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Key Factors Impacting ladle refractory materials Lifespan

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Refractory materials for ladles account for more than 30% of iron and steel metallurgical refractory materials and are the focus of metallurgical refractory material consumption. At present, ordinary small ladles use aluminum-magnesia castables with high aluminum as the main raw material. The service life has reached more than 70 times, and the good ones have reached more than 150 times. Especially when the cold repair mode of cold peeling and then casting restoration is adopted, And repeat.

For medium and large ladles, magnesia-carbon bricks are generally used for the slag line, and corundum-spinel or alumina-magnesia castables or alumina-magnesia carbon bricks are used for other parts. And 2 or 3 slag lines and bottom bricks are balanced with 1 molten pool lining, so that the unit consumption of refractory materials is 2~4kg/t. However, there are also low-grade brick-built ladles with low service life, no repairs, and one-time replacement, resulting in high unit consumption of refractory materials.

As the quality of steel products improves, an increasing proportion of processes such as argon blowing and stirring, oxygen blowing to decarburize, heating, slag addition, alloying elements, and vacuum are required to be carried out in the ladle. These processes lead to longer and longer residence times of molten steel in the ladle, accelerated erosion of the refractory lining and a significant reduction in service life.

This study analyzes and discusses the impact of each refining process parameter on the lining, revealing their influence rules. These rules are used to combine the operating conditions with improving the service life of the ladle to achieve the purpose of increasing the service life of the ladle and reducing the unit consumption of refractory materials.

Key Factors Impacting ladle refractory materials Lifespan

Effect of usage conditions on corrosion of lining refractory materials

1. Effect of temperature

The relationship between the dissolution rate of the ladle lining refractory material and temperature. When the temperature of the molten steel in the ladle is higher, the dissolution and erosion will be faster and the service life will be shorter. Researchers conducted a systematic study on the resistance of refining ladle materials to slag dissolution and obtained very instructive conclusions.

The increase in the temperature of the molten steel in the ladle significantly increases the erosion rate. An important process of ladle refining is that the input of heat energy such as arc heating, electromagnetic stirring heating, adding exothermic agent 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 important reasons why the service life of refining equipment such as VOD and LF is significantly lower than that of ordinary ladles.

On the other hand, temperature uniformity is also an important factor affecting the service life and safety of refining ladles. For the LF furnace, arc heating leads to local hot spots and accelerated damage. Without repair, the fastest erosion determines the service life, thus leading to a reduction in the service 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 service life of the ladle lining. In addition, timely repairs and 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. The corrosion of refractory materials in ladles is mainly caused by slag.

The effect of molten steel on refractory materials is mainly reflected in three aspects:

First, during the processes of tapping, pouring, and argon blowing and stirring, the flow rate and impact force of the molten steel are very large, eroding 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, which affects the quality of the steel.

The second is that the lining refractory material dissolves into the molten steel. Generally, 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 high solubility in steel and dissolve into steel, which affects the production of low carbon steel and ultra-low carbon steel.

Third, certain components of the refractory material interact with certain components in the molten steel (especially some special steel components), causing chemical reactions, resulting in changes in the steel composition and erosion of the refractory material.

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 the tapping process, the impact of the molten steel on the lining causes local erosion loss. At the same time, the strong stirring causes the reaction corrosion between the slag and the refractory material to be intense. During the refining process, the longer the refining time, the more the slag reacts with the refractory material, and the greater the corrosion amount, that is, the lower the service life of the ladle. The service life of the ladle lining decreases linearly with the extension of the refining time. During the residence period, as the time prolongs, the interface reaction layer thickens, and the reactants and products need to diffuse for a long time. The 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 lining refractory material erodes slowly during the residence period. During the steel pouring process, slag rises up and falls through different positions of the ladle, which is the main cause of erosion of the ladle molten pool. However, during the steel pouring process, the contact time between the slag and the lining somewhere is very short, so the erosion of the lining during the steel pouring process is still very small.

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. The corrosion of refractory materials in ladles is mainly caused by slag.

The effect of molten steel on refractory materials is mainly reflected in three aspects:

First, during the processes of tapping, pouring, and argon blowing and stirring, the flow rate and impact force of the molten steel are very large, eroding 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, which affects the quality of the steel.

