temperature of around 1,400 deg C and low tensile strength. Below this area, air gaps are forming, especially near the corners. The air gaps reduce the local heat transfer and lead to a slower shell growth in the affected areas. These weak areas increase the risk of break-out. These weak points which normally occur in square or rectangular sections, can lead to bulging and rhombic deformation eventually producing ‘hinge-cracks’. The risk of thermal induced tension is imminent. Moreover, the solidified material is at that point within the zero-ductility zone. The already grown shell acts as a great barrier for the heat transfer and further shell growth rate is substantially reduced.
An extreme temperature gradient takes place across the Cu plates and this causes geometrical distortions of the mould. Moreover, long hours of operation at high temperatures generates creep. This resultant creep is also associated with a thermal fatigue phenomenon, which is caused by the several room temperature heating and cooling cycles undergone by the mould during the initial and final transitory of the casting sequence.
For smooth stripping of solidifying steel from mould wall, a lubrication mechanism is used in mould. Oil lubrication or powder lubrication methods are used in this purpose. Mould lubrication powder is a mixture of metallic oxides which is added at top of the mould covering the liquid steel. It also protects the steel from re-oxidation inside the mould. The powder gradually melts and enters into the interface of mould wall and solidified steel. There a part of it again solidifies and forms a solid slag layer which hinders the heat transfer process in the mould. The liquid casting powder infiltrates into the gap between solidified shell and mould wall with downward movement of the strand and lubricates the interface for smooth extraction.
For uniform infiltration of lubricants, mould oscillation mechanism is used. During mould oscillation for a certain time downward mould velocity becomes higher than the extraction speed of the strand. This time is called negative strip time which is a very important criteria of oscillation. During negative strip time, negative taper of mould wall exerts a compressive force on the newly formed shell causing it to buckle. During subsequent positive strip, i.e., when mould moves upward and strand moves downward the gap between shell and mould wall opens up causing overflow of liquid steel along with infiltration of lubricant and formation of fresh shell. Hence, an oscillation mark is created in the cast product. Inside mould, the solidification is governed mainly by conduction and radiation across the interfacial casting powder layer and gap between the solidifying steel shell and the mould. The liquid steel after solidification shrinks and loses contact with mould wall creating a gap which causes a resistance to heat transfer. For ensuring proper contact, the inner walls of mould are having an inward taper along the exit direction.
Mould materials – Because of the high thermal conductivity, Cu is selected as primary constituent of mould. For preventing distortion of moulds, alloying elements such as Cr, Ni, P (phosphorus), Ag (silver), Zr (zirconium), Co (cobalt), and Be (beryllium) etc. are added. The inner wall of mould is plated with Cr of 0.1 mm to 0.12 mm thickness for imparting high temperature abrasion resistance.
The common and exotic mould alloys are work hardening alloys such as Cu-Ag, precipitation hardening alloys such as Cu-Cr-Zr, Cu-Ni-Be, Cu-Ni-P, and Cu-Co-Ni-Be. The mould material can be produced by hot and cold rolling, or forging. The heat treatment carried out for mould material consists of solution annealing, precipitation (age) hardening, and stress relieving. The desired properties of mould material are (i) high thermal conductivity, (ii) high strength and hardness for wear resistance, creep resistance, fatigue resistance, and cracking resistance, and (iii) high softening temperature so that the material maintains the desired properties at high temperatures.
The moulds are either uncoated or coated. The coating of the mould is done with Ni, Ni-Co, Ni-Cr, Cr, or ceramic materials. Main reasons for coating of the mould material are prolonging of the mould life for better wear resistance, and improving the quality of the cast product for avoiding star cracks. The side effects of coating are reduced heat transfer (around 1.5 % / mm Ni), and higher wall temperatures (around 15 deg C / mm Ni).
The advances in casting technology have been made possible by the development of high-performance moulds made of Cu materials. The performance requirements which are to be met by moulds and mould materials depend on the specific application and the levels of stress involved. These stress levels are mainly pre-determined by the continuous casting machine and casting parameters, which means that several different cast shapes are needed, depending on the type and construction of the mould. When designing a new mould, the correct profile is to be chosen in order to achieve high product quality, optimal casting speeds, smooth casting operations, and long service life of the moulds.
A good example of this are the requirements placed on modern mould materials for near net-shape casting processes which have been developed in recent years. Here, very high casting speeds are achieved and a much higher proportion of the liquid steel is needed solidify in order to form a sufficiently stable strand shell. The resulting extreme temperatures need moulds with higher strength levels. At the same time, a high alternating thermal stress can occur, e.g., on casting rolls. This wide variety of requirements placed on moulds has to be met by highly developed materials and system expertise. In order to meet the future oriented solutions for the wide variety of different casting technologies and taking into account the constantly changing requirements on moulds and mould materials, the fields of mould technology which is to be constantly studied and improves are mould engineering, mould materials, mould coatings, and mould manufacturing.
