Evaluation of Factors Affecting the MgO–C Refractory Lining Degradation in a Basic Oxygen Furnace

Abstract

Identification of the factors influencing refractory lining wear and its residual thickness in the basic oxygen furnace (BOF) is a prerequisite for optimizing the steelmaking process. In this study, the factors that contribute significantly to the wear of the refractory lining in the most stressed areas of the banded lining (i.e., the trunnion ring area and slag line area) are identified. Knowledge of the rate at which a given factor acts on refractory wear is closely related to the development of technological procedures aimed at limiting its influence. This research evaluates the technological causes and describes the lining wear mechanism and the thermodynamic parameters of the reactions between the MgO–C metal, slag, and gunning material phases. In researching the topic, real operational data were processed using statistical methods and data analysis, which were supported by thermodynamic modeling of chemical reactions. The results show that the combination of technological factors, mechanical action of the raw materials, blowing and free oxygen in the metal, silicon from the pig iron, and slag viscosity have the greatest influence on the residual thickness of the MgO–C refractory lining in BOFs. Refractory gunning material consumption, its effect on campaign length, and the cost-effectiveness of repair work were also analyzed.

1. Introduction

The global refractories consumption market is significantly influenced by the increase in steel production. The development and introduction of new steelmaking technologies are also reflected in the increasing demand for periclase-carbon (MgO–C) refractories for basic oxygen furnace (BOF) linings [1]. The technological processes currently used for steel production place high demands on the refractory materials’ quality parameters. The specific processes that take place during the steelmaking process, such as rapid temperature changes, changes in the composition of the slag, or the impact of heavy pieces of charged scrap, are destructive to the MgO–C refractory lining of the BOF [2]. This is why a great deal of emphasis is placed on maintaining the refractory lining during the campaign, which is associated with the ever-increasing consumption of refractory gunning material [3]. The internal working lining is not exposed to the same wearing factor at each point. For this reason, the BOF is equipped with a zonal lining containing several types of refractories. The properties of the refractories are dimensioned according to their specific use in each zone of the working lining. This ensures uniform wear and increases the life of the BOF refractory lining [4]. Nowadays, the main aspect of the basic refractories production program for the steel industry has shifted to unfired periclase-shaped refractories bonded with organic binders with a carbon content of up to 7% and periclase-carbon-shaped refractories with a carbon content of 7% to 30% [5]. In MgO–C building blocks, a carbon bond replaces the ceramic bond. This bond is provided by organic binders such as coal tar and synthetic resins. Pyrolysis leaves behind a carbon residue, which represents a high-temperature bond and part of the carbon component of the products. The specific feature of the carbon bond is that the graphite surface is not wetted by liquids, and the bond is formed through the periclase (MgO) component and the edges of the graphite crystals [6]. In addition, the MgO–C refractories are characterized by high refractoriness and excellent thermal shock and corrosion resistance, resulting from high thermal conductivity, low thermal expansion, low wettability of graphite, and high MgO refractoriness [7].

The basic MgO–C refractory lining of the BOF is chemically degraded by corrosion, slag penetration, and erosion. Among the mentioned chemical degradation methods, graphite oxidation (corrosion) contributes the greatest to the wear of MgO–C refractories. Oxidation increases porosity and decreases the strength and resistance of the MgO–C refractory to further exposure to the oxidizing agent [8]. Some authors have investigated the direct and indirect oxidation of graphite in MgO–C refractories [9,10]. Direct oxidation (gas phase oxidation) is the main mechanism at temperatures below 1400 °C where carbon is consumed by gaseous oxygen (Equation (1)). The process of direct oxidation and decarburization of MgO–C refractory material with injected oxygen (O₂) starts with the diffusion of oxygen through the gas layer to the decarburized MgO-O₂ phase interface, continues with the diffusion of oxygen through the decarburized MgO layer, followed by Equation (1) at the MgO–C-MgO phase interface, and finally the product of the CO reaction diffuses through the decarburized MgO towards the outer surface of the lining. It is true that with increasing temperature, the thickness of the decarburized layer increases, and the proportion of the total mass loss due to decarburization of the MgO–C refractory also increases. [11]. At temperatures higher than 1400 °C, indirect oxidation, or solid phase oxidation (Equation (2)), becomes the main mechanism, where carbon reacts with oxygen in solid MgO [12]. The gaseous products of the reaction diffuse through the pores of the lining to the refractory surface. Near the MgO–C surface, oxygen is at a higher partial pressure [11] than the products from Equation (2). According to Equation (3), oxidation and sedimentation from the gas phase results in the formation of a dense, fine-grained periclase layer, which slows down the corrosion process of the MgO–C refractory by the barrier effect [13]. Despite the slowing down of corrosion by the dense layer, the reacted MgO (Equation (2)) is restored to the oxide form (Equation (3)), and the process still results in the loss of carbon [14]. Therefore, various forms of antioxidants have been used to mitigate such effects and enhance the oxidation resistance of MgO–C refractories [15,16,17].