The second is that the lining refractory material dissolves into the molten steel. Generally, 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 high solubility in steel and dissolve into steel, which affects the production of low carbon steel and ultra-low carbon steel.

Third, certain components of the refractory material interact with certain components in the molten steel (especially some special steel components), causing chemical reactions, resulting in changes in the steel composition and erosion of the refractory material.

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 the tapping process, the impact of the molten steel on the lining causes local erosion loss. At the same time, the strong stirring causes the reaction corrosion between the slag and the refractory material to be intense. During the refining process, the longer the refining time, the more the slag reacts with the refractory material, and the greater the corrosion amount, that is, the lower the service life of the ladle. The service life of the ladle lining decreases linearly with the extension of the refining time. During the residence period, as the time prolongs, the interface reaction layer thickens, and the reactants and products need to diffuse for a long time. The 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 lining refractory material erodes slowly during the residence period. During the steel pouring process, slag rises up and falls through different positions of the ladle, which is the main cause of erosion of the ladle molten pool. However, during the steel pouring process, the contact time between the slag and the lining somewhere is very short, so the erosion of the lining during the steel pouring process is still very small.

Key Factors Impacting ladle refractory materials Lifespan

3.2. Influence of slag oxidation

At present, magnesia carbon bricks are mostly used in ladle slag refining lines. Magnesia carbon bricks are easy to oxidize and are greatly affected by the oxidation of the slag. The stronger the oxidizing property of the slag, the easier it is to oxidize and corrode magnesia carbon bricks.

The oxidation of ladle slag is mainly caused by the following operating conditions.

1) The slag in the steelmaking furnace is a highly oxidizing 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, that is, good slag-blocking and tapping technology will significantly reduce the erosion of the ladle lining and reduce the amount of deoxidizer.

2) For VOD and AOD furnaces, oxygen blowing decarburization is required, so the iron oxide in the slag is very high. The oxygen in the slag and the oxygen in the molten steel diffuse mutually to form a dynamic equilibrium. This also reflects the oxidation of molten steel.

High oxidation produces the following effects:

① The increase in iron oxide in the slag causes the slag viscosity and melting temperature to decrease, thus accelerating the erosion of the lining;

②An important reaction for carbon-containing refractory materials is the oxidation of carbon. That is [{C}+{O}=CO2↑, {C}+[O]+CO2↑]. This causes decarburization of the lining and the formation of a loose-structured decarburization layer, which causes the lining to be penetrated and eroded by slag, and also accelerates erosion. The erosion of refractory materials by oxidizing slag is very serious.

3) Use deoxidizers and detergents to deoxidize and desulfurize the molten steel in the ladle, causing the slag on the upper surface of the ladle to show reducing properties and changes in composition. This reducing slag is less corrosive to refractory materials. Among molten steel cleaners, there are aluminum calcium slag and calcium silicate slag, which have different corrosion effects on refractory materials. Generally, alumina-calcium slag corrodes magnesia-carbon bricks slightly, but corrodes magnesia-calcium carbon bricks severely. The calcium silicon slag is affected by the calcium to silicon ratio shown in Figure 1. The increase of magnesium oxide in the slag causes the unsaturation and concentration difference of magnesium oxide in the refractory material to decrease, so the driving force for dissolution decreases and the corrosion rate slows down. That is, dolomite or magnesia slag-forming agent can effectively reduce the erosion of the lining and increase the service life.

3.3. Influence of slag viscosity

The decrease in slag viscosity will cause the diffusion layer to become thinner. Because 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 slag viscosity. Therefore, the slag viscosity decreases, resulting in an increase in the diffusion depth, that is, the slag viscosity decreases, which will thicken the reaction deterioration layer of the refractory material, leading to increased erosion. The refractory material layer penetrated by slag decreases in refractory degree, increases in sintering density, and increases 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 service life of the ladle. The viscosity of the slag can be controlled by adding an appropriate amount of dolomite and selecting a reasonable slag-forming agent, thereby reducing the corrosion of refractory materials and extending the service life of the ladle.

