Ladle slag and its role in secondary steelmaking
Ladle slag and its role in secondary steelmaking
Presently the production of liquid steel takes place in two stages. The first stage involves getting a semi-product in primary steel making process furnace such as converter, electric arc furnace or induction furnace, whereas the second stage consists in refining of steel in the secondary steelmaking process such as ladle refining furnace, or vacuum degassing furnace. Both the primary and the secondary steelmaking processes take place in the presence of slag. Irrespective of the secondary steelmaking process used, refining steel in the ladle proceeds under a layer of specially formed ladle slag. The physical and chemical properties of this slag decide about the quality of the produced steel and the technology used influences the economics of the process.
Steel refining in the ladle allows the production of steel with high cleanliness as long as proper slag is used in the steelmaking ladle. Refining slag is to be characterized by certain properties which can ensure the chemical composition of the steel. In order to refine steel efficiently, it is also necessary to use an appropriate quantity of slag in the ladle.
Ladles are used to transport liquid steel in a steel melting shop. A ladle cycle starts as the primary steelmaking furnace starts tapping liquid steel into the ladle. The full ladle is transferred between stations for further processing of the steel. Typically, the ladle treatment starts with a combined deoxidation / desulphurization treatment at a stirring station. A slag rake removes the top slag before the next step. Further processing is then performed at a ladle furnace. After slag building, a final composition adjustment and temperature trimming is performed. The ladle treatment at the secondary steelmaking operations is finished when the desired steel temperature and composition have been achieved. The ladle is then transferred to the continuous casting machine for the casting of the liquid steel. While ladle slag plays very important role in ladle refining, the slag layer also protects the liquid steel from re-oxidation and acts as an insulator to reduce the heat loss from the liquid steel to the surroundings during the casting of the liquid steel. Heat is transferred by conduction, convection, and radiation.
The three important aspects which are to be taken care by the secondary steelmaking are time, temperature, and chemical specification of steel. With the advent of sequence casting, it is imperative that the heat arrive at the casting machine on time. Any delay can cause a disruption in the sequence and result in a bottleneck down the line, ultimately resulting in less heats being cast and a subsequent decrease in the profitability. As regards temperature, with ladle reheating furnaces, temperature can be controlled to within +/- 3 deg C of the target temperature. If the slag is well deoxidized, the carbon and alloying elements aims can be met within very tight tolerances. The slag phase has a tremendous impact on the above steelmaking objectives. The truth of the old maxim is ‘take care of the slag and the steel will take care of itself’, gets clearly demonstrated in the ladle refining process.
Secondary steelmaking is probably the most important part of steelmaking since it is here where the hot iron units from the furnace are changed into a saleable high quality steel product. Ladle refining implies the following (i) deoxidation and alloying of the steel, (ii) temperature and composition homogenization, (iii) desulphurization or resulphurization of the steel, and (iv) improved steel cleanliness (inclusion flotation and sulphide and oxide shape control). The objectives of ladle refining is to deliver a ladle of homogeneous liquid steel to the continuous casting machine on time, at the right temperature, and meeting of total chemical specifications.
Slags can be classified as (i) crusty, (ii) fluffy, (iii) creamy, and (iv) watery. Crusty slag has too much CaO (and / or MgO). Fluffy slag is CaO / MgO saturated, and is satisfactory for refractories but not optimum for desulphurization. Creamy slag is just CaO / MgO saturated, is good for steelmaking and refractories (ideal). Watery slag is too liquid, aggressive to the refractories.
The scoria of a metal or fused material separated during the reduction of metal is called slag. Slag is an ionic solution consisting of molten metal oxides and fluorides which float on top of the steel (completely liquid or partially liquid). A partially liquid slag consists of a liquid fraction and a solid fraction. As the solid fraction of the slag increases the fluidity of the slag decreases and it changes from ‘creamy’ to ‘fluffy’, and eventually to ‘crusty’ or solid. The drive-in recent years is to cut the costs and still produce high quality steels has highlighted the importance of good slag practices in primary and secondary steelmaking processes.
