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Ladle refractories account for more than 30% of iron and steel metallurgical refractories. They are the focus of metallurgical refractories consumption. At present, the main raw material used in ordinary small ladles is high-alumina aluminium-magnesia castables. The lifetime has reached more than 70 times, the good ones more than 150 times. Especially when using the cold repair mode of cold peel and then cast restoration, and so on.
Magnesia-carbon bricks are generally used for the slag line in medium and large ladles, for other parts, corundum-spinel or alumina-magnesia castables or alumina-magnesia carbon bricks are used. 2 or 3 slag lines and bottom bricks balanced by 1 molten pool lining, so that the unit refractory consumption is 2~4kg/t. However, there are also low-grade brick-built ladles with short service life, no repair and one-time replacement, resulting in high unit refractory consumption.
As the quality of steel products improves, more and more processes such as argon blowing and stirring, oxygen blowing for decarburisation, heating, slag addition, alloying elements and vacuum are required in the ladle. These processes lead to longer and longer residence times of the molten steel in the ladle, accelerated erosion of the refractory lining and a significant reduction in service life.
This study analyses and discusses the effect of each refining process parameter on the lining and identifies the rules of influence. These rules are used to combine the operating conditions with the improvement of the ladle life in order to achieve the objective of increasing the ladle life and reducing the unit consumption of refractory materials.
Influence of operating conditions on refractory lining corrosion
1. Effect of temperature
The relationship between the dissolution rate of the ladle lining refractory and temperature. The higher the temperature of the molten steel in the ladle, the faster the dissolution and erosion, and the shorter the service life. Researchers have conducted a systematic study of the resistance of ladle lining materials to slag dissolution and have come to some very instructive conclusions.
Raising the temperature of the molten steel in the ladle significantly increases the rate of erosion. An important process in ladle refining is that the application of thermal energy, such as arc heating, electromagnetic stirrer heating, addition of exothermic agents or oxygen blowing, increases the temperature of the ladle, resulting in accelerated erosion of the ladle lining and reduced service life. This is one of the main reasons why the life of refining equipment such as VOD and LF is significantly shorter than that of ordinary ladles.
On the other hand, temperature uniformity is also an important factor affecting the life and safety of refining ladles. In the LF furnace, arc heating leads to local hot spots and accelerated damage. If not repaired, the fastest erosion will determine the life of the ladle. In this case, reducing the continuous heating time and appropriately reducing the heating intensity is very effective in reducing hot spot overheating and increasing the life of the ladle lining. In addition, timely repairs and a balanced ladle lining are required.
2. Influence of filling time of molten steel
Under normal circumstances, molten steel hardly dissolves refractory materials, or molten steel dissolves refractory materials very slowly. Corrosion of refractory materials in ladles is mainly caused by slag.
There are three main aspects to the effect of molten steel on refractory materials:
Firstly, during the tapping, pouring, argon blowing and stirring processes, the flow rate and impact force of the molten steel are very high, which erodes the refractory lining, resulting in refractory material loss and refractory particles entering the molten steel. Most of the refractory particles entering the molten steel float to the slag, while a small number of tiny particles cannot float and form non-metallic inclusions in the steel, affecting the quality of the steel.
The second is that the refractory lining material dissolves in the molten steel. In general, the solubility of refractory materials in steel is very low and there is little dissolution and erosion. However, some components (such as carbon) in refractory materials have a high solubility in steel and dissolve in steel, which affects the production of low and ultra-low carbon steel.
Thirdly, certain components of the refractory interact with certain components in the molten steel (especially some special steel components), causing chemical reactions that result in changes in the steel composition and erosion of the refractory.
The residence time of molten steel in the ladle is divided into tapping time (2 to 7 minutes), refining time, residence time and steel pouring time. The degree of erosion of the refractory lining during these periods is different. During tapping, the impact of the molten steel on the lining causes local erosion loss. At the same time, the strong stirring causes intense reaction corrosion between the slag and the refractory. During the refining process, the longer the refining time, the more the slag reacts with the refractory and the greater the amount of corrosion, i.e. the shorter the life of the ladle. The life of the ladle lining decreases linearly as the refining time increases. As the residence time increases, the interfacial reaction layer thickens and the reactants and products have to diffuse for a long time. Erosion is controlled by diffusion.
According to the diffusion kinetic equation, the amount of erosion is proportional to the square root of the residence time. Therefore, the refractory lining will erode slowly during the residence time. During the steel pouring process, slag rises and falls through different positions of the ladle, which is the main cause of erosion of the ladle pool. However, during the steel pouring process, the contact time between the slag and any part of the lining is very short, so the erosion of the lining during the steel pouring process is still very small.
3.Influence of slag oxidation
At present, magnesia carbon bricks are mainly used in ladle slag refining lines. The magnesia carbon bricks are easily oxidised and are greatly affected by the oxidation of the slag. The more oxidising the slag, the easier it is to oxidise and corrode the magnesia carbon bricks.
The following operating conditions are the main causes of oxidation of ladle slag.
