Continuous Casting Mould
Continuous Casting Mould
In the continuous casting process, liquid steel flows from a ladle, through a tundish into the mould. The mould is regarded as the heart of the continuous casting process and plays a very important role in the efficiency of the process and the strand quality. It is in the mould that the final cast shape and the strand surface quality are produced. If the conditions are not correct in the mould, then the strand quality cannot be corrected later. Once in the mould, the liquid steel freezes against the walls of the water-cooled copper mould to form a solid shell. Mould is basically an open-ended box structure containing a water-cooled inner lining fabricated from a high purity copper alloy. The box can come in many shapes and sizes in order to cast different semis such as blooms, billets, round beam blanks, slabs, and thin slabs.
Once the liquid steel refining process is completed during steelmaking, the liquid steel contained in the ladle is normally sent to a continuous casting machine. The steel is poured from the ladle to a tundish and then from the tundish into a water-cooled copper mould which induces the formation of a thin, solidified steel shell (Fig 1). Flow between vessels is driven by gravity. Between the tundish and the mould, this driving force is proportional to the head of liquid steel between the top surface of the liquid steel in the tundish and the liquid steel level in the mould. Control of the flow rate into the mould is achieved by metering nozzles, stopper rods or slide gates. The metal level in the mould, which is known as ‘meniscus’, is very important for the surface quality of the cast product. Accurately controlled and consistent conditions are needed for all parameters in the mould for the surface quality of the cast product.
Fig 1 Schematic of mould region of continuous 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, thus, 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.
Metal flow rates are matched with the slab casting speeds using a stopper rod in the tundish, a slide gate, or a metering nozzle just above the shroud to control the delivery rate. Billets are normally cast with fixed metering nozzles, and the strand speed is adjusted to any changes in steel flow rate. It is very important for good surface quality of the cast product that the liquid steel meniscus level is accurately controlled within a tight band of operation, at least within +/- 5 mm of set point and normally within +/- 3 mm. The measurement of level can be achieved by a number of methods.
As there is a relative motion between the strand and the mould wall, some form of lubrication is needed. A thin film of lubricating oil or of lubricating flux is interposed between the mould and the hot liquid phase to prevent its direct contact with the mould, which can potentially endanger and damage the mould itself.
Liquid steel in the slab mould is normally covered with a layer of mould powder (casting powder) to protect the steel from reoxidation and absorb inclusions. The powder has a low melting point and flows over the liquid steel to provide mould lubrication and to help control heat transfer. It also serves to protect the liquid steel against reoxidation, thermally insulate the free surface and absorb any inclusion which can float to the surface. Rapeseed oil, which has since been replaced by synthetic oils, has typically been used to prevent sticking to the mould in case of billet casting.
The development and increasing use of continuous casting have transformed the moulds from pure and simple containers for the liquid steel into the principal component required to attain the goals in terms of quality and production. The choice of ever more sophisticated materials which increase the products life and improve its heat-exchange features, coupled with studies on optimal taper, have successfully turned the mould into an object which is at the cutting edge of modern technology. Its features are specific to each continuous casting machine, and thus being developed continuously for technological improvement.
During continuous casting, the copper mould plates control the shape and initial solidification of the steel product, where quality is either created or lost. Maintaining a reliable, crack-free mould within close dimensional tolerances is also crucial to safety and productivity. The costs associated with mould maintenance are a significant fraction of the operating costs of a casting machine. Thus, it is important to understand the thermal and mechanical behaviour of the mould.
Maintaining a reliable, crack-free mould within close dimensional tolerances is also crucial for safety and productivity. Thus, studies have been undertaken to better understand the complex thermal and mechanical behaviour of the mould. An extreme temperature gradient takes place across the copper plates and this causes geometrical distortions of the mould. Moreover, long hours of operation at high temperatures generate creep. This resultant creep is also associated with a thermal fatigue phenomenon, which is caused by the many room temperature heating and cooling cycles undergone by the mould during the initial and final transitory of the casting sequence 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.
The performance requirements which are to be met by the moulds and the mould materials depend on the specific application and the levels of stress involved. These stress levels are mainly pre-determined by the machine and the casting parameters. This means that many 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 mould.
At the meniscus position, after the initial very short, close contact between the liquid steel and the mould, a fully defined shell is formed. Once the shell exists, there is normally a barrier between the solidified steel shell and the mould wall. This barrier can be either liquid or solid casting flux, an air gap, or a combination of all these. Once the nascent shell is formed, it starts to grow in thickness. However, initially the thickness is very small, and the temperature of the shell is high, so that the shell is very pliable. This means that the mould has to provide support for the shell in order to maintain the needed cross-section of the strand. This support is to be continuously around the circumference and along the length of the mould for a sufficient time until the shell is relatively becomes self-supporting.
The requirements placed on modern mould materials are high 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 to solidify in order to form a sufficiently stable strand shell. The resulting extreme temperatures demand moulds to be with higher strength levels. At the same time, a high alternating thermal stress can occur, for example on casting rolls. This wide variety of requirements placed on the mould has to be met by highly developed materials and system expertise.