$${2\mathrm{C}}_{\left(\mathrm{s}\right)}+{\mathrm{O}}_{2\left(\mathrm{g}\right)}\to {2\mathrm{C}\mathrm{O}}_{\left(\mathrm{g}\right)}$$

(1)

$${\mathrm{C}}_{\left(\mathrm{s}\right)}+{\mathrm{M}\mathrm{g}\mathrm{O}}_{\left(\mathrm{s}\right)}\to {\mathrm{C}\mathrm{O}}_{\left(\mathrm{g}\right)}+{\mathrm{M}\mathrm{g}}_{\left(\mathrm{g}\right)}$$

(2)

$${2\mathrm{M}\mathrm{g}}_{\left(\mathrm{g}\right)}+{\mathrm{O}}_{2\left(\mathrm{g}\right)}\to {2\mathrm{M}\mathrm{g}\mathrm{O}}_{\left(\mathrm{s}\right)}$$

(3)

Regarding kinetics, corrosion depends on the rate at which the slag drains MgO from the lining [18]. Therefore, saturating the slag with MgO is a simple way to reduce the dissolution potential of the major lining component, reduce its wear, and thus prolong the life of the structure [19]. The phase ratios in the system MgO-SiO₂-R₂O₃—RO phase—(P₂O₅) are indicative of the amortization of the base lining from a chemical point of view. In slag with CaO/SiO₂ < 1, the periclase does not coexist and, therefore, dissolves. As the CaO/SiO₂ ratio (basicity) increases, the solubility of periclase decreases significantly, while at CaO/SiO₂ > 2, it changes insignificantly [20]. The corrosion rate of the MgO–C refractory lining is also affected by grain size, where small grains of MgO dissolve in the early silicate BOF slag to form Ca and Mg silicates. It is more difficult for the slag to penetrate the larger grains of MgO; therefore, the slag penetrates the lining along the grain boundaries [21]. Porosity also affects the corrosion rate of the MgO–C refractories. Low porosity reduces the surface area available for reaction with the melt [4]. It is advantageous if the pores are sufficiently filled with carbon compounds, which also increases density and reduces the wettability of the MgO–C material [6]. The depth of BOF slag penetration depends on the refractory density, as it determines the size of the building material’s contact area for reaction with the slag [5,22]. The carbon in MgO–C refractories affects the corrosion process by reducing iron oxides, thus limiting their contact with the refractory lining. As the carbon in MgO–C increases, the wettability by BOF slag decreases; meanwhile, the pore surface area increases due to carbon depletion [23,24,25,26]. The wettability is an important factor influencing the wear kinetics of the MgO–C refractory material. The wettability by molten BOF slag is critically dependent on the BOF slag’s viscosity and surface tension, as well as the chemical reactions and adsorption at the interface MgO–C refractory—BOF slag. From a metallurgical point of view, the lowest possible wettability of the MgO–C material is desirable [4,5,21,27]. The amount of MgO in the BOF slag also influences the corrosion kinetics of the MgO–C refractory. In practice, dolomitic lime, burnt dolomite, or caustically burnt magnesite is added to BOF slag because the low MgO slag has a high solubility capacity with the slag. The usual MgO additions range from 8% to 10% [19,28,29]. A major challenge for carbon-containing refractories is the oxidation of non-oxide constituents, particularly carbon. Various additives are incorporated as antioxidants to protect the castable from oxidation [30]. Metallic silicon and aluminum powder are the most extensively used antioxidants in MgO–C refractory compositions [5,31,32]. The slag that passes through the porous structure of the MgO–C refractory stops its penetration at a depth corresponding to the solid phase crystallization temperature. This phase comprises the slag and building material components [27].

