Initially, the casters were totally vertical but considerable height was required to achieve satisfactory production rates per strand. With the development in the Basic Oxygen Steelmaking (BOS) process having the capacity to produce in excess of 400 tonnes/hour, more strands were required to match the feeding rate with casting rate.
Vertical caster was the natural technique of casting with gravity which offers a symmetric macrostructure but caster productivity is severely hampered due to the machine height. Hence several efforts were put to extend the machine length but reduce the machine-building height by the technique of strand bending and straightening method. Based on the concepts of critical strain and strain rate to prevent inner cracking various concepts were developed which finally led to bending and straightening zones extended over roller pairs.
Some noteworthy contribution was the billet caster by Rowley and a more advanced proposal by Tarquinee and Scovill which had the provision for strand in-line sizing after temperature equalization and this concept was recognized by the U. S. Steel for the South Works pilot (1961) and the Gary Works No. 1 slab caster (1967), respectively. Thus, the slowly new designs evolved ranging from the totally vertical machine to the “low head machine”.
In 1965, the continuous was very simple. 80% of the casters, for slabs, blooms, and billets, were vertical machines. With the development in casting with curved machines, in 1975, 80% of the slab casters and 70% of the bloom and billet casters were of the curved type. This trend continued to progress but towards more complex geometry, with the application of the progressive bending and straightening, which in 1984 was used in 30% of the slab casters and in 20% of the bloom and billet casters.
Some of the disadvantages associated with the vertical casters were:
1. requirement for increased height to achieve higher production rates.
2. extra financial burden in buildings and crane height.
3. The mechanism for turning the slab to a horizontal position after cutting was complex and costly. To reduce building and crane heights the bottom end of vertical casters were often built with deep pits that required subsequent slab lifting after cutting and turning. In the early 1960s in order to simplify and reduce the cost of lifting and turning mechanism, several mechanisms such as bending and straightening pinch rolls after solidification and hence the cast strand was traveling in the horizontal direction prior to cut off. However, this technique was not helpful in significantly reducing the machine height but some benefit was achieved.
4. The other main disadvantage of vertical and vertical with bending caster was the duty on the roller support system due to the action of greater ferrostatic forces caused by the machine heights. This, in turn, increased maintenance involved in order to maintain the roller gap geometries and roller alignments so that tight tolerances are ensured.
The machine-building height was further reduced after the advent of the curved mold design casting principle after the pioneering plant trials at Mannesmann Huckingen and Von Moos Stahl in 1963. This development was the initiation of the rapid growth of Continuous casting application especially for small casting shops and thus emerged mini-mill where steel was produced through the EAF route.
In the early years, the curved mold machine has been widely used. This enables the radius of such a machine to be typically 8 to 12 meters depending on the product size and the thickness. This, in turn, reduces the ferrostatic forces whilst achieving the throughput requirements and, in many machines, the solidification position can be 30 to 40 meters from the meniscus without increasing the ferrostatic force beyond the machine radius.
Curved mold got wider acceptance and even today, these are considered for casting bloom and billets. However, the curved mold compensates on the quality front.
Moreover, its widespread application in slab casters got halted in recent times owing to accentuated quarter band accumulation of macro inclusions and/or argon bubbles (with inclusions attached) which leads to high reject rates on a cold-rolled sheet of ultra-low carbon (ULC) steels.
Thus, recently developed casters are built as a straight mold with the continuous bending type(V-B) and old curved mold casters are revamped in order to meet more stringent requirements on product cleanliness. The strands with liquid core are supported unless self-supporting to avoid creep or critical shell deformation and this applies for rounds and billet casting. Whereas, for rectangular sections especially slabs roller support is provided for wide faces for some distance keeping in mind that strands don’t get squeezed by driven rolls in the withdrawal system. At the same time uniform cooling is recommended in order to avoid shell deformation due to thermal stress. Thus, roller support arrangements and cooling profiles form the basis for caster performance and maintenance in the future. However, internal cracking is bound to happen under critical conditions of high-speed casting even for perfectly aligned and uniform cooling profiles and then stronger/hard cooling remains the remedy.
As far as internal quality is concerned another concern was the formation of mini-ingot formation, i.e., intermittent center porosity and macrosegregation. The contributors being the dendrite bridging in the case of columnar growth, and liquid core “pumping” due to strand bulging near the crater end in the case of slab sections. The central porosity and macrosegregation inducing columnar- to-equiaxed transition can be reduced by the application of electromagnetic stirring in the mold and/or strand and/or final solidification zone.
Mechanical soft reduction proposed by NKK in 1974 near the crater end in slabs brings about controlled strand squeezing, which has been found effective in improving central soundness and is now increasingly applied for large bloom sections. For small cross-sections like billets and blooms hard cooling near the crater has been found equally effective. Recent casters have the provisions for direct dispatch of the cast and cut product to the rolling, thereby utilizing the heat content of the cast product and reducing the carbon dioxide emissions and promoting shorter lead time and reduced stock volume.
Product inspection is greatly reduced and quality assurance is based on computerized on-line monitoring of the process variables by means of an expert system. The computer-assisted quality assurance system also allows dynamic scheduling, i.e., diversion to other orders, if steel cleanliness or inner soundness is predicted as not conforming to original order requirements.
Nowadays, multi-radius machines are used which helps in further reducing the ferrostatic forces but factors like quality and mold teeming difficulties often restrict minimum achievable height. The strand could become totally horizontal but difficulties arise with the liquid steel feed arrangement. Significant efforts have been put to develop a horizontal caster and some machines do exist but are limited to billet casting.
This evolution reflects the need for higher casting speeds with the trend towards building longer machines but without increasing the machine height. With the circular arc machines, it is possible to increase the supported length without further increasing machine height and for slab machines, the strand needs full support until after complete solidification.
Slabs for strip and plate can be cast in a wide range of widths varying according to the type of final product such as tin plate, strip for deep drawing, hot-rolled strip, heavy plates or tubes. Typically, the required slab width can range from 800mm to 2000 mm in increments of 50mm. Until about a decade ago, slab width changes on the caster were only possible during the non-casting time which restricted the sequence length and productivity. Various methods have been developed to deal with the requirement of a large number of widths and which reduces the range of production rates.
Variable mold widths- Systems have been developed for changing width during casting. This technology can certainly increase sequence length and hence overall productivity but cannot achieve the sufficiently short casting times when casting the narrow widths unless combined with twin casting.
Twin and Triple casting– This development enables two or three narrow sizes to be cast on one strand of the slab machine. This is achieved by replacing the single wide slab mold by 2 or three small molds. Twin casting can be done by the use of two separate molds or by a water-cooled copper divider in the slab mold. In the latter case, each of the two narrow slabs can be varied in width by the use of the variable width technology.
Edge reduction in the Rolling mill- In the hot strip mill, some limited width reduction can be achieved by the use of edger mill ahead of the roughing mill.
Slab Longitudinal Slitting- This provides considerable flexibility in achieving a wide range of widths for only a limited range of cast widths without detriment to an overall production rate of the steel plant and caster. However, a significant yield penalty occurs due to torch cutting.
