**1. Introduction**

The quest for billets and blooms production in the continuous casting of carbon steels with more stringent quality demands in internal soundness, free from surface defects, and internal porosity has increased the need for more insight in the appraisal of the phenomena associated with solidification under industrial conditions. Medium and high carbon steels cover a broad range of manufacturing products; nevertheless, the production of this type of material with a constant high percentage of prime-choice products remains a tough subject to analyze, understand, and more difficult to attain in practice. These carbon steels exhibit relatively low liquidus and solidus temperatures, with these values decreasing even further as carbon levels increase. It is understood that the blooms produced must be 100% crystallized (solid) at least before cutting to the delivered lengths is accomplished. It is realized that the larger the crosssection of the produced cast product the more time it requires in order to solidify completely. Big-sized cast products (or blooms) are more desirable for mainly two reasons: (a) large crosssections are associated with large values of mass per unit length, hence, productivity is favored, and more important (b) large cross-sections are subject to larger values of area reduction once rolled, giving products with smaller possibility to quality degradation. On the other hand, the proper control of the cooling intensity in the secondary cooling zones (air-mist spray zones) upon the solidifying product in order to avoid surface/sub-surface defects in the unbending regions of the caster, places limitations on the casting speeds, and therefore productivities are not always at the desired levels. In practice, fundamental operation parameters like casting speed, casting temperature, and cooling-water consumption at the secondary (spray) zones

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per produced mass of steel are among the most critical ones that the operator should keep in mind, once these have been analyzed, and their impact upon quality has been realized. Furthermore, from the early stages of medium and high-carbon steels casting, the operator has appreciated that it has been impossible to attain the high levels of productivity as with low and medium-low carbon steels without loss of internal quality. This is so because the low solidus temperatures that these grades exhibit become even lower in a dynamic way due to the local cooling rates that affect the solidification mechanism. Indeed, micro-segregation phenomena become more pronounced for these types of steels reducing even further the temperature at which the product becomes 100% solid, or in other terms, the solid fraction becomes one. There are some correlations for the solidus temperatures that unfortunately do not hold appreciably well, under varying cooling conditions, as it normally happens in the industrial continuous casting process. Nevertheless, the liquidus temperature is computed with great precision based upon the liquid-steel chemical analysis, and in this way, the industrial parameter known as superheat, which is the difference between the casting and liquidus temperatures, is calculated correctly; it is known that superheat is of paramount importance in the casting process. In this study, an attempt was carried out in order to shed some light into the effect of the various casting parameters upon the internal quality of the produced blooms in the continuous caster of Stomana, Pernik, Bulgaria. The biggest size of blooms produced currently in this caster is 300 x 250 mm x mm, and most medium and highcarbon grades are produced in this size. A heat transfer and a micro-segregation model were coupled and put into effect in order to facilitate the analysis of solidification along the caster length. Consequently, the solid fraction in the mushy zone, which is actually the intermediate zone between liquid and solid, was computed across a bloom section at any point along the caster, or in other words, from the meniscus level in the mold till the point of analysis. In addition to this, temperature and local-cooling rate distributions were also computed in a similar manner. Different operating conditions were fed into the simulation model in order to compute the required metallurgical lengths, or in other words, the effective cutting lengths that obeyed the unity solid fractions in the mushy zone along the centerline of a bloom.

#### **2. Quality problems associated with internal soundness/central porosity**

From the early times of the continuous casting for medium and high carbon steels, it has been realized that central porosity as a quality problem seems inevitable. Figure 1 shows a picture from a macro-etched high carbon bloom cross-section.

It is pointed out that for high carbon steels the tendency for central porosity generation is very large no matter how well liquid steel is treated, and how successful the casting process is performed. With time, the continuous casters manufacturers realized that this problem can be abided if electromagnetic stirring (EMS) was applied not only in the mold but in specific positions along the strand, mostly known as strand (S-EMS) and final (F-EMS) positions. F-EMS position is considered the position at which the mushy zone along the centerline becomes solid, or the position around the final solidification of the product. This is an effective technical solution to lessen the problem and is currently applied in some caster installations worldwide.