Penetration of BOF slag through cracks and pores into the MgO–C refractory results in degradation of the refractory’s surface layer through the formation of a decarburized layer. Slag passes through this layer to a depth of approximately 5 mm; below this layer, the MgO–C refractory is mechanically degraded to a depth of approximately 10 mm [4]. The transition between the bonded working layer and the original unchanged refractory is sharp [5]. By crystallizing the solid phase, stresses are generated at the interface, leading to the formation of cracks and, upon temperature change, to peeling of the bonded layer and exposure of the bonded shard [6]. The filtered slag reacts with the MgO lining to form easy-to-melt magnesium silicate phases: mixed Ca and Mg silicates with melting points from 1320 °C to 1580 °C [4]. Penetration of the slag forms a zonal structure, which is the cause of delamination. The depth of the bonded layer and, thus, the formation of the zonal structure can be reduced by adding anti-wetting agents, such as carbon, SiC, or spinel particles [21].

The interaction of the three-phase MgO–C-slag-metal system and the formation of CO bubbles are the main causes of local corrosion and, hence, wear of the BOF refractory lining. Local corrosion is strongly influenced by the formation location and number of CO bubbles. The CO bubbles formed at the phase interface of the ternary MgO–C-slag-metal system inhibit the progress of local corrosion. Meanwhile, gas bubbles formed at the MgO–C—metal interface enhance the local corrosion process [33]. Equation (4) is the main reaction involved in the formation of carbon monoxide. The FeO in Equation (4) is produced by the reaction presented in Equation (5). Iron oxide (FeO) passes from the slag–metal phase interface to the slag-refractory phase interface, where it reacts with carbon to generate CO bubbles, according to Equation (4) [34]:

$$\left(\mathrm{F}\mathrm{e}\mathrm{O}\right)+{\mathrm{C}}_{\left(\mathrm{s}\right),\mathrm{r}\mathrm{e}\mathrm{f}\mathrm{r}\mathrm{a}\mathrm{c}\mathrm{t}\mathrm{o}\mathrm{r}\mathrm{y}}\to \left[\mathrm{F}\mathrm{e}\right]+\left\{\mathrm{C}\mathrm{O}\right\}$$

(4)

$$\left[\mathrm{O}\right]+\left[\mathrm{F}\mathrm{e}\right]\to \left(\mathrm{F}\mathrm{e}\mathrm{O}\right)$$

(5)

An additional factor that affects the wear rate of the MgO–C refractory is the effect of the flow of molten slag and steel. The movement of the slag film is caused by intense swirling and turbulent flow due to high-intensity oxygen blowing; this is a significant factor causing local corrosion of the oxygen converter lining. The movement of the slag film accelerates the dissolution of the MgO–C refractory and acts as an abrasive on the BOF lining [21,27,35].

The factor determining the efficiency of the mass and heat transfer process in BOF slag is viscosity, primarily a function of temperature and the size and mobility of the anions present. Basic BOF slags have a very low viscosity at the temperatures of the steelmaking process, which increases significantly as the temperature decreases. BOF slags differ from other iron or steelmaking slags containing FeO due to their elevated FeO content and high CaO/SiO₂ ratio [36]. Only five studies in the literature address the viscosity of BOF slags or slags with compositions similar to BOF slags [37,38,39,40,41]. The viscosity of basic (BOF) slags is substantially increased by the addition of components with the ability to form large complexes (SiO₄⁴⁻, Si₂O₇⁶⁻, PO₄³⁻) and is decreased by the addition of ions with small dimensions (Ca²⁺, F¹⁻, Na¹⁺). The viscosity can be calculated according to various formulas [6,21,42], represented by Equations (6) and (7):

