Productivity and Product Quality

 

In the relatively short time span since the commercial application of continuous casting in the early 1960's, there have been a wide variety of new process developments directed at improving productivity and product quality. These developments include new machine design concepts, metallurgical practices, and the application of process control and automation by computer systems. The main driving force behind these developments has been the recognition that substantial yield and energy savings are possible which have a dramatic effect on operating cost. Through these developments major quality improvements have been obtained such that the product is fully equivalent to and exceeds that of ingot steel. Today, essentially all grades of steel, including the highest qualities for critical applications, can be efficiently produced by continuous casting.

 

PRODUCTIVITY

 

Productivity improvements have been directed at decreasing the caster downtime and thus increasing the time that the machine is actually casting utilization time while maintaining the ability to produce the variety of product sizes and steel grades. There are five major factors which contribute to downtime that have been addressed:

 

• machine set‑up time following the completion of a cast;
• mold changing for casting different section sizes;
• casting machine or strand stoppage because of failures such as strand breakout, tundish nozzles blocked by frozen metal or inclusion build up, and uncontrolled flow of metal from the ladle (e.g., a running stopper);
• outwtspe6hcation heat composition and temperature and
• machine maintenance.

 

In addition to improved steelmaking control practices and techniques, the influence of these factors has been reduced by the development of new operating concepts and equipment designs. The major changes in operating concepts include:

 

• Sequence casting to reduce machine set‑up time.
• Slab slitting to reduce the frequency of mold changes and to reduce mold inventory.
• Variable‑width adjustable molds to reduce mold changing time
• Divided or split molds to reduce mold changing time and mold inventory and to increase casting rate (tons per hour per strand)
• Top‑fed dummy bar to reduce set‑up time
• Hot charging and direct rolling

 

Sequence Casting ‑Casting machine setup, after the completion of a cast, is time consuming since it involves feeding the dummy bar through‑the entire length of the casting machine into the mold cavity and packing the dummy bar head to prevent leakage between the mold walls and head. Sequence casting was developed to reduce the frequency of setting the dummy bar by casting a series of heats in succession without interrupting the casting process.

This practice has been widely adopted and it is not uncommon in slab casting for a series of 40 heats to be cast successively representing several thousand tons; strings of several hundreds of heats, tens of thousands of tons have been cast. However, this practice also demands precise heat scheduling, high‑machine reliability and the ability to rapidly change ladles (within one or two minutes), tundishes and refractory tube shrouds between the ladle and tundish and between the tundish and mold‑ Special purpose equipment has been designed to provide this capability. One example is the use of rotating ladle turrets which can have a single rigid dewing arm carrying two ladles, or two individual dewing arms, carry one ladle each. In addition, some designs include a ladle lifting mechanism and weighing equipment. Another example is rapid‑change tundish cars which also have lifting devices to facilitate shroud changing.

 

Slab Slitting ‑ One problem experienced in casting a long series of heats successively is that of scheduling the different slab sizes required by the finishing mills for different customer applications. Rather than interrupting a string of heats to change the mold size, which in itself represents a loss of casting time, a practice has evolved in which a small number of "master" slab sizes are cast with the slab product being slit longitudinally in a separate operation using mechanized oxy‑natural gas torches. Sophisticated computer‑assisted programs have been developed to minimize both the potential yield loss and inventory, as well as meet the scheduling requirements.

 

Adjustable Mold Width ‑ To minimize both the time required to change a mold as well as the mold inventory, stepless‑width adjustable molds were first developed for slab casting which could be adjusted without the mold being removed from the casting machine. The adjustment could be made either manually, electro‑mechanically or hydraulically while the previously cast slab was being removed from the machine. More recently, as an alternative to slab slitting, the slab width can be changed during the actual casting operation.

 

Divided Molds ‑ Another development which increases the productivity of a casting machine is the use of mold inserts. For example, in a new slab caster permanent divider in the center of the mold permits the casting of two narrow slabs simultaneously on a single strand using common containment and withdrawal units.

 

Top‑fed Dummy Bar ‑ Several systems have been developed for inserting the dummy bar into the mold. The shortest set‑up time is obtained by inserting the dummy bar through the top of the mold (as opposed to the older methods where it is inserted through the entire machine into the bottom of the mold). With the top‑fed design, the dummy bar can be inserted before the previously cast strand has passed through the machine. In addition to this advantage, the top‑fed dummy bar is also shorter than the more conventional bottom‑fed type.

