The ladle was primarily used as a transfer vessel to transport liquid steel from the converter steelmaking area to the caster. But in present times due to stringent quality demands it serves more as a process vessel where functions like temperature control, deoxidation, carburizing, ferroalloys addition and inclusion floatation and modifications have come into the picture.
With the increasing demand for value-added grades of steel and advancement in the secondary metallurgy operations the ladle lining suffers from severe conditions like arc/chemical heating and high steel temperatures. The ladle has to resist wear due to the bath movement when argon purging is used, degassing, ferroalloy additions and addition of synthetic slag. Further, it has to tolerate higher holding and arcing time. At the same time, the ladle should prevent heat loss from the liquid. Proper preheating of ladles prior to the first heat, and reducing circulation time and skull formation can minimize heat losses.
The refractory apart from meeting such requirements should remain inert to the liquid steel and should not deter steel cleanliness. Cleanliness is affected when oxygen is picked up from the lining. Linings containing SiO2 in uncombined forms can contribute to deteriorating steel cleanliness.
The basic design of the steel ladles comprises of the top lip ring, slag line, safety wall and bottom, barrel, working wall/bottom. The bottom is protected against a breakout by the use of safety and working lining and has porous plugs for inert gas purging and also has a slide gate system for teeming of steel.
The proper degree of compression is to be maintained on the ladle refractories throughout their service. Excessive compressive forces can result in cracking/ buckling whereas lower compressive stresses can lead to the formation of cracks or joints, which may allow steel or slag infiltration. Ladle finite element analysis has been found successful in establishing improved behavior of refractory during the course of their service.
The thickness of the lining varies from zones to zones with thickness of working lining ranging from 150 mm to 250 mm in the barrel and 225 mm to 300 mm in the bottom whereas the safety lining thickness can vary from 50 mm to 150 mm. The refractory materials used for lining can be dolomite bricks, magnesia chrome bricks, magnesia carbon bricks, high alumina bricks, aluminium carbon bricks, and high alumina castable. The high alumina refractories are preferred for most areas except the slag line. But the selection depends largely on the operating conditions.
The selection is made keeping in mind the wear mechanism in the various zones of the ladle: Wear in the bottom is severe due to the impact of the tapping stream which again depends on the height of the tapping stream and the angle of impact. To protect against such severe wear the safety/working bottom thickness is increased and an impact pad is installed to take care of this issue. Sometimes the side bottom wall is affected by such wear. Thus refractories with high wear resistance are to be selected based on hot strength.
The bottom and the lower barrel area suffer from wear due to erosion at the time of rinsing. Physical damage during deskulling also contributes to serious erosion. Damage from skull removal can be severe and in extreme case, the bottom has to be repaired. The barrel area comparatively suffers from less wear.
The wear in the slag line is attributed to the corrosion from the slag. The slag composition can be wide-ranging and include high iron oxide (FeO) slag carried over from the steelmaking furnace, artificial slag introduced after partial slag skimming, slag added or formed during specific metallurgical purposes such as rinsing or injection, and slag formed or circulated during degassing. The different types of slag are normally basic in nature so basic refractories are compatible.
Arc reheating contributes immensely to the corrosion of refractories. The erosion is higher owing to the higher temperatures of the slag (40 deg C to 150 deg C above the steel temperature). Although techniques are implemented to control slag erosion. Some of these are controlling slag basicity, alumina content, and MgO additions to the slag. Slag rinsing further controls slag superheat. Dolomite, magnesite chrome or MgO-C bricks are normally the preferred choices.
The typical gate system includes well block, upper nozzle, the fixed and sliding slide gate plates, and collector nozzle. The plates should be able to withstand thermal shock and steel erosion. The composition of these plates may vary from alumina to zirconia, to complex oxide–carbon systems. The material of the plates is chosen depending on the grade of steel being cast and the life that the shop demands. The composition of the steel-like Ca ppm can exponentially lower down plate life due to the Ca vapor attack. The life of the plates can vary from one heat to more than 15 heats depending on the quality of the material used in the plates.
Porous plugs are used in ladle bottoms for rinsing with inert gas mainly argon. Plugs having cross-sections with different directional mechanisms are used to provide controlled argon flow in the ladle. High alumina or burnt magnesia materials are the preferred materials.
The selection of materials for lining varies from shop to shop and depends on a number of factors like circulation time, secondary metallurgy route, steel grade, shop logistics and the cost of refractory per ton of steel produced. The thermal chemical and mechanical property requirements are different for different zones and are also to be kept in mind. So the selection of the material is difficult and demand for better quality has increased in recent times. Although the refractory maker should look into meeting these requirements and increasing campaign life at the same time try to minimize refractory cost as far as the profit margins are concerned.