$$\mathsf{\eta }=\mathrm{A}\cdot {\mathrm{T}}^{\frac{\mathrm{B}}{\mathrm{T}}}$$

(6)

$$\mathsf{\eta }={\mathrm{T}}^{\frac{\mathrm{B}}{\mathrm{T}+\mathrm{A}}}$$

(7)

where η is viscosity in Pa.s⁻¹; T is temperature in K; A and B are coefficients. Dynamic mechanical loading of the MgO–C lining, which includes abrasion, impact, and vibration, occurs mainly during the charging of steel scrap into the oxygen converter. During the raw material feed, the refractory lining is most stressed in the impact area. However, under operating conditions, mechanical wear factors do not act on the refractory lining alone but in combination with thermal or chemical factors [1,43].

The use of gunning refractories, slag coating, or slag splashing is an important factor in maintaining the structural integrity of the refractory in BOFs. Repair procedures help protect the MgO–C refractory lining from wear caused by mechanical and corrosive effects while also helping extend the life of the refractories and, therefore, the BOF campaign [3]. Regardless of the treatment method, it is important to note that each lining has a specific critical level of durability beyond which the application of any repair method becomes economically unviable. Gunning refractories are one of the most effective ways to extend the life of an oxygen converter’s refractory lining, increasing the number of heats per campaign. The strategic objective of pneumatically applying monolithic refractories to a precisely defined area of the BOF lining is to avoid slowing down the production process due to excessively long breaks for lining repairs, thus, preventing a reduction in steel productivity [1]. Wear evaluation of the BOF refractory is achieved by comparing the MgO–C refractory lining thickness data. The first measurement of the converter’s internal profile, made after constructing the new working lining, is taken as the reference. This data shall be considered the benchmark for the entire campaign. Each subsequent measurement shall be compared with the reference measurement to determine the loss (refractory wear) or gain (skulls formation) of refractory in BOFs.

During the steelmaking process, a mixture of liquid steel and slag forms deposits on the refractory lining, oxygen nozzle, converter throat, and inner wall of the flue gas duct [44]. These deposits are collectively known as skulls. Determining the chemical composition of the skulls is difficult because the chemical composition of steel and slag changes significantly during the steelmaking process [1]. The aim of treating the refractory lining in the BOF is to reduce the skulls on the structure and maintain the MgO–C lining at an optimum level. The size of the skulls is determined by measuring the lining using laser equipment. A special oxygen nozzle is used in the BOF to remove the skulls. The holes in the copper head can be sealed with copper screws as required to allow the oxygen stream to reach the skull area without hitting the unaffected MgO–C lining [45]. Skulls on the oxygen nozzle cause complications when handling the nozzle. It is possible to reduce the formation of skulls on the copper head and the nozzle by introducing a post-combustion sub-lance [1], controlling the molten metal level during the heat process using radar sensors [44], and controlling the height of the oxygen lance during the steelmaking process [45].

Lime and dolomitic lime are the main slag-forming components and are added to form a primary basic slag. The formation of an active high-base slag depends on the flux (CaO and MgO) transfer to the slag. The rate of lime assimilation by the slag is determined by the reactivity of the lime and the dolomitic lime. To delay the formation of high-base slag in the converter and thus excessive wear of the MgO–C lining, soft-burnt lime with high reactivity is used as a slag-forming flux [1,27]. Another factor adversely affecting the MgO–C lining is the high silicon content of the hot metal. A high silicon content in the pig iron prolongs the formation of high base BOF slag, while acidic slag with a high silicon oxide content significantly damages the basic MgO–C lining. Moreover, high silicon in the pig iron decreases the steel yield due to increased metal rejection. In addition, the increased splashing of molten metal and slag causes the formation of skulls on the refractory lining [46].