The flow-through water-cooled copper mould is the key element of the continuous casting machine. Special attention is to be given to problems associated with the design and material requirements for the moulds. A number of different designs have been used, including thin-wall tube-type moulds, solid moulds, and moulds made from plate. Plate moulds have been found to provide good mould life and to avoid the necessity for making of moulds from solid copper blocks.
For ensuring optimal performance, moulds are to keep their original specifications at mean operating temperatures as long as possible, and is to, above all, have adequate heat transfer capacity. Thermal stresses, which arise mainly on the hot faces in the area of the meniscus, result in more or less rapid and permanent deformation of the mould, thus cutting short its life. The seriousness of this phenomenon is related to the temperature level inside the mould, and to temperature differences between the hot faces and the cold faces, and between the area of the meniscus and the area immediately below it. The appropriate solution for each of the various operating conditions depends on correctly choosing the material for the mould.
Steel and brass, as well as copper, have been used for moulds, but the most outstanding material is nearly pure copper with small additions of alloying elements which promote precipitation hardening or raise the recrystallization temperature, because both effects apparently provide longer mould life. Popular mould materials are DHP (deoxidized high phosphorus) copper, CuAg, CuCrZr, CuNiBe, and CuNiP. Mould coatings are applied to extend service life.
The properties of DHP copper material are widely known. DHP copper is still today the most widely used material to manufacture moulds for the continuous casting of billets, where the thermal flow is normally moderate and the thickness of the moulds not excessive.
Silver bearing copper material is obtained by adding 0.10 % silver to the copper. This increases the recrystallization temperature by around 100 deg C. Because of its properties, this alloy is used to manufacture moulds for the casting of blooms and slabs, where the temperature at the meniscus reaches and exceeds 300 deg C. Such high temperatures are due to the considerable thickness of the walls and to the high thermal flow inside the mould.
The fact that this material maintains its initial hardness (HB higher than 80) for long periods of exposure at 300 deg C, also makes it possible to re-process plates which have been subjected to repeated wear before reaching the minimum prescribed thickness. Silver-bearing copper is also widely used for moulds producing billets in special conditions, such as weakly sequential casting, high casting speeds, cooling conditions which are not optimal, high temperature delta of the cooling water, and others.
For improving the mechanical properties at high temperatures of copper alloys with high thermal conductivity, metallurgical specialists have turned to structurally hardened alloys. The main elements used for these copper alloys, whose solubility generally varies according to temperature variations, are mainly Be – Cr – Co- Cd – Fe – Mg – Mn – Ni – Nb – P – Si – Sn – Ti – Zr. There are several alloys which can be obtained in saturated solution of these elements, but results are not always compatible with industrial requirements, such as coping with pollution problems, high costs, and excessive loss of thermal conductivity. Hence, the number of alloys which can be used in practice is considerably reduced. The percentage of addition of elements is further restricted by the need to harmonize a high level of hardness with a high degree of thermal conductivity. The CuCrZr (copper-chromium-zirconium) alloy satisfies all the above-mentioned requirements, and is used also because its excellent properties allow it to maintain its hardness for long exposure periods at high temperatures.
The remarkable success in further developing the continuous casting process has greatly increased the need to carry out adjustments to the mould which enables the technology of continuous casting to fulfill the expectations of the players in this field. Presently, rising to the challenge, the range of traditional materials has been broadened with a new alloy (CuNiP), whose chemical composition can be altered in accordance with single applications, thus personalizing each type of mould to meet the specific requirements of each user. The alloy in question allows to correctly combining thermal conductivity and mechanical resistance at high temperatures, in an effort to minimize the problem of temperature variations across the mould’s entire perimeter. This has obvious advantages for solidification conditions without excessive thermal stress in the solidified skin, as well as in the mould itself. The controlled thermal conductivity of this new alloy considerably diminishes the critical state of the cooling conditions, which are normally linked to three variables namely (i) thickness of the lubricating film, (ii) thermal flow, and (iii) shrinkage of the solid skin. As a result, excessive thermal stress and problems of cracking are both eliminated.
The surface of the copper mould coming in contact with the hot liquid steel is frequently plated to provide a harder working surface and to avoid copper pick-up on the surface of the cast strand, which can facilitate the development of the surface cracks on the cast product. A chromium and nickel coating is generally used, often with an intermediate layer of nickel for improved coherence. The technology of chromium coating has advanced considerably. Now the deposit, with the thickness appropriate to the various needs, guarantees complete satisfactory results at all the levels.
At present, both metallic and ceramic plating are available. The ceramic plating allows for an increased mould lifespan but it is not widely used due to its high cost and low thermal exchange. To the contrary, metallic plating is either nickel or chromium-based. Despite its brittleness and low wear resistance, chromium is the most used metallic element in mould plating.