Slags are used for refining purpose in the primary and secondary steelmaking to collect impurity oxides, to bind impurities (sulphur, phosphorus) and to capture deoxidation products i.e., oxide inclusions from the steel liquid steel. Slags have even a protecting role in ladle, tundish, and mould when they act as barriers against atmospheric oxidation and heat escape. For these functions, the slags are designed to optimize the positive interaction with steel, and the passive, protecting role between the steel and the environment. On the other hand, slags can have negative impacts in ladle metallurgy and casting. Harmful macro inclusions in steel frequently originate from the slag through reoxidation mechanism of unstable oxides, principally FeO (ferrous oxide) and MnO (manganese oxide), or due to emulsified slag particles, directly.
There has been a gradual realization that the slag phase in steelmaking is not a necessary evil but a crucial part of modern steelmaking practices. Neither goal of producing high quality steel nor low costs can be achieved by poor slag practices. The concept of ‘slag engineering’ or ‘slag optimization’ is becoming more common in several steel melting shops, as the need to implement these concepts are driven by more stringent steel quality requirements.
The importance of slag has been very well understood by the steelmaking industry since it helps in cutting the cost of producing quality steel. The slag phase is a necessary part of the steelmaking process. Tailoring the slag chemistry using the concepts like ‘slag engineering or slag optimization’ to improve the steelmaking process is a crucial part of making cost effective, quality steel. Enormous improvements in steel manufacturing techniques have been achieved through thoughtful mechanism of metal, slag, and gas reaction. Since the primary purpose of slag is to absorb metallic impurities, the composition of slag is largely governed by the composition of the input material and alloy recipe.
Techniques to remove impurities in the primary steel making process and modification through ladle refining techniques result in better yields, better quality steels, and lower cost. Remarkable benefits achieved during ladle refining such as improved rate of melting, electrical efficiency, and reduction in arc radiation have led to the perception of ‘make the slag and the steel will make itself’.
Slag forming materials (lime), reaction products and impurities dissolve in the achieved slag. Ladle slag used in secondary steelmaking plays a very important role in (i) steel desulphurization, (ii) steel deoxidation, (iii) purification of steel from non-metallic oxide inclusions, (iv) ensuring proper chemical composition of oxide inclusions in ready-made steel, (v) protection from the atmospheric influences, and (vi) thermal insulation. The process of removing impurities from steel is the most important of the all above mentioned roles.
Slag conditioning and the maintenance of slag quality in the ladle refining process have an added advantage to produce better quality steel economically. Carry-over slag from the primary steelmaking furnace and flux addition during tapping promotes the development of slag in the ladle furnace.
The functions of slag include (i) to cover the arcs in the ladle furnace and protect the refractories from arc-flare, (ii) to control the steel oxygen levels and chemistry of the steel, (iii) can protect steel from oxygen and nitrogen (N2) pickup and can influence hydrogen (H2) pickup, (iv) to improve the quality of the steel by absorbing deoxidation products (silica and alumina) and inclusions (clean the steel), (v) to desulphurize steel in the ladle, (v) to protect the metal from oxidation, (vi) to insulate the steel to minimize heat loss, and (vii) be fully compatible with the refractory lining
A ‘bad’ slag does nothing to improve the quality of the steel. It is incompatible with the ladle refractories and dissolve it to satisfy its solution requirements. A ‘bad’ ladle slag contain a large proportion of reducible oxides (FeO and MnO) which react with the steel to cause aluminum (Al), silicon (Si), and manganese (Mn) disappearing. The viscosity (or fluidity) of the slag is also an important property. However, here there are somewhat conflicting requirements. A good slag for the metallurgical practice is required to have a high fluidity (low viscosity), whereas, a good slag in terms of refractory wear is required to have a low fluidity (high viscosity) to ensure minimum penetration and reaction, and good coating formation.