The volume and intensity of technical oxygen blowing directly influence the wear rate of the MgO–C refractory lining, the level of slag oxidation, and the slag formation process. Dissolution of the lime is inhibited during the melting process by forming silicate coatings on the surface of the lime pieces. The density of the dicalcium silicate packing depends on the iron oxide content of the slag; if it is <20%, a dense packing of 2CaO.SiO₂ forms on the lime pieces, preventing the lime from dissolving. It is only when the iron oxide content is above 20% that the dicalcium silicate layers are released, and lime dissolution is initiated. The iron oxide content can be increased by increasing the position of the nozzle or by increasing the intensity of the oxygen blown. This method is particularly important in the first few minutes of melting when the formation of high-base slag must be accelerated [47,48]. Excessively long heat, downtime, and oxygen leakage also adversely affect the overall life of the MgO–C refractory lining of the BOF. Re-blowing oxygen not only increases the temperature in the BOF but also causes oxidation of the BOF slag and the overall atmosphere in the BOF. In such an atmosphere, there is an increased oxidation of carbon from the MgO–C lining and, thus, a reduction in the resistance of the BOF lining to the attack of slag [1].

The global trend of increasing BOF heating during a campaign poses high demands on the MgO–C refractory lining. To sustain this trend and optimize service life, it is essential to align the MgO–C refractory’s chemical, physical, and thermomechanical properties with the operational factors causing wear. This study explores BOF MgO–C refractory lining wear in a comprehensive manner not previously covered in existing literature.

2. Materials and Methods

2.1. Description of Analyzed Campaign

During the campaign under analysis, 17,669 heats were performed, and 751 measurements of the refractory lining were taken with non-contact laser equipment from Ferrotron Technologies, GmbH. The wear analysis of the MgO–C refractory lining of the BOF (Figure 1) was focused on the vertical direction in the zone between 75 and 105° (labeled as S3) and the zone between 255 and 285° (labeled as S9). In the horizontal direction, the analysis focused on the 4–5.2 m range (trunnion ring area) and the 5.2–6.2 m zone (slag line area). The monitored areas and zones are evident in Figure 1. Out of 751 refractory lining measurements, 160 were selected to include detailed measurements from the surveyed areas. The random selection of these measurements was made so that they covered the entire campaign in terms of time and that the time interval between the selected data was approximately the same throughout the campaign.

2.2. Methodology for Evaluating the Wear Factors of the Lining and Data

Each measurement of the remaining BOF refractory lining thickness contained data for the heat number according to which the measurement was made, as well as the date and time of the record. From this information, it was possible to associate suitable heat data with the measurement and link the refractory thickness data with the chemical composition and temperature data of the molten pig iron, slag, and gunning refractory material data throughout the campaign. All parameters were correlated with average refractory loss values in specific monitored areas.

The observed factors were processed in a Microsoft Excel 2021 spreadsheet with statistical add-in Lumivero XLSTAT 2019. All factors were included in the correlation dependency, which established a quantitative dependency, where changes in one variable led to changes in the other variables. The so-called regression function expresses this dependence. According to the shape of the regression function, a positive and negative correlation dependence was recognized. Covariance was used to detect the presence of linear correlations in the bivariate statistical set. Pearson’s correlation coefficient was used to determine the degree of linear correlation, with values of <−1.1>.

$$\mathsf{{\rm P}}\left(\mathrm{X},\mathrm{Y}\right)=\frac{\mathrm{cov}\left(\mathrm{X},\mathrm{Y}\right)}{\sqrt{\mathrm{D}\left(\mathrm{X}\right)\cdot \mathrm{D}\left(\mathrm{Y}\right)}}$$

(8)

where ρ is the Pearson correlation coefficient, cov(X,Y) is the covariance between variable X and Y, D(X) and D(Y) are dispersion or variances of variables X and Y. Based on the correlation coefficient of the bivariate basic set, it was possible to determine the order of the factors with the greatest influence on the wear of the refractory structure. The three most significant factors were studied for each analyzed area (trunnion ring/slag line) and zone (S3/S9).

2.3. Gunning Refractories Consumption in BOF

In order to verify the theoretical consideration of excessive refractory lining wear in the area of the trunnion ring and slag line, it was necessary to evaluate the consumption of the gunning refractory material for other zones of the BOF lining. By cross-referencing the amount of repair mixture applied over the campaign, the areas of greatest BOF refractory lining wear were clearly identified. The areas of wear included the trunnion ring area between 4 and 5.2 m and in the 75°–105° range (S3 zone) and an opposite range of 255–285° (S9 zone), then the slag line area between 5.2 and 6.2 m and in the of 75–105° range (S3 zone) and an opposite range of 255–285° (S9 zone). The monitored areas and zones are shown in Figure 1. The consumption of gunning refractories in the above areas was compared with those used in the BOF’s tap pad, charge pad, tap hole, and cone area.