The range of mould materials developed and produced these days allow appropriate selection of the optimum Cu alloy for individual applications. However, in order to achieve high performance, optimum steel quality, and a long service life of the moulds in the continuous casting machines, further engineering work is normally necessary, particularly when casting facilities are operated on system parameters which have been changed from the original concept in order to achieve higher casting outputs or produce special types of steel. This is where mould engineering comes into play, supporting in upgrading continuous casting moulds and optimizing system parameters and mould constructions. Using FEA (finite element analysis) for calculating the mould stresses based on 3-D (three dimension) CAD (computer aided design) modelling allows accurate simulation of the mechanical and thermal stress factors involved in each case. Mould dimensioning, tapering, and the specification of cooling conditions are based on the results of these calculations.
For the optimization of the moulds, the parameters which are to be considered are (i) chemistry of cast steel, (ii) mould flux, (iii) casting speed, (iv) mould taper, (v) wall thickness, (vi) cooling conditions (water quality, flow rate, and velocity), (vii) adjustment of strand guide, (viii) adjustment of oscillating unit, and (ix) width changes etc. Hence, it is necessary to carefully look at each individual case for fine tuning.
The most suitable length for a continuous casting mould has been found to be in the range of 510 mm to 915 mm, a range which seems to remain constant regardless of section size. Fig 2 gives casting speed with respect to the mould length. This surprising result can be explained by the higher rates of heat removal achieved with smaller sections and higher casting rates. Also, a thinner skin can be permitted for smaller sections leaving from the mould than for larger sections since bulging of the solidifying shell is less severe. At higher casting rates, the use of an increased taper in the mould is necessary for maintaining high heat removal rates, particularly for the narrow faces of slab moulds.
Fig 2 Mould length and casting speed
The mould is basically an open-ended box structure containing a water-cooled inner lining fabricated from a high purity Cu alloy. The box can come in several shapes and sizes in order to cast different semis such as blooms, billets, round, beam blanks, slabs, and thin slabs. There are three alternatives which normally apply for the continuous casting mould arrangements. These are (i) plate moulds for slabs and larger blooms and beam blanks, (ii) tube moulds for billets, rounds, and smaller blooms and beam blanks, and (iii) block moulds with drilled cooling channels which are used for complex shapes like beam blanks. For thin slab casting in the compact strip production, funnel shaped mould is used. Plate and tube moulds are popular types of moulds while the block and funnel moulds are expensive because of the quantity of Cu used and the extent of machining needed for the mould production. Fig 3 shows tubular moulds and plates for plate moulds.
Fig 3 Tubular moulds and plates for plate moulds.
Tubular moulds are used for the continuous casting of billets, rounds, blooms, and beam blanks. Conventional tubular moulds consist of a Cu tube surrounded by a steel jacket. In the gap between the two elements cooling water flows. In order to improve the thermo-mechanical performance, a different design configuration of the mould has been developed. It consists of a thicker Cu tube provided with drilled holes for cooling. In this way, a high stiffness with excellent heat transfer capacity is achieved at the same time. Another new development in the tubular mould is the wave tube moulds.
The production of tubular moulds starts with the casting of bars with a circular cross-section. These are subsequently hot-extruded, or forged. The extruded tube is then cold-drawn and formed for achieving the geometrical and mechanical features needed by the technical specifications, which of course also include taper. For the latter step, which is by far the most crucial one in the production cycle, a well-equipped powerful and best equipped forming press is used. Forming is accomplished with special steel equipment, which is specific to each mould and is produced with CNC (computerized numerical control) machines. Finally, the tube mould undergoes machining and is then chromium-plated internally, before it being inspected and measured.
For tube moulds, there is no discontinuity around the circumference of the mould Cu, the mould being formed by a Cu tube. There is hence no need to clamp the individual plates together. A water jacket is arranged around the complete tube circumference. It is necessary to centre the mould tube within the water jacket. Tubes can typically be 10 mm to 12 mm thick for small billets and up to 30 mm or 40 mm thick for large section casting.
In a tube mould, the cooling is achieved by an annulus of water around the complete circumference of the tube. The thickness of the annulus has to be even in order to achieve uniform flow of the water around the complete circumference and hence uniform heat transfer. Normally, the water flows from the bottom to the top of the mould in much the same way as the plate mould. Since the tube mould use thinner Cu than the plate moulds, it is necessary to operate at higher water velocities in order to suppress nucleate boiling. Typical velocities can be in the regions of 11 metres per second (m/sec) to 13 m/sec.