A compromise is hence needed, i.e., a slag which is still fluid enough to refine the liquid steel but not to fluid to cause accelerated wear on the refractory, i.e., slag with a ‘creamy’ consistency. A creamy slag has the consistency of latex paint. The building blocks of slag is its composition which is normally expressed in terms of the component oxides (or fluorides) on a percent basis. For example, a slag can have a typical composition of CaO – 55 %, SiO2 – 20 %, MgO – 8 %, Al2O3 – 12 %, and CaF2 – 5 %. These components come from lime (98 % CaO), dolomite (58 % CaO and 39 % MgO), calcium aluminate (45 % CaO and 53 % Al2O3), refractories, steel oxidation (2O + Si = SiO2, 3O + 2Al = Al2O3, and O + Mn = MnO), and fluorspar (CaF2)
Slag used for secondary steelmaking processes is characterized by (i) it emulsifies easily in liquid steel which happens when slag has low cohesion work, low viscosity and low melting temperature, (ii) emulsified slag particles easily flow out of steel which depends on density and adhesion work, and (iii) has good refining properties in relation to sulphur and non-metallic inclusions present in steel. This slag is to moisturize steel and non-metallic oxide inclusions and its adhesion work is to be higher for inclusions than for steel.
Slag forming is a complicated process which depends on several factors such as the quality of materials used and the selected process. Slag with determined properties is achieved because of the use of an appropriate quantity of slag forming materials during the secondary steelmaking process. It is also necessary to use of good quality materials. These materials in combination with the products of oxidation of iron, admixtures together with impurities form slag.
Formation of ladle slag
In a nutshell, the basic rule to optimize the slag for the ladle metallurgy facility is (i) to limit slag carry-over to the ladle, (ii) to reduce FeO and MnO content to less than 1 %, which is corrosive to ladle brick and carries oxygen to the steel, (iii) to maintain basic slag with the basicity ratio of 2.5 or higher by having creamy consistency of slag, and (iv) to have slag with the correct quantity of lime and MgO to improve desulfurization and to protect refractories. In practice, the appearance of a slag provides useful information on the quantity of reducible oxide it carries. It is always appreciated to avoid dark oxides like FeO, MnO, and Cr2O3 for cleaner low oxygen content steel.
In order for the ladle slag to fulfill all the tasks, it is necessary to use slag with an appropriate chemical composition. Carefully selected chemical composition allows the achievement of desirable properties of slag and at the same time it ensures that the secondary steelmaking process proceeds to fulfill the quality requirement of the liquid steel. Secondary steelmaking process normally involves the use of slag based on CaO and Al2O3. This slag has non-oxidizing character, which means that when in use it does not cause oxidation of steel components with high oxygen affinity. It also ensures proper desulphurization and removal of non-metallic oxide inclusions as well as the production of low oxygen steel.
For achieving proper ladle slag, it is necessary to select the main components carefully. It is important that after the dissolution of the components slag has the lowest melting temperature. This allows the secondary steelmaking process to be conducted efficiently. It is a rule that the melting temperature of such slag is lower than the temperature of steel. Decreasing the melting temperature can be done by adding aluminum oxide to the composition of the forming slag. Ladle slag formed on the basis of CaO and Al2O3 (containing CaO in the optimal range) ensures a decrease in the content of oxygen, sulphur and non-metallic oxide inclusions after the secondary steelmaking process.
Silica is an important component of the ladle slag. It enters the ladle slag during tapping from the primary steelmaking furnace into the ladle or it is the product of silicon oxidation in steel. Silica causes the refining properties of slag to worsen as it reduces the assimilation of oxide phases in slag. It is believed that the silica content in slag intended for secondary steelmaking process do not exceed 10 %. On the whole, ladle slag is basic with main components as CaO, Al2O3, and SiO2. Fig 1 shows a three-component phase system of the CaO-Al2O3-SiO2 type. This is the basis for the composition and phase analysis of the ladle slag.