2.4. Analysis of Technological Factors Influencing Refractory Lining Wear

A detailed analysis of the technological and architectural layout of the steelworks and basic oxygen furnace, as well as the individual work processes, is important to understand the operational factors that affect the wear of the MgO–C refractory lining. The method of charging the converter and managing the oxygen lance during the heat were analyzed.

2.5. Effect of BOF Slag Viscosity on MgO–C Lining Wear

Individual slags were compared to understand chemical composition changes. The maximum and minimum compound levels were identified in the converter slag during the target period. This data established the heat number and full chemical composition of both slags and allowed the dynamic viscosity calculation using Formula (7) and the IRSID-Riboud model [36], which correlates the viscosity with the temperature fluctuation. Consequently, the maximum and minimum influence of the compound on converter slag viscosity was determined.

2.6. Wear Mechanism of MgO–C Refractory Lining of BOFs and Effect of Gunning Refractories

Knowing the chemical composition of the BOF slag, MgO–C refractory, and gunning material makes it possible to identify the chemical compounds that have attacked the refractory during the steelmaking process and which form a protective layer on the BOF refractory lining during repair operations, thus reducing the wear mechanism and extending the life of the BOF lining. The mentioned mechanism of BOF refractory lining was analyzed using the thermodynamic software HSC Chemistry 5.11 from Outokumpu (Metso Outotec, Helsinki, Finland). Using thermodynamic modeling, the most likely oxides of the chemical reactions in the BOF slag were formed from the most represented chemical elements in the converter slag (Ca, Fe, Si, Mg, O). Based on the chemical composition of the gunning material and particular temperatures, the most probable chemical compounds were modeled.

3. Results

3.1. Thickness Evolution of MgO–C Refractory in BOFs

The evolution of the MgO–C refractory lining thickness over the course of the analyzed campaign in the trunnion ring area (S3 and S9) is graphically shown in Figure 2a. The course of the polynomial curves reflects that the thickness of the MgO–C lining in both monitored zones had the same value several times during the campaign.

Figure 2b shows the evolution of the lining thickness during the campaign in the slag line zone (S3 and S9). The graph shows that the slag line zone in the S3 area has worn much more than in the S9 area.

The cross-section of the basic oxygen furnace vessel in Figure 3 shows the internal profile evolution of the MgO–C refractory lining during the analyzed campaign. A detailed view of the monitored vertical fields of the BOF lining in the area of the trunnion ring and slag line within zones S3 and S9 are shown in Figure 4a,b.

From Figure 3 and Figure 4a,b, it is clear that the most massive skull formation relative to the brand-new MgO–C refractory lining occurred in the slag line area during the first third of the campaign. It is evident that the formation of the most massive gains (skulls) relative to the thickness of the new lining occurred in the slag line area during the first third of the campaign (up to heat 6055). In terms of time, the formation of the largest gains in the trunnion ring area occurred mainly toward the end of the MgO–C lining life (from heat 16,400) due to massive gunning of the refractory.

The greatest loss of refractory lining in the slag line area (zone S3) was observed from heat 9253 to 16,400 when massive refractory gunning was deployed. During the campaign, the slag line within zone S9 was not affected by such extensive degradation of the MgO–C lining. Degradation of the BOF lining within the trunnion ring area was more visible in zone S9 throughout the campaign. The greatest loss of refractory occurred between heats 14,739 and 16,400. Massive refractory gunning was used from this time until the end of the campaign (Figure 5).

3.2. Identification and Analysis of Factors Influencing the Wear of the MgO–C Refractory Lining of BOFs

Detailed synchronization of the multiple databases was crucial for the unambiguous determination of the parameters that influence the wear of the BOF refractory lining.

3.2.1. Trunnion Ring Area (S3 Zone)

Table 1. Ranking of factors causing wear of the BOF lining in the trunnion ring area (S3 zone).