1) The slag in the steelmaking furnace is a highly oxidising slag containing more than 20% iron oxide. When tapping steel from the steelmaking furnace to the ladle, if the slag is not properly blocked, part of the steelmaking slag will enter the ladle, which will not only affect refining, consume more deoxidizer, but also accelerate the erosion of the ladle lining. Therefore, during the steelmaking and tapping process, slag-free technology should be adopted, i.e. good slag blocking and tapping technology will significantly reduce the erosion of the ladle lining and reduce the amount of deoxidiser.
2) VOD and AOD furnaces require oxygen blowing decarburisation, so the iron oxide in the slag is very high. The oxygen in the slag and the oxygen in the molten steel diffuse with each other to form a dynamic equilibrium. This also reflects the oxidation of the molten steel.
The following effects result from high oxidation
1.The increase in iron oxide in the slag results in a decrease in slag viscosity and melting temperature, thus accelerating the erosion of the lining;
2.An important reaction for carbonaceous refractories is the oxidation of carbon. This is [{C}+{O}=CO2↑, {C}+[O]+CO2↑]. This causes decarburisation of the lining and the formation of a loose decarburisation layer, which allows slag to penetrate and erode the lining, and also accelerates erosion. The erosion of refractory materials by oxidising slag is very serious.
3. Deoxidising and desulphurising the molten steel in the ladle using deoxidisers and detergents causes the slag on the top of the ladle to exhibit reducing properties and changes in composition. This reducing slag is less corrosive to refractory materials. Molten steel cleaners include aluminium-calcium slag and calcium-silicate slag, which have different corrosive effects on refractory materials. In general, alumina-calcium slag corrodes magnesia-carbon bricks slightly, but magnesia-calcium-carbon bricks severely. The calcium-silicon slag is affected by the calcium to silicon ratio shown in Figure 1. The increase in magnesium oxide in the slag causes the unsaturation and concentration difference of magnesium oxide in the refractory to decrease, reducing the driving force for dissolution and slowing the corrosion rate. This means that dolomite or magnesia slag forming agent can effectively reduce the erosion of the lining and increase the service life.
3. Influence of slag viscosity
The decrease in slag viscosity will cause the diffusion layer to become thinner. As the erosion rate is inversely proportional to the diffusion layer, the reduction in slag viscosity will accelerate the erosion rate.
On the other hand, the relationship formula between slag viscosity and slag penetration depth into refractory materials shows that the slag penetration depth into refractory materials is inversely proportional to the square root of the slag viscosity. Therefore, as the slag viscosity decreases, the penetration depth increases, i.e. the slag viscosity decreases, which causes the reaction deterioration layer of the refractory to thicken, resulting in increased erosion. The layer of refractory material penetrated by the slag will decrease in refractoriness, increase in sinter density, increase in thermal expansion and other performance differences with the original layer of refractory material. During the intermittent use of the ladle, cracks and spalling of the slag penetration layer were caused, resulting in the loss of the refractory lining. Therefore, increasing the viscosity of the slag can reduce the erosion of the refractory lining and increase the life of the ladle. The viscosity of the slag can be controlled by adding a suitable amount of dolomite and selecting a suitable slag forming agent, thereby reducing the corrosion of the refractory materials and extending the service life of the ladle.
4. Effect of vacuum treatment
Many refineries have vacuum processing functions such as LF-VD, VOD, RH and DH etc. Vacuum conditions have a great influence on the loss of refractory materials, especially carbonaceous refractory materials. According to the principle of chemical equilibrium, the following reactions are promoted to the right under vacuum conditions, causing internal vaporisation of the refractory material. As a result, the carbonaceous refractories become internally loose, their strength is reduced, and they are even pulverised, causing the service life of the lining to decrease linearly with the increase in VD ratio and processing time. Therefore, under high temperature vacuum conditions, it is not advisable to choose additives such as aluminium powder, silicon powder and boron carbide, which are prone to redox reactions with magnesium oxide. Not only will they not increase the life of the ladle, they will reduce it. CaO is not susceptible to redox reactions with carbon, so under certain conditions MgO-CaO-C is more suitable than magnesium carbon.
5. Effect of ultra-high temperature
Ultra-high temperatures are required during the stainless steel melting process, i.e. high temperatures in excess of 1700°C often occur in AOD and VOD refining furnaces. The increase in temperature significantly increases the erosion rate of refractory materials, so ultra-high temperatures cause severe erosion of refractory materials. Ultra-high temperature not only reduces the viscosity and increases the solubility of the slag, resulting in accelerated corrosion, but is also serious for carbonaceous refractory materials. According to the principle of chemical equilibrium, increasing the temperature causes the reaction to move to the right, with the same consequences as under vacuum conditions. That is to say, at ultra-high temperatures, magnesia carbon brick lining containing additives such as aluminium powder and silicon powder will not have a good use effect, or even worse. Therefore, when using refining ladles, the ultra-high temperature and the time at high temperature should be controlled.