Fig 1 CaO-Al2O3-SiO2 phase system
Analyzing the phase diagram, one can see a situation in which the quantity of CaO or Al2O3 in slag is higher than 50 %. What is more, it overlaps with the content of SiO2 (right and left corner of the diagram). There are also certain areas in the diagram where slag has higher melting temperature than steel. In the middle part of the diagram within the entire range of SiO2 share one can see complex compounds which have a considerably lower melting temperature than the metal which is being refined. As mentioned above, a considerable share of SiO2 in slag makes it impossible to achieve the needed secondary steelmaking parameters. The main reason for this is the decrease in the basicity of slag despite achieving low melting temperature of such slag. Secondary steelmaking practice shows that it is possible to select slag components depending on the types of steel produced. Fig 2 shows typical chemical composition areas of secondary steelmaking ladle slag in the CaO-Al2O3-SiO2 phase system.
Fig 2 Typical chemical composition areas of secondary steelmaking ladle slag
The areas marked I,II,III, and IV in Fig 2 characterize the four types of ladle slag namely (i) slag for ‘super pure steel’ with low content of Al subject to calcium treatment, which has desulphurizing and deoxidizing properties, (ii) wollastonite slag for steel intended for wires (cold treatment, e.g. tyre cords) and for high sulphur free machining steel, (iii) slag used in the steel refining process to achieve products with high requirements when it comes to the quality of their surface (hydraulic cylinders, beating rolls, tracks of roller bearings) but has no desulphurization potential and has high absorption of Al2O3, (iv) ladle slag for steel used to produce roller bearings, deoxidation of carbon and silicon without the addition of aluminum, no calcium treatment (high absorption of alumina).
Using the above-mentioned information concerning the chemical composition parameters, it is possible to determine the basic chemical composition of the refining slag. Carefully selected chemical composition can affect considerably the final content of impurities in steel by improving the efficiency of desulphurization, removing non-metallic oxide inclusions, and ensuring low content of oxygen in steel in the course of its refinement. Several steel melting shops have assumed that such slag is required to have a chemical composition CaO – 45 % to 55 %, Al2O3 – 25 % to 30 %, SiO2 below 10 %, MgO – 6 % to 8 %, and FeO below 1 %. Also, in case of the secondary steelmaking process whose aim is to get liquid steel with low content of sulphur and non-metallic oxide inclusions, it is desirable to use slag fulfilling the requirements of Mannesmann index, M=(CaO/SiO2)/Al2O3. Depending on the assumed technological conditions in a particular steel melting shop it can be that the secondary steel making operators prefer lower content of Al2O3 than that assumed in the composition of slag or that there are conditions in which slag can have low (several percent) content of SiO2.
However, the final condition, i.e. the lowest FeO content in slag, is the main requirement when it comes to processing of steel in the ladle efficiently in the secondary steelmaking facilities and achieving low content of oxygen in steel as well as minimal losses of components with high oxygen affinity. Also, it is important to maintain low content of MgO, TiO2 and Cr2O3 as they are detrimental to the flow and worsen the refining properties of slag. However, the presence of MgO in the presented range is technologically founded because of its role in improving the durability of the refractory lining of the ladle. Achieving slag with the above-mentioned composition ensures the production of very clean steel. Ladle slag is formed because of the addition of CaO, bauxite, and specially made slag forming mixtures containing CaO and Al2O3. In order to form slag. It is also possible to add slag made outside the furnace.
One of the formulations which has always eluded metallurgists and engineers is finding a universal compositional parameter which can be applied to slag in a fashion similar to which the pH formulation has been applied to aqueous solutions. Slag component oxides can be classified as acidic, basic, or amphoteric (can act as a base or acid). However, the physical measurement of the basicity of a slag is still not possible, even in these modern times. A number of formulations have been used to express the basicity of a slag. The most common approach is the use of a basicity index in which the basic oxides are placed on the numerator and the acid oxides on the denominator. Of these, by far the most commonly used is the ‘V’ ratio (% CaO/% SiO2). While this is useful as a first approximation, in slags containing appreciable quantities of oxides other than CaO and SiO2 it is a very imperfect expression of basicity.