Normally Ni-Cr special coatings are used for the coating of the extruded copper mould tubes. This coating consists of a double layer coating of nickel and chrome. The component in contact with the extruded copper hot face is a nickel alloy, which is then overlaid with a layer of hard chromium. This approach is derived from the experience gained in producing four piece plate moulds. Accordingly, the fundamental process of applying a layer of nickel between the chromium and copper to achieve a much higher service life for the mould is particularly useful for avoiding the formation of cracks in the chromium coating, especially in the meniscus zone of the mould. The nickel alloy, in fact, has a coefficient of thermal expansion which is almost double that of the chromium. Hence, the nickel alloy coating is better able to tolerate the greater expansion of the copper which takes place in the meniscus zone during the casting process.
Until now, nickel plating the inner surface of a single piece extruded mould tube has proven to be particularly difficult, with the technical issues focusing on the regularity and surface quality of the nickel coating. With the advent of the Ni-Cr coating process developed, it is now possible to achieve a smooth and consistent nickel plated surface in both the corners and flat surfaces of the mould tube hot face. Comparative field tests in a number of steel plants have confirmed a significant increase in the average life of mould tubes plated with the Ni-Cr coating, as compared with the moulds tubes plated with the conventional chrome coating.
It is a well known operational fact that the removal of an extruded single piece mould tube from service is determined by the wear conditions of its inner dimensions. If not addressed, these wear conditions can lead to solidification problems, and/or defects in the final cast product. It is also a well known operational fact that the corners of the mould tube tend to wear faster than other areas within the mould tube which is a function of the rapid solidification that takes place in the corners. For addressing this problem, a method of applying the chromium coating has been developed with specific geometric characteristics, wherein the chromium thickness in the corners is thicker.
This unique plating geometry provides a coating which better withstands corner wear, while at the same time preserves the heat transfer properties which are necessary for the proper solidification of the flat faces. Comparative laboratory and field tests have confirmed that this new and unique coating geometry considerably addresses the corner wear problem.
Chromium coating is still widely used for plate moulds for blooms and beam blanks, and the technology is well known. Plate moulds for slab casting are generally nickel coated, and have varying degrees of thickness and diverse configurations. A thin chromium layer is sometimes applied to increase the durability of the nickel coating.
As well as Nickel which is available in two different hardness configurations, new nickel and cobalt alloys have recently been tried, and these afford better wear resistance. Choosing of one of the standard solutions described above depends entirely on the specific operating conditions each user adopts, and to the moulds’ maintenance and re-machining requirements. Through comparative tests, the user is to establish the best coating thickness to increase availability and cost-effectiveness, which in turn ensures longer mould life.
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, 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 because bulging of the solidifying shell is less severe. At higher casting rates, the use of an increased taper in the mould is necessary to maintain high heat removal rates, particularly for the narrow faces of slab moulds.
Fig 2 Mould length and casting speed
There are three alternatives which normally apply for the continuous casting mould arrangements. These are (i) plate moulds for slabs and larger blooms (Fig 3), (ii) tube moulds for billets, smaller blooms, and rounds (Fig 4), 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 (Fig 4). Plate and tube moulds are popular types of moulds while the block and funnel moulds are expensive because of the amount of copper used and the extent of machining needed for the mould production.
Production of plate moulds involves the 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, to achieve 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 copper 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 to be cast product. Cooling is achieved by water cooling in slots behind the copper plate. The fastening of the copper plate is normally done by bolts, fastening into the copper 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 copper 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 copper 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 therefore 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 originally 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 2 Continuous casting slab mould
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 to attain the geometrical and mechanical features required by the technical specifications, which of course also includes taper. For the latter step which is by far the most crucial one in the production cycle, a well equipped powerful and best equipped press is to be used. Forming is accomplished with special steel equipment, which is specific to each mould and is produced with CNC machines. Finally, the tube mould undergoes machining and is then chromium-plated internally, before being inspected and measured.
For tube moulds, there is no discontinuity around the circumference of the mould copper, the mould being formed by a copper tube. There is therefore 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 upto 30 mm or 40 mm thick for large section rounds 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 therefore 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. As the tube mould use thinner copper 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 m/sec to 13 m/sec.
In tube moulds also, tapers are applies 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 (upto 6 m/min), and the shrinkage is more pronounced. Parabolic tapers have beneficially being 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 aim to reduce the effect of the shell pull away in the corners and therefore 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 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 is the first concretization of this concept. Possibly imagined by a rugby player, the shape was designed to accommodate the submerged nozzle, a compulsory technology for casting clean Al-killed carbon steels.
Oscillating moulds have been adopted almost universally, although fixed moulds can be successfully used with efficient lubricating systems. The oscillation is usually sinusoidal, a motion which can be achieved easily with simple mechanical arrangements. A fairly short stroke and a high frequency are used to provide a short period of ‘negative strip’ during each oscillation, in which the mean downward velocity of the mould movement is greater than the speed of withdrawal of the casting strand in the casting direction. Oscillating frequencies are being increased from 50 cycles per minute (cpm) to 60 cpm upto 250 cpm to 300 cpm, with the benefits of shallower oscillation marks, less cracking and reduced conditioning requirements.