Factor Rank	Factor	Correlation Coefficient R	Factor during Campaign	Influence on MgO–C Refractory Lining
1	Dolomitic lime consumption	−0.4517	Figure 6a	Figure 6b
2	Overall oxygen consumption	−0.4087	Figure 7a	Figure 7b
3	Lime consumption	−0.3009	Figure 8a	Figure 8b

shows the ranking of the most significant factors causing wear of the magnesium-carbon refractory lining in the trunnion ring area (S3 zone).

As can be seen from Figure 6a, the consumption of dolomitic lime was cumulative over the course of the campaign. Figure 6b shows that as the consumption of dolomitic lime increases, the thickness of the lining in the area studied decreases. This may be due to oxidation of the MgO–C liner during skull removal using the 20-hole oxygen nozzle. Excessive consumption of dolomitic lime causes skull growth on the BOF refractory lining. The consumption of dolomitic lime thus indirectly affects the wear of the MgO–C refractory due to skull formation. Oxygen acts on the MgO–C refractory lining by oxidizing the carbon component. The MgO–C material is affected by oxygen from the slag, free oxygen from the molten crude steel, and oxygen blown from the oxygen lance. During the campaign, the oxygen consumption tended to increase (Figure 7a), which was negatively reflected in the BOF thickness of the refractory lining in the investigated area and zone (Figure 7b). The effect of lime (Figure 8a,b) is similar to that of dolomitic lime in that it causes a decrease in the viscosity of the converter slag. Thus, they also reduce the ability of the slag to flow into the pores and cracks of the refractory. Meanwhile, slags concentrated with lime and dolomitic lime cause excessive skulls on the refractory material, which can only be removed by oxygen, which also oxidizes the MgO–C lining.

3.2.2. Trunnion Ring Area (S9 Zone)

Table 2 shows the ranking of the most significant factors causing wear of the MgO–C refractory lining in the trunnion ring area (S9 zone).

Table 2. Ranking of factors causing wear of the BOF lining in the trunnion ring area (S9 zone).

Factor Rank	Factor	Correlation Coefficient R	Factor during Campaign	Influence on MgO–C Refractory Lining
1	Dolomitic lime consumption	−0.3479	Figure 9a	Figure 9b
2	Overall oxygen consumption	−0.3467	Figure 10a	Figure 10b
3	Lime consumption	−0.3376	Figure 11a	Figure 11b

Table 2. Ranking of factors causing wear of the BOF lining in the trunnion ring area (S9 zone).

Factor Rank	Factor	Correlation Coefficient R	Factor during Campaign	Influence on MgO–C Refractory Lining
1	Dolomitic lime consumption	−0.3479	Figure 9a	Figure 9b
2	Overall oxygen consumption	−0.3467	Figure 10a	Figure 10b
3	Lime consumption	−0.3376	Figure 11a	Figure 11b

The consumption of dolomitic lime was cumulative over the course of the campaign, as can be seen in Figure 9a. Figure 9b shows that as the consumption of dolomitic lime increases, the thickness of the lining in the study area decreases. This may be due to BOF refractory lining oxidation during the removal of skulls by the 20-hole oxygen nozzle. In this case, the oxidation effect of the MgO–C lining is stronger than the protective function of the dolomitic lime against oxidation. In addition, excessive consumption of dolomitic lime causes a build-up of MgO–C refractories. During the campaign, oxygen consumption tended to increase (Figure 10a), which had a negative effect on the BOF lining thickness in the study zone (Figure 10b). In this area, the technological and operational factors related to excessive oxidation of the lining due to oxygen blowing had a significant impact. Thus, consumption of dolomitic lime and lime indirectly impacts the wear of MgO–C refractory due to skull removal. Again, the effect of lime is similar to that of dolomitic lime in that it reduces the viscosity of the BOF slag. This also reduces the ability of the BOF slag to flow into the pores and cracks of the MgO–C lining. Meanwhile, the dense slags cause excessive build-up (skulls) on the refractory, which can only be removed with the help of oxygen, which also oxidizes the MgO–C refractories.

3.2.3. Slag Line Area (S3 Zone)

Table 3 shows the most important factors that cause the MgO–C refractory lining in the slag line area (S3 zone) to wear.