Other formulations such as (% CaO + % MgO)/% SiO2 or (%CaO + % MgO)/(% SiO2 + % Al2O3), and several more, are also sometimes used. The first problem with these expressions is that they involve an arbitrary decision as to whether a component is basic or acidic and does not incorporate the differences in the relative basicities (acidities) of the different oxides. A second problem with these ratios is that the assessment of basicity becomes impossible in slags free of any recognized acid component. However, the two basicity formulations which are commonly used to define oxidized slags and reduced slags are basicity index for oxidized slags [% CaO/(% SiO2 + % Al2O3)], and basicity index for reduced slag [(% CaO + % MgO)/(% SiO2 + % Al2O3 + % CaF2)]. It is common to also add FeO and MnO to the acidic components in the reduced slag ratio (denominator) provided that their sum (FeO + MnO) is less than 5%.
Recently, a more fundamental indicator of slag basicity, namely optical basicity, has been established. Using experimentally determined spectrographic information on a large number of glasses and Pauling’s electronegativity data, the optical basicity for several components have been calculated. Table 1 shows optical basicity values for the common ladle slag components.
|Tab 1 Optical basicity of slag components|
|Oxide||Optical basicity, Lambda|
|Sodium oxide, Na2O||1.15|
|Calcium oxide, CaO||1|
|Magnesium oxide, MgO||0.78|
|Calcium fluoride, CaF2||0.67|
|Titanium oxide, TiO2||0.61|
|Aluminum oxide, Al2O3||0.61|
|Manganese oxide, MnO||0.59|
|Chromium oxide, Cr2O3||0.55|
|Ferrous oxide, FeO||0.51|
|Ferric oxide, Fe2O3||0.48|
|Silicon oxide, SiO2||0.48|
The optical basicity concept has been used with great success in correlation with sulphide and phosphorous capacities. While the optical basicity is presently the best compositional parameter available which can be used to describe the composition (basicity) of a slag, it does not say anything about the physical properties of a slag. For example, considering the ‘slag’ composition of CaO-62 %, MgO-8 %, and SiO2-30 %, the optical basicity of the slag can be calculated as lambda – 0.756, which can be considered sufficiently basic to ensure good sulphur removal and minimum refractory wear. However, this number is meaningless since this slag is completely solid at steelmaking temperatures, and it starts to melt at 1,790 deg C and gets fully molten at 1,950 deg C. The application of the optical basicity concept to steelmaking slags is only useful if the slags are completely molten. Hence, considerable more information, such as solidus and liquidus phase relations and viscosity data, is necessary for completely evaluating the slags which are suitable in the ladle furnace.
The fluxing effect and the concepts of solidus and liquidus temperatures – The solidus temperature is defined as the temperature where the first drop of liquid start to form and the liquidus temperature is the temperature where the mixture is completely molten. Slags typically consist of the oxide components such as SiO2, CaO, MgO, Al2O3, FeO, MnO, and Cr2O3. The pure individual component oxides melt at very high temperatures and melt at a specific temperature. In this case, the solidus temperature is equal to the liquidus temperature. Slags typically consist of a solution (mixture) of more than one oxide. In case of a solution or mixture then the solidus temperature is not equal to the liquidus temperature.
Besides the major slag components consisting of CaO, MgO, SiO2, Al2O3, CaF2, and FeO, there are several other minor components such as Cr2O3, MnO, TiO2, and P2O5, which have also an impact on the phase relations of slags. The levels of these components are not very high (less than 5 %) in typical steelmaking slags. Also, the effect of some of these components on the phase relations is so similar that they can be considered equivalent. A good example is FeO and MnO which behaves almost identically in slags. The only difference is that slags involving FeO have much lower melting than slags involving MnO.
Ladle slag is formed because of the fluxing action of the flux. The extent of fluxing is dependent on the quantity of the flux added. In secondary steelmaking, extensive ‘fluxing’ between the component oxides has to occur at steelmaking temperatures in order to form liquid slags. The extent of fluxing between the different oxides is normally shown graphically on phase diagrams. A phase diagram is really a plot showing the fluxing effect of one oxide on another as a function of temperature and composition. For example from the phase diagram of the components CaO and SiO2, it can be determined exactly how much SiO2 is to be added to CaO to have a completely liquid slag (solution) at 1,600 deg C.