4. Effect of vacuum treatment

Many refining equipment have vacuum processing functions, such as LF-VD, VOD, RH and DH, etc. Vacuum conditions have a great impact on the loss of refractory materials, especially carbon-containing refractory materials. According to the principle of chemical equilibrium, under vacuum conditions, the following reactions will be promoted to the right, causing internal vaporization of the refractory material. As a result, the carbon-containing refractory materials are internally loose, their strength is reduced, and they are even pulverized, causing the service life of the lining to decrease linearly with the extension of the VD ratio and processing time. Therefore, under high temperature vacuum conditions, it is not advisable to choose additives such as aluminum powder, silicon powder and boron carbide that are prone to redox reactions with magnesium oxide. Not only do they fail to increase the service life of the ladle, but they reduce it. CaO is not prone to redox reactions with carbon, so under certain conditions, MgO-CaO-C is more suitable for these special conditions than magnesium carbon.

5. Effect of ultra-high temperature

Ultra-high temperatures are required during the smelting process of stainless steel, that is, high temperatures above 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 will 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 carbon-containing refractory materials. According to the principle of chemical equilibrium, increasing the temperature causes the reaction to proceed to the right, which results in the same consequences as vacuum conditions. That is to say, at ultra-high temperatures, magnesia carbon brick lining containing additives such as aluminum powder and silicon powder will not have good use effect, or even worse. Therefore, during the use of refining ladles, the ultra-high temperature and the time at high temperature should be controlled.

Key Factors Impacting ladle refractory materials Lifespan

6. Effect of argon blowing and stirring

During the ladle refining process, argon is generally blown and stirred, which causes the diffusion layer at the interface between the slag and the refractory material to become thinner, so the diffusion of the erosion medium accelerates, that is, the erosion rate becomes faster. If the sample is eroded by rotation in the slag, the erosion speed of the sample is proportional to the 0.7th power of the rotation speed (J=A+Bn0.7). However, argon blowing causes the oxygen concentration on the surface of the ladle slag to decrease, and argon itself is not corrosive to the furnace lining, so argon blowing can reduce the oxidation of carbon-containing refractory materials. Therefore, the impact of argon blowing on the ladle on the erosion of the refractory material of the ladle lining is not very serious.

7. Effect of intermittent operation

The ladle is in the alternating process of filling steel – transporting – refining – staying – pouring steel – turning over slag – repairing – waiting (preheating). The temperature of the ladle can continuously change from room temperature to 1700°C. This temperature fluctuation can cause significant stresses in the refractory lining. Refractory materials are brittle materials at low temperatures. Under such temperature fluctuation conditions, they are prone to cracks and peeling, resulting in abnormal losses and reducing the service life of the lining. Therefore, in order to reduce spalling and abnormal loss, the ladle turnover should be accelerated and the ladle preheating and insulation should be strengthened to prevent the temperature drop of the ladle while waiting for steel connection. This can increase the life of the ladle by more than 30%.

8. Influence of different refining equipment

Different refining equipment handles molten steel differently, that is, the refining conditions are different, and the erosion of the ladle lining is also different.

The magnesia carbon bricks used in the slag line of ordinary ladles can have a one-time service life of up to 120 times. The general LF furnace adds operating processes of argon blowing, deoxidizer, synthetic slag and arc heating. The molten steel filling time has also increased from about 60 minutes for ordinary pouring ladles to about 100 minutes. In this way, the one-time service life of the 100% LF furnace lining slag line is about 60 times, and the erosion rate has doubled, that is, the service life has been reduced by 50%. If the ladle LF treatment ratio is R1, the one-time service life of the magnesia carbon brick ladle slag line is S=120-60R1.

At present, many steel mills use LF-VD, which adds vacuum treatment to the LF treatment, resulting in a decrease in service life of about 50%. The service life of 100% VD treated LF furnace lining is half of that of pure LF furnace, about 30 heats. As the VD ratio increases, the service life decreases linearly, that is, the relationship between the service life (S2) and the VD ratio (R2) of LF-VD is approximately S2=60-30R2.