In tube moulds, tapers are applied to the tube cooling faces in order to compensate for the shrinkage of the newly defined shell / strand cross-section. In case of billet casting, the casting speeds are quite high (up to 6 metres per minute), and the shrinkage is more pronounced. Parabolic tapers have beneficially been applied in order to give good support the shell / strand cross-section.
Historically, when no complex tapers were applied, combined with higher casting speeds, the very thin shell of the billet would shrink and pull away in the corner region of the mould. This then led to a reduction in heat transfer and a retarding of the shell growth in the corners, which in turn gave either potential break-out conditions or the danger of the quality problems such as cracking close to the corners. The newer complex cross-section aims to reduce the effect of the shell pull away in the corners and hence give a more even shell growth. In case of tube moulds, the life limiting factor is normally the loss of taper due to the distortion close to the meniscus. Fig 4 shows tube and funnel moulds
Fig 4 Tube and Funnel mould
In thin slab casting, the most innovative piece of technology embodied is the liquid core reduction concept (LCR). The funnel-shaped mould (Fig 4) is the first concretization of this concept. Possibly imagined by a rugby player, the shape was designed to accommodate the sub-merged nozzle, a compulsory technology for casting clean Al-killed carbon steels.
The wave mould has a patented design which super-imposes a series of undulations onto the hot-face side of the mould, causing a mirror image to be formed on the billet surface as it begins to solidify. These two surfaces interlock, and the shell is guided through the length of the mould while restraining any movement from side-to-side. The mould and shell are hence ‘coupled’ together to such a degree that a more equal heat extraction, and hence uniform shell growth, occurs during this critical time. The result is improved billet shape and internal quality, as well as increased mould life. Fig 5 shows wave and beam blank moulds.
Fig 5 Wave and beam blank moulds.
Plate moulds are produced by the assembly of Cu plates and are used for the continuous casting of blooms, slabs and bloom blanks. The design and manufacture of mould plates can be with cooling slots or deep-hole drills. The Cu mould in continuous casting extracts heat from the liquid steel by means of cooling water flowing through rectangular and / or circular channels, and also supports the solidifying shell to determine its shape. The mould assembly consists of two wide faces, two narrow faces, and their respective water boxes. The steel water boxes, either machined single-piece slabs or built up from several slabs, serve to circulate the cooling water in the mould, and also increase the rigidity of the assembly to control the thermal distortion of the mould when it heats up to the operating temperature.
Production of plate moulds involves casting of a slab which is subsequently hot rolled (or forged) and then cold rolled. The entire plate is then ultrasonically inspected. Only plates which have passed the test 100 % are then worked with high precision CNC machines, for achieving compliance with the strictest tolerances set down in the technical specifications. This stage also includes welding of the steel studs when the plate moulds are designed for this type of configuration. Finally, galvanic wear-resistant coating is applied if called for, after which the plate mould goes for final inspection.
In plate moulds, mould plates are made of Cu and are typically 30 mm to 60 mm thick. These are mounted on the water jackets. These plate assemblies are then clamped together to form the necessary faces of the mould defining the cross-section of the product to be cast. Cooling is achieved by water cooling in slots behind the Cu plate. The fastening of the Cu plate is normally done by bolts, fastening into the Cu plates.
The moulds normally use a close circuit water cooling system. The cooling water is circulated past the mould plates in machined slots on the cold surface of the Cu plate. Water is routed through the mould frame to a distribution chamber at the bottom of the mould, then up cooling slots to the top of the mould and into a collection chamber before returning through the mould frame to the water treatment plant. The cooling slots can be located in the Cu or located in the backing water jacket.
When the initial solidification of the shell occurs at the meniscus, the steel undergoes a phase change from liquid to solid together with associated volume shrinkage. The strand cross-section hence shrinks, following the initial solidification at the meniscus. To follow the shrinkage of the solidifying material, and to support the newly created strand, the mould plates have tapered strand section and width. The tapers earlier followed a simple linear profile. Today, much more complex tapers with multiple or parabolic profiles are being applied which more closely follows the product shrinkage. Typical values for slab narrow faces are 0.9 % to 1.2 % per meter and for slab wide faces values are 0.35 % to 0.45 % per meter. Fig 6 shows continuous casting slab mould.
Fig 6 Continuous casting slab mould
When designing the size of moulds for slabs, blooms or billets, each case is to be considered individually. The main variables which play a role are the types of steel to be cast, the cooling conditions, and the desired casting speed. For mould taper, the type of steel, the construction of the casting machine, and the casting parameters are the main factors which are to be taken into account when specifying the mould taper. From a theoretical approach, the optimal taper of a mould can only be specified for one type of steel and for one specifically defined casting conditions, i.e. super-heat of the liquid steel, and casting speed, etc. For this reason, there is always an element of compromise in the taper actually used, especially in the case of non-adjustable moulds.