Slag balance – The slag components are divided into two groups namely (i) refractory oxides, and (ii) fluxing oxides. Refractory oxides are CaO and MgO. The fluxing oxides are SiO2, Al2O3, CaF2, and Iron oxide. The individual components CaO (melting point 2,600 deg C) and MgO (melting point 2,800 deg C) are very refractory, but even when these components are combined, minimal fluxing occurs between them and a liquid phase only forms above 2,300 deg C. Therefore, dolomite, is a refractory material. Hence, the addition of other fluxes (component oxides or fluorides) to CaO and MgO is needed to form a liquid slag at steelmaking temperatures. In very simple terms, slag engineering can be defined as the balance between the refractory oxides and the fluxing oxides. This balance is typically expressed as a basicity ratio. Basicity ratio is the ratio of the refractory oxides and fluxing oxides [(% CaO + % MgO)/(% SiO2 + % Al2O3 + % CaF2 + % FeO)]
As a general guideline, a minimum basicity ratio of 1.5 is needed to achieve a reasonable ladle slag. However, while a basicity ratio of 1.5 can be adequate for silicate slags, it is too low for slags containing considerable quantities of Al2O3 and CaF2. The solubility of CaO in the slag increases as the Al2O3 contents of the slag increases. In order to maintain a slag with a ‘creamy’ consistency, higher quantities of lime is to be added. The basicity ratio for an aluminate slag (around 25 % Al2O3) is around 1.8. When the slag contains considerable quantities of fluorspar, the needed basicity ratio can be around 2 since fluorspar increases the solubility of CaO and MgO in the slag.
SiO2, Al2O3, and CaF2 are the three main fluxing components in case of secondary steelmaking. The fluxing potential of these components are not equal. SiO2 is the ‘weakest’ flux to bring CaO into solution, whereas CaF2 is the most potent flux to bring CaO into solution. One important point to note is that if compared on a two-component basis (CaO-SiO2, CaO-Al2O3, or CaO-CaF2), there is not much difference in the quantity of CaO which the different fluxes can dissolve at a specific temperature. However, when the fluxes are combined (SiO2 + Al2O3 or SiO2 + CaF2), the solubility of CaO in the slag increases dramatically.
The slag requirements include improving the quality of the steel, and to be compatible with the refractories. These are not opposing goals. Since a basic slag practice is used for most grades of steel, the best slag for steelmaking quality is also to be the best slag for refractory compatibility. These optimum slags have a ‘creamy’ consistency and are normally just saturated with respect to CaO, or MgO, or both.
The significance of the quantity and composition of primary steelmaking furnace carry-over slag into the ladle – The extent to which oxidized carry-over slag from the primary steelmaking furnace can be tolerated in the ladle depends largely on the type and quality of steel produced. For example, for some grades of steel, such as reinforcement bar, the slag in the ladle plays a limited metallurgical role because of the fairly high phosphorus and sulphur specifications of the steel. Here the carry-over slag has an advantage as it provides the necessary slag volume required for arc-flare protection. A major benefit of utilizing and optimizing furnace carry-over slag is that the carry-over slag is already hot, and mostly liquid, so that it can act as a fluxing precursor, improving the dissolution kinetics of other flux additions. A suitable quantity of lime is normally added to ‘neutralize’ the slag to improve refractory compatibility.
One of the primary concerns regarding slag carry-over is the reversion of phosphorus for steel grades with very low phosphorus specifications. In these cases the carryover slag is either minimized during tap or removed after tap and a complete ‘synthetic’ slag mixture is used. For most aluminum killed steel grades, carry-over slag from the primary steelmaking furnace has to be minimized. Carry-over slag contains considerable quantities of FeO, MnO, SiO2, and P2O5, which are reduced back to the steel by the dissolved aluminum in the steel during stirring operations. The FeO and MnO are particularly responsible for aluminum fade and inclusion formation, whereas the reversion of silicon and phosphorus can be detrimental for some grades of steel. Majority of the aluminum killed steel producers use some kind of method to eliminate or minimize the carry-over of furnace slag into the ladle.