Another refining equipment, VOD, blows oxygen into the ladle to warm and decarburize the molten steel, and at the same time, a desulfurizer is added for desulfurization and vacuum degassing. The composition of the slag changes greatly, the alkalinity of the slag changes from acidic to alkaline, and the slag also changes from strong oxidizing to reducing. Its operating conditions are even worse than those of LF-VD, which results in the one-time service life of the ladle lining being further reduced by about 50%, that is, the one-time service life is about 15 times. If mixed with LF-VD, the VOD ratio is R3, then the relationship between the service life (S3) of this VOD furnace and the VOD ratio (R3) is approximately S3=30-15R3. If mixed with LF, the one-time service life is about S3=60-45R3.

It is worth pointing out that 120 times, 60 times, 30 times, and 15 times change with the maintenance of the ladle, the design of the lining, the quality of the refractory material, and the residual thickness. The above numbers are currently relatively good or normal results.

To sum up, due to the different refining conditions of ordinary ladle, LF, LF-VD and VOD, the difference in erosion speed is: ladle: LF: LF-VD: VOD≈1:2:4:8. Or the difference in service life is: ladle: LF: LF-VD: VOD=(100~130): (50~70): (20~30): (12~20)≈8:4:2:1. That is, the unit consumption of refractory materials is 2.5, 5, 10 and 20kg/t.

The above-mentioned difference in erosion rate is reflected in key parts such as slag lines, which only account for a very small part of the lining. If a small amount of repair material is used for repair and maintenance, the service life will be increased and the unit consumption of refractory materials will be greatly reduced. Therefore, the non-uniformity of metallurgical lining erosion brings a lot of room for reducing the unit consumption of refractory materials.

1.The increase in ladle temperature significantly increases the erosion rate of refractory materials; the ladle lining corrosion increases with the prolongation of refining time.

2.Slag is the main medium that corrodes the ladle lining. The enhanced oxidation of slag and the reduced viscosity and alkalinity all lead to accelerated corrosion of refractory materials.

3.Ultra-high temperature and vacuum treatment of the ladle not only lead to accelerated melting corrosion, but also cause the oxidation-reduction reaction of internal gasification of carbon-containing refractory materials to occur, thereby making the erosion and loss of refractory materials more rapid.

4.Under long-term vacuum treatment and ultra-high temperature smelting conditions, carbon-containing refractory materials are not suitable. Carbon-containing refractory materials containing traditional additives have worse use effects.

5.Argon blowing does not significantly increase the erosion of the lining, while oxygen blowing greatly accelerates the erosion of the refractory material.

6.Different refining equipment has different effects on refractory material erosion. The difference in erosion speed of different refining equipment is: ladle: LF: LF-VD: VOD≈1:2:4:8. The service life of the lining decreases linearly with the refining ratio.

Through good slag blocking and tapping technology and controlling the oxidation of slag, controlling the viscosity, alkalinity and composition of the slag by controlling the addition of synthetic slag is a very effective method to reduce the corrosion of refractory materials.

Appropriately lowering the steelmaking temperature and preventing the steelmaking temperature from being too high is one of the effective methods to increase the service life of the ladle.

Smooth operation is an effective way to reduce localized overheating and severe erosion.

Rapid turnaround and ladle insulation to reduce temperature fluctuations can significantly increase ladle life.

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As professional one-stop solution provider, LIAONING MINERAL & METALLURGY GROUP CO., LTD(LMM GROUP) Established in 2007, and focus on engineering research & design, production & delivery, technology transfer, installation & commissioning, construction & building, operation & management for iron, steel & metallurgical industries globally. 

Our product  have been supplied to world’s top steel manufacturer Arcelormittal, TATA Steel, EZZ steel etc. We do OEM for Concast and Danieli for a long time.

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As professional one-stop solution provider, LIAONING MINERAL & METALLURGY GROUP CO., LTD(LMM GROUP) Established in 2007, and focus on engineering research & design, production & delivery, technology transfer, installation & commissioning, construction & building, operation & management for iron, steel & metallurgical industries globally. 

Our product  have been supplied to world’s top steel manufacturer Arcelormittal, TATA Steel, EZZ steel etc. We do OEM for Concast and Danieli for a long time.

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