These days a multitude of tapers are used in the design of mould tubes, which include a range of linear tapers with single, double, and triple taper formats. In addition, the more modern trend is to use a parabolic taper which can be tailor-made to meet the casting parameters. If a limited range of steel types with a similar chemical composition and similar shrinkage behaviour are to be cast, it makes sense to adjust the taper of the mould more closely to the shrinkage behaviour of the steel. Especially for billet and bloom mould tubes, it has been shown that parabolic tapers better conform to the shrinkage behaviour of the strand than linear tapers and hence contribute to an improvement in strand quality (off-squareness / oscillation marks).
Another important factor is the adjustment of the cooling conditions and casting parameters in order to ensure good system productivity and product quality. For this purpose, CFD (computational fluid dynamics) calculations of the water flow between cold face of the mould and the water box are performed. In combination with the thermal load calculation of the hot face, this gives a detailed analysis of the thermal and mechanical stresses on the mould during the casting process.
Studies have been carried out to better understand the complex thermal and mechanical behaviour of the mould. An extreme temperature gradient takes place across the Cu plates and this causes geometrical distortions of the mould. Moreover, long hours of operation at high temperatures generates creep. This resultant creep is also associated with a thermal fatigue phenomenon, which is caused by the several room temperature heating and cooling cycles undergo by the mould during the initial and final transitory of the casting sequence. Some studies have attempted to simulate this mechanical behaviour and to predict the potential damage to thin slag-mould systems in order to better understand the role played by the machine dynamics in the mould damage process.
In addition, friction phenomena can potentially occur between the strand and the mould. Friction between the solidifying steel and the mould is basically sliding (with a small fraction of sticky friction). These damages can end up having catastrophic consequences. At present, both metallic and ceramic coatings are available. The latter allows for an increased mould life-span but it is not widely used because of its cost and low thermal exchange. To the contrary, metallic coating is either Ni-based or Cr-based. Despite its brittleness and low wear resistance, Cr is the most used metallic element in mould coating.
The challenge for developing the best performing mould systems lies in the making of the best suitable designs for the particular needs of each individual steel plant. Firstly, basic parameters like steel grades cast, end application, section size and the planned annual production define a certain mould system. Secondly, the philosophy of service, replacement, refurbishing is to be considered before a decision is made e.g., plate mould, tube mould or the newly developed integrated mould system (Fig 7b).
A state of art mould system is a combination of 3 to 4 parts which need to be maintained and operated friendly. The only changeable part is the cartridge insert. All other items remain permanently in place and include the mould level control system. Integrated mould consists of a stainless back-up construction which provides the needed rigidity and stiffness for supporting and holding the Cu moulds liner at the inner (steel) face. The water-cooling channels are placed between the two materials. Integrated mould which combines a Cu liner with a rigid back-up structure is made from stainless steel. Fig 7 shows mould components and mould types.
Fig 7 Mould components and mould types
The mould design is based on the FEM (finite element method) calculation method and provides uniform surface temperature of the solidified shell at its entire circumference. It means that the casting powder can melt with same conditions at faces and corners. The result is a crack free product with favourable large radius corners for the rolling process.
Billets, blooms and slabs at the mould exit as cast with overcooled and dark colour at corners. i.e. heat flux has occurred in two directions (x-direction, and y-direction) at the same time. This is because of the early loss of contact at all four corners. The faces keep in contact with the Cu mould, and there is heat flux only in one direction (x-direction). These faces of semi-products (billets, and blooms) are defect free but corners, in particular, the off-corner parts can show subsurface cracks.
Round sections typically suffer from longitudinal face depressions and/ or longitudinal cracking. The absence of a ‘guiding face’ such as existent with square or rectangular shapes gives the possibility to contract totally uncontrollable and contact with the Cu cooling face is hence non-uniform i.e. incidental. Contrary to the logical theory, shell growth is very irregular and tension in the steel shell is unavoidable.
In the case of beam blanks moulds, the initially formed shell can shrink freely from all mould faces except in the four fillet parts where it ‘clamps’ onto the faces. The result can be longitudinal cracks in the web area. Another phenomenon is a ‘wash-out’ where the open liquid steel stream is not centered, too close to the fillet and prevents the shell growth.
Convex moulds (Fig 7c) which have been introduced to the steel plants around 1995, shows some remarkable performance in terms of their popularity. This success is based on the versatile use and practical application of this unique mould geometry. For the first time it combines a (known) vertical taper with a horizontal taper at the first portion of the mould cavity resulting in an increase of contact area strand / mould. Hence, the productivity increase has been substantial. Convex moulds with its unique shape, allow to cast a large variety of steel grades with 0.02 % C (soft wire) up to 1 % C (bearing steel) with the very same mould geometry. Understandably, this universal use is highly appreciated by the production people.