However, for the majority of steel grades, a certain quantity of slag carryover can be tolerated, and can even be beneficial, provided that it is controlled and conditioned by the addition of suitable fluxes and deoxidation agents. The composition of the primary steelmaking furnace slag has a major impact on the type of slag formed in the ladle, i.e., the carry-over of a ‘balanced’ primary steelmaking furnace slag can result in a good slag in the ladle. The control of the carryover quantity of primary steelmaking furnace slag and deoxidation of the slag is necessary.
The significance of first making slag then making steel – The goal of secondary steelmaking is to deliver a ladle of homogeneous liquid steel to the casting machine on time, at the right temperature, and meeting total chemical specifications. This is only possible if the slag is conditioned and optimized first. Tab 2 shows the sequence of events and the benefits which can be realized for operations where the slag is not skimmed off before ladle refining starts.
|Tab 2 The sequence of events and the benefits which can be realized for operations|
|Add suitable fluxes for good fluidity and slag volume during tapping.||Reactions in a liquid slag is always faster than in a crusty or solid slag.|
|Add suitable slag deoxidizers during tapping (or after tapping) so that addition of (FeO + MnO) is less than 3.||Better alloy recoveries (yields) and less fading of the steel.|
|Further deoxidizing of the ladle slag can be needed for certain grades of steel so that addition of (FeO + MnO) is less than 1.||Stable and consistent steel chemistries|
|A liquid (creamy) deoxidized slag five minutes after ladle refining started.||Improved desulphurization 1. Crusty slag do no desulphurize very well. 2. Black or brown slags do not desulphurize.|
|Less trim additions are needed for steel specification since the slag is now ‘inert’ towards the steel.|
|Improved thermal efficiency – A liquid slag with sufficient volume now covers the arc from the beginning of power-on so that arc-flare is minimized.|
|Cleaner steel – The sooner the slag is liquid and deoxidized the sooner it can absorb inclusions.|
|Add lime to maintain a creamy slag in the ladle. As the slag heats up it becomes more liquid.||Improved refractory life – Very fluid (watery) slags is an indication that these slags are not saturated with the refractory oxides and hence tend to dissolve them from the refractories.|
Slag volume and fluidity requirements for optimum ladle furnace refining – The efficiency of heating in a ladle furnace is a function of stirring and slag depth at a given voltage setting. Typically the arcs of 50 mm to 75 mm is to be buried in slag which is slightly deeper. The slag weight can be calculated using a general equation which incorporates the size and shape of the ladle as shown in Fig 3.
Fig 3 Calculation of slag weight
A typical slag density value for gas free ladle slags is around 2.65 tons per cubic meter (t/cum). Heavy metal oxides (FeO, and MnO etc.) increases the density value. A 10 % heavy metal content changes the density by around 0.21 t/cum. A typical furnace slag has a density of around 3.18 t/cum. Carbon containing slag deoxidizers can be used in the ladle to ‘artificially increase the slag volume due to CO2 gas evolution. This is a very important consideration when the equation given in Fig 3 to calculate the slag weight to achieve a certain slag thickness. For foamy ladle slags a slag density value of between 2.12 t/cum to 2.47 t/cum is then to be used in the calculation (the foamier the slag the lower the density value).
Slag fluidity requirements for operations with a ladle furnace – All the fluxing oxides are to be added to ladle as soon as possible, preferably during tapping. The quantities of lime, dolomite, and fluxing oxides are to be sufficient to cover the arc in the ladle furnace (75 mm to 100 mm). Mixtures of pure components can be used (lime + bauxite or lime + fluorspar + silica sand). The slag is to be superheated by the arc and the heat spends enough time at the ladle furnace station for all the components to go into solution. The slags are to be fluid, but not watery (slags with a ‘creamy’ consistency). The ‘creaminess’ of the slag is to be maintained throughout the heat by adding lime to slag if it becomes too watery.