 

Hot Charging and Direct Rolling ‑ Although the practice of hot charging a semi‑finished shape into the reheating furnace of the finishing mills is not necessary a productivity improvement attributable to continuous casting, it has, nevertheless, received wide attention because of the potential fuel savings. In the early development of continuous casting the product was cooled to ambient temperature, inspected for defects and, if necessary, conditioned to remove the surface defects (a practice that is comparable to that used for many ingot‑rolled, semi‑finished products). The product was then reheated and further processed in the finishing mills which involved an appreciable consumption of energy. By charging hot continuously cast product into the finishing mills, the sensible heat of the product is utilized with significant energy savings. This practice may avoid reheating altogether or require some intermediate reheating. However, it demands close coordination between the caster and finishing area. It also demands excellent surface quality because on‑line hot inspection and conditioning of the cast material is not yet fully developed.

 

PRODUCT QUALITY

 

The quality of continuously cast steel is dependent on the steelmaking and casting practices employed. It is affected by the interaction of chemical and physical factors which must be closely controlled to obtain the full potential of the process.

 

Typical defects experienced in continuous casting have included:

  

• Surface
• Deformed cross‑section (including concavity and convexity)
• Cracks (longitudinal and transverse)
• Laps, scale and entrapped inclusions and slag
• Oscillation marks

• Sub‑surface: Pinholes and blowholes

• Inclusions
• Cracks

• Internal: Cracks (central, diagonal and half‑way)

o        Porosity

• Inclusions
• Segregation

 

Crack formation is related to a wide variety of physical causes. Techniques employed to eliminate or reduce the occurrence of external and internal cracks include:

 

• Surface cracks: mold and secondary cooling, mold lubrication, mold coatings, mold wear control, machine alignment and casting speed.
• Internal cracks (and porosity): machine type, machine alignment, electromagnetic stirring, in‑line reductions, multi‑point straightening, compression casting, liquid steel temperature and casting speed.

 

Laps and scabs are related to casting speed control and the integrity of the pouring stream between the tundish and mold. Oscillation marks are a function of the steel grade cast and the type of mold oscillation.

 

Pinholes and blowholes are controlled by deoxidation and tundish stream shrouding. Center line segregation has been minimized by low casting temperature, electromagnetic stirring and casting speed.

 

The frequency of inclusions, whether at the surface, sub‑surface or in the interior of the cast sections, has been progressively reduced through improvements, for example, in steelmaking, deoxidation and shrouding practices, and equipment design. These improvements form an integral part of a continuing effort to further upgrade the quality of cast products.

 

The most significant recent developments in improving product quality include:

 

(1)     the concept of "clean" steels;

(2)     the application of electromagnetic stirring; and

(3)     air‑mist cooling to further reduce the incidence of surface cracks.

 

One of the primary objectives, in the case of flat‑rolled steels, is to produce a cast surface which does not require conditioning prior to further processing.

 

"Clean" Steels ‑The concept of "clean" steels is multifaceted and involves a number of techniques and practices designed to: minimize the number of inclusions formed during deoxidation; avoid inclusion formation by reoxidation; minimize inclusion pick‑up from refractories; and facilitate the separation and removal of inclusions. Additional important objectives are the ability to desulfurize steel to low sulfur levels (including sulfide shape control) and oxide inclusion modification (primarily in aluminum‑killed steels) to minimize inclusion deposition in the tundish nozzle. The full spectrum of practices (several of which have been discussed in the preceding section) extend from furnace tapping through fluid flow control in the mold. They are:

 

• Slag‑free tapping
• Secondary refining in the ladle (ladle metallurgy)
• Complete shrouding between the ladle, tundish and mold
• Tundish design
• Refractory selection for ladle and tundish
• Mold powders
• Electromagnetic stirring

 

Slag‑free tapping techniques have been designed to retain steelmaking slag in the steelmaking vessel until the steel has been tapped into the ladle. The absence of slag in the ladle avoids a loss of deoxidation elements, and permits a closer control of deoxidation additions as well as avoiding a source of inclusions. It also provides a closer control in secondary refining operations such as desulfurization.

 

Secondary refining serves a number of functions depending on the type of processes installed. They can include: temperature homogenization (by mixing) as well as the addition of heat; vacuum treatment (oxygen and hydrogen removal) to minimize the number of inclusions formed when solid deoxidizers are subsequently added; the addition of deoxidizers (either as bulk additions or via wire feeders); addition of oxide‑inclusion modifiers, such as calcium wire; and the addition of desufurizing agents and sulphide‑shape modifiers.

 

Shrouding between the ladle, tundish and mold has been discussed previously. Refractory shrouds, tundish, nozzles and stopper heads have been designed to permit the introduction of inert gases to both prevent air infiltration as well as to minimize the deposition of aluminum‑type inclusions on the walls of these components and subsequent blockage (Fig. 1).

 

Large capacity, deep tundishes are used to increase the steel residence time to facilitate inclusion removal, as well as to provide a "buffer" in sequence casting. Dams and weirs have also been incorporated to assist inclusion removal and to improve fluid‑flow conditions to the nozzles.