In case of convex moulds for rounds, the round sections, in fact, have a big advantage over square ones, since they have no corners. Hence, there are also no corner-related defects. With the convex round mould there is not only 15 % more heat flux than normal moulds, but also (i) rounds have a higher heat transfer than rectangular sections, which means there is still an unused potential in terms of casting speed, (ii) the heat transfer is uniform over the length of the mould, (iii) the heat flow uniformity along the perimeter is achieved because of an improved mould-to-billet contact and because of a guidance of the strand inside the mould which prevents any ‘rotation’ or ‘twist’ of the strand as experienced on conventional moulds for round sections.
In case of convex moulds for beam blank sections, the deficiency of the ‘fillet’ part can be eliminated by the introduction of a convex shape. The intention is to ‘push’ from the centre by the balloon shaped faces and thereby taking the force which acts at these parts. Also, the web structure can profit from an accelerated solidification because of the better contact of strand to mould.
Integrated mould (Fig 7b) is a type of a solidification ‘reactor’ which is all in one. This composite and integrated mould body contains an ‘endless’ mould lining since the Cu liner for rapid heat dissipation is very thin and directly connected to the structural body with the necessary mechanical strength. It is a patented mould with a compact design and full of all the desired features. The highlights of the integrated mould are (i) uniform temperature-field along the perimeter of a mould-section including the corner area, (ii) higher heat transfer with thin copper layer, (iii) precision machined mould faces for achieving close to perfect mould geometry, (iv) higher mechanical stability against thermal deformation, (v) improved penetration of electro-magnetic field generated by mould electro-magnetic stirrer (MEMS), (vi) renewable Cu part, (vii) integrated mould instrumentation, (vii) simplified maintenance, (viii) higher productivity, and (ix) lower running cost.
In case of tube moulds, the classical construction of a Cu tube with a concentric cooling gap does not permit to adopt different cooling intensity to different areas of the billet (corners, faces, top, or bottom). With the new system cooling channels are integrated in the mould construction. This design counteracts the normally inherent two-dimensional heat-flux in the corner areas. The Cu part is thinner thanks to the design, which allows direct contact with the supporting steel body. Hence, the Cu wall temperature is lower, which results in considerably reduced heat load and longer service life.
The mould can be fitted with temperature, friction, mould powder thickness, and other sensors to constitute and ‘instrumented mould’ for detailed real-time process control (prediction of quality, solidification modeling, diagnostic purposes, and break-out prevention etc.). The Cu part of the mould is no longer conceived as a wear part but is instead in permanent use by refurbishing it. The refurbishing includes copper deposition followed by precision machining and Cr-coating, Ni-coating or composite coating for recovering the inner geometry.
The evolution of the convex concept has proceeded in the direction for ensuring the production of SBQ (special bar quality) steel-grades with best quality, at the highest productivity, at industrial scale and repeatability. The emphasis here is first of all on quality. The computer designed ‘all-convex’ mould produces a much more uniform temperature distribution in the billet shell compared to the other mould designs. This is achieved by a new geometry of the corner areas, which is achieved by a so-called super-circle profile instead of the standard corner radius.
This completely new geometry for a mould-cavity has developed for the purpose of producing high quality steels (SBQ steel-grades) in square, and rectangular section respectively at the highest possible productivity rate (high casting speed). An eye-catcher is definitely the generous and specially dimensioned corner rounding. Contrary to any other previously applied corner rounding, this one has neither a circular shape nor is it constant along the whole mould length. It is of a so-called super-elliptic ‘geometry, which varies over the mould length and appears as a rather large ’radius’. This perfect strand-mould contact provides uniform shell-growth especially in the corners and the areas near them. Weak spots are eliminated by better temperature distribution and homogeneity of the macro-structure is promoted. Larger super-elliptic corner radii of the ‘all convex’ mould permit a more uniform solidification of the strand shell, overcoming the issues related to the solidification of square billets.
The FEM analysis of solidification demonstrates that with a traditional mould shape the area of an initially cast rounded corner shrinks and forms a smaller radius, which consequently leads to the formation of the air gap. The change of shape along the ‘convex’ funnel to the exit of the mould reduces the quantity of the air gap and provides a ‘near to perfect’ mould- strand contact.
Fluid flow in the mould – Because of its essentially turbulent nature, several important aspects of flow in continuous casting are transient and difficult to control. However, the time-averaged flow pattern in the mould is mainly influenced by the nozzle geometry, sub-mergence depth, mould dimensions, argon (Ar) injection rate, and electro-magnetic forces.