Slag fluidity requirements for operations without a ladle furnace – All the slag components are to be added to ladle during tapping. Pre-fused materials (calcium aluminate) or lime/spar/sand mixtures (70 %/15 %/15 %) is to be used. The mixtures is required to melt immediately and go into solution very fast after a few minutes of vigorous stirring. Since no external source of heat is available to heat the slag but the steel, the aim chemistry of the slag is to be chosen so that it is fully liquid at around 1,570 deg C. Since the tapping temperatures are normally higher for these operations, the slag initially is to be very liquid but then become creamy as the steel cools down to teeming temperatures.
Stirring and argon flow rates – Gas stirring can aggravate refractory erosion. Too little stirring leads to superheating of the slag and surface layers of the steel, low heating efficiency, and also increased refractory erosion. Excessive stirring causes metal splashing, short-circuiting, carbon pickup, electrode ‘seeking’ and low heating efficiency. Short-circuiting and electrode seeking can destabilize the arc, causing arc flare and refractory damage. Higher stir rates are typically used to open the eye for alloy additions and desulphurization. Better thermal efficiency and inclusion floatation are normally achieved at lower stirring rates.
Heating and arc control -The electrode arcs are to be electrically balanced and proper aligned to prevent excessive arc impingement on the ladle walls. The temperatures of carbon arc plasmas are over 3,310 deg C and well melt quickly through any type of refractory lining. If the arcs are imbalanced electrically or are physically misaligned, this can cause the arc flare to overheat a small section of the ladle slag line leading to early failure of the slag line. Serious damage can also occur if the arc length is long and the arc is not covered by the slag layer in the ladle. The maximum heating rate which is typically used is 3 deg C/min to 4 deg C/min.
Sulphide capacity of slag – Sulphide capacity of slag describes the slag’s potential ability to remove sulphur from steel. This important property of slag is very useful in prediction and control of the desulphurization process. Sulphide capacities are expressed in log units. The less negative the logarithmic unit, the better is the sulphur removing capacity of the slag. Sulphide capacities for CaO-Al2O3-MgO and CaO-Al2O3-SiO2 systems, can be determined experimentally. However, these experimentally determined sulphide capacities are not useful for complex industrial slags, where a general expression of ‘slag capacity’ as a function of slag composition and temperature is useful. Correlation of optical basicity with the sulphide capacity has shown very good correspondence over a wide range of slag composition.
Several adjustments have been proposed in an effort to improve accuracy of the correlation with optical basicity. Alternate correlations which are not based on optical basicity are also available, but are less useful. The optical basicity correlation is easy to use and shows good approximation for different ranges of slag composition.
The ratio of sulphur content in the slag and the metal phase normally describes an estimation of the sulphur reduction in a desulphurization process. The sulphur distribution ratio is a function of temperature, sulphide capacity, oxygen activity and sulphur content of the molten steel. If the sulphur and oxygen content in steel are known, the sulphur distribution ratio can be calculated using values of the sulphide capacity for a particular slag. It is always assumed that the sulphur distribution ratio values are to be large for better sulphur removal. This can be achieved by having high temperature, large sulphide capacity value, and low oxygen activity. Since the sulphide capacity and sulphur distribution ratio are functions of temperature, it is very important to maintain high temperatures for a better desulphurization process.
Slag and metal weight and initial sulphur content in the metal (% S) affect the final sulphur content. Also, to have lower final sulphur content in the metal, the value of the sulphur distribution ratio is to be high or slag volume is to be high, or alternatively the initial sulphur content is to be low. Also, the degree of sulphur removal during the desulphurization treatment is a very capable tool to characterize the efficiency of the treatment with slag.
One of the most important aspects which is frequently overlooked when evaluating and discussing slags, is that most slags consist of two fractions, i.e., a liquid fraction and a solid fraction. Slag fluidity in basic slags is controlled by the liquid and solid fractions of the slag. The higher is the solid fraction of the slag, the lower is the fluidity of the slag (higher viscosity). A basic slag which is completely liquid has maximum fluidity.