The impacts of corroson damage - Effect of ladle lining refractory on steel quality

P. GALLIANO; C. CICUTTI

Corrosion of Refractories: The Impacts of Corrosion

Góller Verlag GmbH

Lugar: Baden-Baden; Año: 2018; p. 343 - 374

Since the beginning of the modern steel industry, ladle refractories showed a permanent evolution in close relation with the advent of new steelmaking technologies. During the last 50 years, ladle refractories characteristics and their resulting performance have been changing to play a more active role in the steel production process [1]. This role was strongly promoted by the consolidation of continuous casting technology, which was firstly introduced in the steel shops in the 1960s. After that, new process conditions were required in the ladles, such as long metallurgical treatments, higher steel temperatures, new reheating facilities, and the development of more basic CaO-rich slags, among others. These conditions became necessary to achieve desired targets of steel deoxidation and desulfurization, inclusion removal, and homogenization of both steel temperature and composition before the continuous casting step [2]Nowadays ladles are not transfer vessels anymore, and their refractories are deeply involved in the steelmaking process promoting the efficiency of the secondary metallurgy, as was well described in previous reviews [2-5]. For that purpose, specific refractories were developed and optimized according to changing steel process requirements. In this regard, the impact of ladle working lining materials on steel quality is outstanding. Lining refractories are in direct contact with both liquid steel and slag during the whole ladle metallurgy process and must guarantee a suitable performance to prevent oxide inclusions and defects in the steel. This last requirement is mandatory in particular for the production of special high performance ultra clean steels with exigent quality standards, being the main criteria for lining design definition and refractory selection. Besides, lining refractories must also assure a safe ladle performance while providing other requirements such as maximum steel capacity and low thermal losses. Changes in the ladle lining refractories due to steelmaking process modifications were significant in the last years and involved raw materials, chemical composition, bond type and installing methods. Before secondary refining was well-established, high-silicate ladle lining bricks had been mainly used. These materials were firstly replaced by zircon bricks and high-alumina bricks obtained from bauxite or andalusite [1-2]. All these materials are not used anymore as working lining materials under present steelmaking conditions since some of their components (e.g: silica) are not compatible with required deoxidation practices in ladle metallurgy, as will be shown see in the following sections A following step in the transition from acid silicate lining materials to neutral alumina-based ones was the development of high alumina ladle lining castables with tabular alumina and corundum as main raw materials. These lining products started to be used in the late 1970s with the aim to extend lining mean life through castable reparations. Further developments of alumina-spinel and spinel-forming castables were done with the aim to reduce refractory wear due to their good infiltration and spalling resistance, as well as their ability to absorb Fe and Mn cations from the slag. Both types of castables are still in use at present in specific areas of the ladle with minor contact to steelmaking slag [1-2,5]Finally, basic materials became an excellent alternative due to their high compatibility with basic slags and liquid steel, in this last case due to their low oxidation potential. First trails of basic materials in ladles were done in the early 1960s. Since that time pitch and tar bonded bricks of doloma, magnesia-doloma and magnesia began to be used. In the early 1980s resin bonded MgO-graphite materials were developed. This technological breakthrough strongly improved working lining refractory performance due to the low wettability and high thermal conductivity provided by the graphite, and the protective effect of in-situ MgO/C reactions. Nowadays the application of MgO-C brick refractories is probably the largest utilization of these type of refractories in the steel industry [6-7].At present both monolithic and shaped brick refractories are currently used as ladle working lining materials, being oxide-C bricks and high alumina-based castables the largest used ones in each group. Moreover, high alumina precast shapes can also be found in specific places such as ladle bottom or upper sidewall lip [8-9]. Working lining materials are not necessary the same in the whole ladle, and differences between bottom, barrel or metal line (lower sidewall) and slag line are usual. Materials selection depend on lining concepts, operative practices and specific conditions for each plant or geographic region. Some general lining characteristics of each ladle zone are briefly described as follows.Main differences in lining design are observed in the ladle floor and lower sidewall (barrel or metal line) where the refractories are in contact with liquid steel. Alumina-containing materials including Al2O3-MgO-C bricks (AMC), precast shapes and castables are the predominant options for the bottom impact area due to their improved mechanical resistance to steel jet. Monolithic alumina-spinel castables are also employed for the same applications, being the prevailing option in certain countries (e.g: Japan) [1,9] and worldwide in certain specific applications in the ladle floor [10]. Working lining bricks of MgO-C, Al2O3-MgO-C, Doloma-C and combinations are frequently used in bottom and barrel zones showing suitable performances. AMC bricks are currently found in both sidewall and floor lining. MgO-C bricks with carbon contents lower than 10% are usually present in both bottom and lower sidewall except for the bottom impact pad. In the case of Doloma-C bricks, they are mainly used when Si- and Mn-killed steels are produced [11]. Both last basic oxide-C bricks are preferred for clean steel production and Ca-treated steels.The situation is much more homogenous in the case of slag working lining, where MgO-C bricks are clearly the predominant material due to their outstanding resistance to both slag corrosion and spalling. Slag-line MgO-C bricks with or without antioxidant additives are mainly used, showing higher carbon content than in the barrel zone, with premium fused magnesia and flake graphite as main raw materials. MgO-C bricks are employed even for ladles that are subjected to exigent working conditions such as vacuum degassing treatments. MgO-C refractories with low carbon contents can be found in the ladle barrel zone for the production of ultra-low carbon steels [12]In summary, at present MgO-C bricks are the standard material in ladle slag line and one of the commonly used ones in the rest of the working lining. In this chapter, the impact of MgO-C ladle refractories on the steel quality will be reviewed. The different reactions and corrosion mechanisms that takes place in the magnesia-carbon refractories during ladle cycle in the steelmaking process will be surveyed, covering their consequences on various steel quality issues that impact in the final product. Lining corrosion and reactions in steel containing ladles and in the empty vessel under preheating conditions are both considered.