Effect of Ar gas injection – One of the important factors controlling flow in the mould is the quantity of Ar injected into the nozzle for the control of clogging. Since the injected gas heats quickly to steel temperature and expands, the volume fraction of gas bubbles becomes significant. Those bubbles which are swept down the nozzle into the mould cavity create a strong upward force on the steel jet flowing from the nozzle, owing to their buoyancy. A few models have been applied to simulate this complex flow behaviour.
Effect of electro-magnetic forces – Electromagnetic forces can be applied to alter the flow in continuous casting in several different ways. A rotating magnetic field can be induced by passing electrical current through coils positioned around the mould. This forces electro-magnetic ‘stirring’ of the liquid in the horizontal plane of the strand. Alternatively, a strong DC (direct current) magnetic field can be imposed through the mould thickness, which induces eddy currents in the steel. The resulting interaction creates a ‘braking’ force which slows down the fluid in the flow direction perpendicular to the imposed field. Slower flow has several potential benefits such as slower, more uniform fluid velocities along the top surface, more uniform temperature, less inclusion entrapment in the solidifying shell below the mould, and the ability to separate two different liquids to cast clad steel, where the surface has a different composition than the interior.
Electro-magnetic phenomena are modeled by solving Maxwell’s equations and then applying the calculated electro-magnetic force field as a body force per unit volume in the steel flow equations. Significant coupling between the electro-magnetic field and the flow field can occur for DC braking, which then needs iteration between the magnetic field and flow calculations. Idogawa and coworkers applied a decoupled model to suggest that the optimal braking strategy is to impose a field across the entire width of the mould in two regions namely above and below the nozzle inlet. Care is to be taken not to slow down the flow too much, or the result is the same as angling the ports to direct the jet too steeply downward i.e., defects associated with freezing the meniscus. In addition, the field also increases some velocity components, which has been modeled to increase free surface motion in some circumstances.
Others have examined the application of electro-magnetic fields near the meniscus for changing the surface micro-structure. Finally, electro-magnetic stirring both in and below the mould is reported to reduce centre-line macro-segregation, presumably because of the flow effects on heat transfer and nucleation.
Transient flow behaviour – Transient surges in the steel jets leaving the nozzle parts can cause asymmetric flow, leading to sloshing or waves in the liquid pool. Jet oscillations are periodic and increase in violence with casting speed, making them a particular concern for thin slab casting. Huang has shown that a sudden change in inlet velocity creates a large transient flow structure, which appears to be a large vortex shed into the lower region of the liquid cavity. Recent transient models have re-produced periodic oscillations of the jet, even with constant inlet conditions.
Consequences of fluid Flow in the mould – The steady flow pattern in the mould is not of interest directly. However, it influences several important phenomena, which have far-reaching consequences on strand quality. These effects include controlling the dissipation of super-heat (and temperature at the meniscus), the flow and entrainment of the top surface powder layers, top-surface contour and level fluctuations, the motion and entrapment of sub-surface inclusions and gas bubbles. Each of these phenomena associated with flow in the mould can lead to costly defects in the continuous-cast product. Design compromises are needed to simultaneously satisfy the contradictory requirements for avoiding each of these defects.
Super-heat dissipation – An important task of the flow pattern is to deliver liquid steel to the meniscus region which has enough super-heat during the critical first stages of solidification. Super-heat is the sensible heat contained in the liquid steel represented by the difference between the steel temperature entering the mould and the liquidus temperature.
The dissipation of super-heat has been modeled by extending the 3-D finite-difference flow model to include heat transfer in the liquid. The effective thermal conductivity of the liquid is proportional to the effective viscosity, which depends on the turbulence parameters. The solidification front, which forms the boundary to the liquid domain, has been treated in different ways. Some studies have modelled flow and solidification as a coupled problem on a fixed grid. However, this approach is subject to convergence difficulties and needs a fine grid to resolve the thin porous mushy zone. In addition, properties such as permeability of this mush are uncertain and care is to be taken for avoiding any improper advection of the latent heat. An alternative approach for columnar solidification of a thin shell, such as found in the continuous casting of steel, is to treat the boundary as a rough wall fixed at the liquidus temperature. This approach has been shown to match plant measurements in the mould region, where the shell is too thin to affect the flow considerably.
Incorporating the effects of Ar on the flow pattern are very important in achieving the reasonable agreement observed. The temperature drops almost to the liquidus by mould exit, indicating that the majority of the superheat is dissipated in the mould. The hottest region along the top surface is found midway between the SEN and narrow face. This location is directly related to the flow pattern. The coldest regions are found at the meniscus at the top corners near the narrow face and near the SEN.
The cold region near the meniscus is a concern since it can lead to freezing of the meniscus, and encourage solidification of a thick slag rim. This can lead to quality issues such as deep oscillation marks, which later initiate transverse cracks. It can also disrupt the infiltration of liquid casting powder into the gap, which can induce longitudinal cracks and other surface defects. The cold region near the SEN is also a concern since, in the extreme, the steel surface can solidify to form a solid bridge between the SEN and the shell against the mould wall, which frequently causes a break-out. For avoiding meniscus freezing issues, flow is required to reach the surface quickly. This is why flow from the nozzle is not to be directed too deep.
