Tundish deskulling is a major problem in the continuous casting of steel, and this problem occurs because of the chemical reactions between the refractory lining and the slag carry over from ladle to tundish.
The slag carry over from ladle to tundish has a direct effect on refractory corrosion in the tundish. The portion of tundish where the skull gets locked into the backup lining is dependent on the slag basicity and amount of alumina present in the slag inside the tundish. Thus a direct corrosion mechanism takes place between acidic slag and refractory when alumina percentage is less than 30% but an indirect corrosion mechanism occurs between acidic slag and refractory when the percentage of alumina exceeds 30 percent.
The basic spray mass or dry vibro mass materials used in the tundish provide a working lining which acts as an inert barrier between the steel and back-up lining, and also acts as a parting wall for easy deskulling or cleaning out the residual steel and slags from the used tundish. This working lining act as an impenetrable barrier which prevents direct contact reactions with the steel, slag and protects the back-up lining as well. Usually, magnesia-based materials are preferred as tundish working linings.
The working lining of a tundish remains in contact with both slag and molten steel. The back-up linings are usually high-alumina brick or high alumina castable materials that contain free alumina and silica or get combined to form mullite. The reaction between the magnesia and/or magnesium silicate aggregates with steel/slag, usually used in tundish materials, is slow and can be ignored. The major concern is the reaction between the working lining material and tundish slag, especially in shops where the longer sequence is practiced. Ideally, the working lining materials should react as little as possible with the slag, but, in practice, one of the pertaining problems is tundish lining corrosion. This occurs because of penetration of steel and slag through the working lining and infiltration causes the steel/slag to get stuck on the permanent lining as well, which causes difficulties in skull removal, increases consumption of materials, and repair of permanent linings becomes a more expensive affair. Attack on tundish linings is a complex phenomenon that depends on the particular system and involves chemical wear (corrosion) and physical or mechanical wear (erosion) processes that may interact synergistically.
There are still some obscure points concerning the relevant parameters which may influence this problem. Therefore, it is important to determine the dominant factors affecting this problem.
(1) The volume of slag carry over from ladle to tundish has a direct effect on the refractory wear in the tundish. It was shown that the portion of tundish where the skull was locked into the back up lining increased with the amount of slag present in the tundish.
(2) The portion of the tundish where the skull was locked into the backup lining where acidic slag is presently decreased with the increase in the amount of Al2O3 present in the slag whereas when the lining comes in contact with basic slag it was found to be independent of alumina content.
(3) Depending on the slag basicity and amount of Al2O3 in the slag in the tundish, the corrosion mechanism of refractory by slag was changed:
· The acidic slag comes in contact with a low amount of MgO.SiO2, 2MgO.SiO2 and SiO2 phases in the refractory and get dissolved into the penetrating slag which forms a low melting phase consisting of 3CaO.MgO.2SiO2 and CaO.MgO.SiO2, but the slag may not fully penetrate into a solid refractory as a single liquid phase.
· When the acidic slag with a high amount of alumina comes in contact with the alumina castable of the backup lining, the deskulling problem in tundish is not serious due to the formation of spinel layer at the MgO/slag interface during the process of penetration and interaction with slag. The dense layer of spinel prevents the working lining material from further slag penetration during service. In this case, the corrosion takes place by the method of indirect penetration and dissolution of refractory in slag.
· When the basic slag was in contact with the castable, because of the existence of low melt phases such as Mayenite, slag penetrated into the working lining via open pores, and also MgO dissolved directly into the slag. Capillaries, such as open pores and microcracks, are the main channels for initial slag penetration into the refractory. In the penetration process of the slag system, calcium silicate was formed on the surfaces of MgO grains, and around them by the reaction between grain boundary and Mayenite, as a main phase of slag with a low melting point. Also, the slag as a single liquid phase is driven into the refractory by the capillaries and the penetration. No dusting and 2CaO.SiO2phase were observed.
· It has been shown that the direct corrosion mechanism occurs in basic slag and also in acidic slag containing low alumina, but the indirect corrosion mechanism occurs in acidic slag containing high alumina.
Effect of Al2O3 content on the percentage of locked skull tundish:
a) Acidic slags and b) Basic slags
Measures for easy deskulling
· Strictly maintaining the slag basicity greater than 2.5.
Greater than 30% alumina in slag composition is to be ensured when rice husk is being used as a tundish covering compound which contains about 90% alumina and forms a highly acidic slag.
· The maximum tolerable limit for slag thickness is 40mm. If the tundish contains more slag volume then excess slag is to be deslagged from the tundish especially when opting for higher sequence.
· Refractory composition needs to be properly maintained.
Apparent porosity and bulk density of working lining refractory materials are the keys in preventing slag penetration. If the apparent porosity becomes more than the required specification, then these pores provide easy pathways for the slag to initially enter into the working lining.
· High viscosity slag works out best as far as penetration of slag is concerned.
· Maintaining steady-state and smooth casting operation must be ensured. Alumino-silicate phases are formed in the tundish, due to ladle slag carryover.
· Tundish flow modified devices installation technique and composition are important as far as deskulling problem, penetration and corrosion is concerned.
· Tundish level fluctuation needs to be minimized. Level fluctuation causes the slag to get chilled on the top of the tundish which hinders deskulling.
· The casting duration is to be strictly monitored. The particular working lining thickness works best for a specified casting duration and creates a nuisance in deskulling if the duration is exceeded.
· Tundish leftover weight after the termination of casting is to be maintained low.