Evaluation of Solidification Times for Medium and High Carbon Steels Based upon Heat Transfer… http://dx.doi.org/10.5772/60706 317

per produced mass of steel are among the most critical ones that the operator should keep in mind, once these have been analyzed, and their impact upon quality has been realized. Furthermore, from the early stages of medium and high-carbon steels casting, the operator has appreciated that it has been impossible to attain the high levels of productivity as with low and medium-low carbon steels without loss of internal quality. This is so because the low solidus temperatures that these grades exhibit become even lower in a dynamic way due to the local cooling rates that affect the solidification mechanism. Indeed, micro-segregation phenomena become more pronounced for these types of steels reducing even further the temperature at which the product becomes 100% solid, or in other terms, the solid fraction becomes one. There are some correlations for the solidus temperatures that unfortunately do not hold appreciably well, under varying cooling conditions, as it normally happens in the industrial continuous casting process. Nevertheless, the liquidus temperature is computed with great precision based upon the liquid-steel chemical analysis, and in this way, the industrial parameter known as superheat, which is the difference between the casting and liquidus temperatures, is calculated correctly; it is known that superheat is of paramount importance in the casting process. In this study, an attempt was carried out in order to shed some light into the effect of the various casting parameters upon the internal quality of the produced blooms in the continuous caster of Stomana, Pernik, Bulgaria. The biggest size of blooms produced currently in this caster is 300 x 250 mm x mm, and most medium and highcarbon grades are produced in this size. A heat transfer and a micro-segregation model were coupled and put into effect in order to facilitate the analysis of solidification along the caster length. Consequently, the solid fraction in the mushy zone, which is actually the intermediate zone between liquid and solid, was computed across a bloom section at any point along the caster, or in other words, from the meniscus level in the mold till the point of analysis. In addition to this, temperature and local-cooling rate distributions were also computed in a similar manner. Different operating conditions were fed into the simulation model in order to compute the required metallurgical lengths, or in other words, the effective cutting lengths that obeyed the unity solid fractions in the mushy zone along the centerline of a bloom.

316 Heat Transfer Studies and Applications

**2. Quality problems associated with internal soundness/central porosity**

From the early times of the continuous casting for medium and high carbon steels, it has been realized that central porosity as a quality problem seems inevitable. Figure 1 shows a picture

It is pointed out that for high carbon steels the tendency for central porosity generation is very large no matter how well liquid steel is treated, and how successful the casting process is performed. With time, the continuous casters manufacturers realized that this problem can be abided if electromagnetic stirring (EMS) was applied not only in the mold but in specific positions along the strand, mostly known as strand (S-EMS) and final (F-EMS) positions. F-EMS position is considered the position at which the mushy zone along the centerline becomes solid, or the position around the final solidification of the product. This is an effective technical solution to lessen the problem and is currently applied in some caster installations worldwide.

from a macro-etched high carbon bloom cross-section.

**Figure 1.** A macro etched cross-section from a 300 x 250 mm x mm bloom. The central porosity, which creates an inter‐ nal-soundness quality problem, is apparent for this high-carbon cast product.

Another promising technical solution is the dynamic soft reduction, in which the part of the bloom which approaches final solidification is subject to a compressive force that slightly reduces its size in one direction but mechanically eliminates the central porosity problem. It is interesting to note that there are installations worldwide that currently apply both technical solutions for internal porosity minimization. However, no matter whether an installation applies one technical solution or another, it is really intriguing to try to figure out why and how this happens, and most importantly up to what extent, depending upon the various casting operating conditions. Without some knowledge upon this problem for a specific installation, one may not appreciate all the phenomena involved in, and maybe a definitive solution may not be successfully attained even after the installation of the discussed technical solutions. That is why it was decided to get some extra information on the subject before the installation of any technical solution might be applied at the Stomana bloom caster.