The jet of the liquid steel exiting the nozzle delivers majority of its super-heat to the inside of the shell solidifying against the narrow face. The large temperature gradients found part-way down the domain indicate that the maximum heat flux delivery to the inside of the solidifying shell is at this location near the mould exit. If there is good contact between the shell and the mould, then this heat flux is inconsequential. If a gap forms between the shell and the mould, however, then the reduction in heat extraction can make this super-heat flux sufficient to slow shell growth and even melt it back. In the extreme, this can cause a break-out. Break-outs are very common at mould exit just off the corners, where contact is the poorest. This issue is worse with higher flow rates and non-uniform flow from the nozzle.
Top-surface shape and level fluctuations – The condition of the meniscus during solidification has a tremendous impact on the final quality of the steel product. Meniscus behaviour is highly affected by the shape of the top ‘free’ surface of the liquid steel, and in particular, the fluctuations in its level with time. This surface actually represents the interface between the steel and the lowest powder layer, which is liquid. If the surface waves remain stable, then the interface shape can be estimated from the pressure distribution along the interface calculated from a simulation with a fixed boundary given by equation ‘standing wave height = (surface pressure – 1 atm) / (steel density – flux density) g’.
When casting with low Ar and without electromagnetics in a wide mould, the interface is normally raised around 25 mm near the narrow face meniscus, relative to the lower interface found near the SEN. This rise is caused by the vertical momentum of the jet traveling up the narrow face and depends largely on the flow pattern and flow rate. The rise in level increases as the density difference between the fluids decreases, so water / oil models of the steel / flux system tend to exaggerate this phenomenon. Recent models are being developed to predict the free surface shape coupled with the fluid flow. Additional equations are to be solved for satisfying the force balance, at the interface, involving the pressure in the two phases, shear forces from the moving fluids, and the surface tension. Numerical procedures such as the ‘volume of fluid method’ are used to track the arbitrary interface position.
The transient simulation of level fluctuations above a turbulent flowing liquid is difficult to model. However, a correlation has been found between the steady kinetic energy (turbulence) profile across the top surface and level fluctuations. For typical conditions, the most severe level fluctuations are found near the narrow face, where turbulence and interface level are highest. These level fluctuations can be reduced by directing the jet deeper and changing the flow pattern. It is interesting to note that increasing Ar injection moves the location of maximum level fluctuations (and accompanying turbulence) towards the SEN at the central region of the mould. Casting at a lower speed with a smaller mould width means that steel through-put is less. This increases the volume fraction of Ar, if the Ar injection rate (litres / second) is kept constant. It is likely that the higher Ar fraction increases bubble concentration near the SEN, where it lifts the jet, increases the interface level near the SEN, and produces the highest-level fluctuations.
Sudden fluctuations in the level of the free surface are very detrimental since they disrupt initial solidification and can entrap casting powder in the solidifying steel, leading to surface defects in the final product. Level fluctuations can deflect the meniscus and upset the infiltration of the casting powder into the gap, building up a thick powder rim and leaving air gaps between the shell and the mould. Together, this can lead to deep, non-uniform oscillation marks, surface depressions, laps, bleeds and other defects. The thermal stress created in the tip of the solidifying shell from a severe level fluctuation has been predicted to cause distortion of the shell, which further contributes to surface depressions.
Top surface powder / flux layer behaviour – The flow of the steel in the upper mould highly influences the top surface powder layers. Casting powder is added periodically to the top surface of the steel. It sinters and melts to form a protective liquid powder layer, which helps to trap impurities and inclusions. This liquid is drawn into the gap between the shell and mould, where it acts as a lubricant and helps to make heat transfer more uniform. The behaviour of the powder layers is very important to steel quality and powder composition is easy to change. It is difficult to measure or to accurately simulate with a physical model, so is worthy of mathematical modeling. A 3-D finite-element model of heat transfer and fluid flow has been developed of the top surface of the continuous casting machine for predicting the thickness and behaviour of the powder and flux layers.
The bottom of the model domain is the steel / flux interface. Its shape is imposed based on the measurements in an operating casting machine, and the shear stress along the interface is determined through coupled calculations using the 3-D finite-difference model of flow in the steel. Uniform consumption of flux is imposed into the gap at the bottom edges of the domain along the wide and narrow faces. The model features temperature-dependent properties of the flux, with improved viscosity in the temperature range where sintering occurs and especially in the resolidified liquid powder, which forms the rim. Temperature throughout the flux is calculated, including the interface between the powder and liquid flux layers.