 

Fig. 1: Shrouding techniques using inert gas to prevent nozzle clogging by alumina clusters [1]

(a ‑ porous ring; b ‑ stopper; e ‑ micropose; d ‑ slit)

 

High‑quality refractories are used in the ladle and tundish to avoid inclusion pick‑up from erosion, to extend the service life and, in the use of vacuum treatment, avoid chemical breakdown of the refractory constituents. High‑alumina and basic refractories are being used to eliminate the pick‑up of oxygen from the silica in fireclay refractories.

 

Mold powders, discussed previously, are used in casting aluminum‑killed steel to absorb nonmetallic inclusions from steel in the mold, to enhance heat transfer to the mold wall, protect the liquid steel from reoxidation, and as a thermal insulation on the liquid steel surface.

 

Electromagnetic Stirring ‑The potential benefits to be derived from electromagnetic stirring of liquid steel during solidification are receiving wide attention. Improvements reported include:

 

• Internal quality (reduced segregation, cracking and porosity) through a preferred solidification structure.
• Sub‑surface and internal cleanliness through a modified metal flow pattern.
• Reduced criticality of casting parameters (temperature and casting speed).
• Increased productivity through increased casting speeds.

 

Electromagnetic stirrers were initially installed on billet casters to reduce centerline segregation. This was achieved by a change in solidification structure: the area of the central equiaxed crystal zone was increased with a corresponding decrease in the area of the outer columnar crystal zone. Subsequently, the other improvements listed were recognized. In a recent survey, it was reported that over 100 stirrers are in operation, of which over 60 are on billet and bloom machines with approximately 40 on slab casters.

 

There are two basic types of stirrers, (rotary and linear) which can be installed either in or below the mold. In a rotary system installed in the mold of a billet caster, a rotating magnetic field produced by the coils imparts a circular motion to the liquid steel. The centrifugal force developed results in a sound skin, with the lighter phases (i.e., inclusions) moving towards the center. The central equiaxed zone is enlarged because the rotational flow promotes the fracturing of the tips of the columnar dendrites which serve as nuclear for equiaxed crystal formation in the central zone.

 

Fig. 2: Electromagnetic stirrer on a billet or bloom caster [1]

 

With the linear system, electromagnetic coils are installed along the side of a strand (below the mold) which produce a vertical circulation pattern (Fig. 2). The increase in the central equiaxed crystal zone is obtained by a similar mechanism as that obtained by the rotary stirrer. Inclusions, which are normally concentrated in a band close to the upper surface in curved mold machines, are more uniformly distributed.

 

In another application of electromagnetic principles on a slab caster, a magnetic field is used as a brake to modify flow patterns within the mold in certain areas and to create flow patterns in others, with a subsequent improvement both in internal cleanliness and surface quality. This effect is achieved through the interaction of a moving steel stream in a stationary magnetic field. The metal stream moving through the magnetic field produces induced currents which, together with the stationary field, creates forces which brake the steel streams. In addition, steel between the streams and the poles of the electromagnet is accelerated which provides a strong stirring action. Thus, the velocity of a metal stream exiting the ports of a refractory tube shroud is reduced as well as the depth of penetration into the liquid crater. Under these conditions, inclusion concentrations are reduced and a more uniform shell growth occurs around the periphery of the mold which lessens the possibility of surface defects.

 

Mist Cooling ‑ Conventional water sprays in the secondary cooling section can aggravate the occurrence of surface cracks initiated in the mold because the cooling is relatively uneven both in the longitudinal and transverse direction. Local overcooling can also occur, for example, in a slab casting machine from water trapped by a containment roll and the slab surface. A reduction in spray intensity by the use of smaller nozzles ("mild" spray cooling) is difficult to achieve because smaller nozzles are easily blocked.

 

Cooling by an air‑water mist is being adopted with improved cooling characteristics being reported: heat transfer is improved, resulting in significantly lower water volumes (steam is constantly removed by the compressed air); and greater cooling uniformity which reduces both longitudinal temperature variation (minimal water retention at the containment rolls) as well as slab‑edge‑to‑slab edge temperature variations. Air-water mist, by cooling the strand more uniformly, reduces the thermal stresses which can enlarge surface defects into cracks during casting.

 

The mist is produced by an atomized nozzle in which cooling water and compressed air are premixed; the mist is discharged through a slit outlet from a pressure chamber. Nozzle blockage is minimized because the discharge area is approximately 100 times larger than nozzles used for "mild" spray cooling.

 

 

References

[1] The making, Shaping and Treating of Steel, 1985, US Steel.

[2] The making, Shaping and Treating of Steel, 2002, AISE Steel Foundation.