When the liquid steel flows rapidly along the steel / flux interface, it induces motion in the flux layer. If the interface velocity becomes very high, then the liquid flux can be sheared away from the interface, become entrained in the steel jet, and be sent deep into the liquid pool to become trapped in the solidifying shell as a harmful inclusion. If the interface velocity increases further, then the interface standing wave becomes unstable, and huge fluctuations contribute to further issues.
The casting powder layer is found to greatly reduce the steel velocity at the interface, because of its generally high viscosity relative to the steel. However, the rapid flow of steel along the interface is calculated to drag the liquid powder, in this case towards the SEN. This induces recirculation in the liquid powder layer, which carries powder slowly toward the narrow face walls, opposite to the direction of steel flow. The internal recirculation also increases convection heat transfer and melts a deeper liquid flux layer near the SEN. The result is a steel flux interface which is almost flat.
The thickness of the beneficial liquid flux layer is observed to be highly non-uniform. It is normally thin near the narrow face since the steel flow drags majority of the liquid towards the centre. The thinnest liquid powder layer is found around 150 mm from the narrow face. In this region, a flow separation exists where liquid powder is pulled away in both directions i.e., toward the narrow face gap, where it is consumed, and towards the SEN. This shortage of powder, compounded by the higher, fluctuating steel surface contour in this region, makes defects more likely near the narrow face for this flow pattern. This is since it is easier for a level fluctuation to allow the liquid steel to touch and entrain solid powder particles. In addition, it is more difficult for liquid flux to feed continuously into the gap. Poor casting powder feeding leads to air gaps, reduced, non-uniform heat flow, thinning of the shell, and longitudinal surface cracks. It is important to note that changing the flow pattern can change the flow in the powder layers, with corresponding changes in defect incidence.
Motion and entrapment of inclusions and gas bubbles – The jets of liquid steel exiting the nozzle carry inclusions and Ar bubbles into the mould cavity. If these particles circulate deep into the liquid pool and become trapped in the solidifying shell, they lead to internal defects. Inclusions lead to cracks, slivers, and blisters in the final rolled product. Trapped Ar bubbles elongate during rolling and in low-strength steel, can expand during subsequent annealing processes to create surface blisters and ‘pencil-pipes’.
Particle trajectories can be calculated using the Langrangian particle tracking method, which solves a transport equation for each inclusion as it travels through a previously-calculated liquid steel velocity field. The force balance on each particle can include buoyancy and drag force relative to the steel. The effects of turbulence can be included using a ‘random walk’ approach. A random velocity fluctuation is generated at each step, whose magnitude varies with the local turbulent kinetic energy level. To get considerable statistics, the trajectories of several hundred individual particles are to be calculated, using different starting points. Inclusion size and shape distributions evolve with time, which affects their drag and floatation velocities. Models are being developed for predicting this, including the effects of collisions between inclusions and the attachment of inclusions to bubbles.
Majority of the Ar bubbles circulate in the upper mould area and float out to the casting powder layer. This is since the flotation velocities are very large for 0.3 mm to 1 mm bubbles. Several inclusions behave similarly. Bubbles circulating in the upper recirculation region without random turbulent motion are predicted to always eventually float out. Bubbles with turbulent motion frequently touch against the solidification front. This occurs on both the inside and outside radius with almost equal frequency, particularly near the narrow faces high in the mould, where the shell is thin (less than 20 mm). It is likely that only some of these bubbles stick, when there is a solidification hook, or other feature on the solidification front to catch them. This entrapment location does not correspond to pencil pipe defects.
A few bubbles manage to penetrate into the lower recirculation zone. Here, a very small Ar bubble likely behaves in a similar manner to a large inclusion cluster. Specifically, a 0.15 mm bubble has a similar terminal velocity to a 208-micrometers solid alumina inclusion. These particles tend to move in large spirals within the slow lower recirculation zones, while they float towards the inner radius of the slab. At the same time, the bubbles are collecting inclusions, and the inclusions are colliding. When they eventually touch the solidifying shell in this depth range, entrapment is more likely on the inside radius. This can occur anywhere along their spiral path, which extends from roughly 1 m to 3 m below the meniscus. This distance corresponds exactly with the observed location of pencil pipe defects. There is a slight trend that smaller bubbles are more likely to enter the lower zone to be entrapped. However, the event is relatively rare for any bubble size, so the lack of sufficient statistics and random turbulent motion mask this effect.
Sudden changes to the inlet velocity can produce transient structures in the flow which contribute to particle entrapment. After an inlet velocity change, the large zone of recirculating fluid below the jet is modeled to migrate downwards like a shedding vortex. The particles carried in this vortex are then transported very deep into the liquid pool. Hence, they are more likely to become entrapped in the solidifying shell before they float out.
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