Table 3. Most important causes of BOF lining wear in the slag line area (S3 zone).

Factor Rank	Factor	Correlation Coefficient R	Factor during Campaign	Influence on MgO–C Refractory Lining
1	Overall oxygen consumption	−0.3889	Figure 12a	Figure 12b
2	Dolomitic lime consumption	−0.3784	Figure 13a	Figure 13b
3	Slag basicity	−0.3594	Figure 14a	Figure 14b

Table 3. Most important causes of BOF lining wear in the slag line area (S3 zone).

Factor Rank	Factor	Correlation Coefficient R	Factor during Campaign	Influence on MgO–C Refractory Lining
1	Overall oxygen consumption	−0.3889	Figure 12a	Figure 12b
2	Dolomitic lime consumption	−0.3784	Figure 13a	Figure 13b
3	Slag basicity	−0.3594	Figure 14a	Figure 14b

Similar rules apply to the effect of oxygen in the slag line area as in the trunnion ring area. However, the effect of metal oxides in the slag is more pronounced. From this, it can be concluded that the greater the amount of oxygen acting on the system, the more negative the impact on the development of the MgO–C lining thickness in BOFs. This is also demonstrated by the curves in Figure 12a,b.

Dolomitic lime is particularly important in the slag line area (S3 zone) (Figure 13a). In addition to its chemical action (with the subsequent formation of skulls), the mechanical action of the embedded dolomitic lime was also evident in the periclase-carbon refractory material. The impact and abrasive action of lime damages the walls of the MgO–C refractory lining, which has a significant effect on the wear of the MgO–C lining, especially in the area of the slag line in the S3 zone (Figure 13b). The amount of dolomitic lime and lime added has a major influence on the slag basicity value; the greater the amount (flux additions) added, the higher the slag basicity (Figure 14a). However, this also results in a higher amount of skulls on the site that must be removed. This results in excessive oxidation of the lining. Higher consumption of slag-forming additives also results in a higher mechanical wear rate of the MgO–C lining, as the more slag-forming additives, the higher the abrasive effect. Thus, the trend of the curve in Figure 14b should be understood as a representation of the indirect effect of slag basicity on the residual thickness of the refractory lining.

3.2.4. Slag Line Area (S9 Zone)

Table 4 shows the most important factors that cause the periclase-carbon refractory lining in the slag line area (S9 zone) to wear.

Table 4. Most important causes of BOF refractory lining wear in the slag line area (S9 zone).

Factor Rank	Factor	Correlation Coefficient R	Factor during Campaign	Influence on MgO–C Refractory Lining
1	Lime consumption	−0.2370	Figure 15a	Figure 15b
2	Silicon in pig iron	−0.1811	Figure 16a	Figure 16b
3	Overall oxygen consumption	−0.1795	Figure 17a	Figure 17b

Table 4. Most important causes of BOF refractory lining wear in the slag line area (S9 zone).

Factor Rank	Factor	Correlation Coefficient R	Factor during Campaign	Influence on MgO–C Refractory Lining
1	Lime consumption	−0.2370	Figure 15a	Figure 15b
2	Silicon in pig iron	−0.1811	Figure 16a	Figure 16b
3	Overall oxygen consumption	−0.1795	Figure 17a	Figure 17b

The effect of lime on wear in this area of the BOF refractory lining must be seen primarily in terms of the formation of excessive skulls, which must be removed with a 20-hole oxygen lance, causing oxidation of the MgO–C lining and thus excessive lining wear (Figure 15a,b). Lime consumption thus has an indirect effect on the wear of MgO–C in BOFs.

Silicon is the acidic component of molten pig iron, which, after oxidation by oxygen, passes as an oxide into the early BOF slag, where it causes an increase in slag viscosity, resulting in increased slag clogging in the pores and cracks of the refractory lining (Figure 16a,b). The effect of SiO₂ on the slag is only temporary, as the addition of basic flux additives, such as dolomitic lime and lime, eliminates the effect of silicon and its compounds. As in the previous cases, the increased oxygen consumption causes oxidation of the refractory components (Figure 17a,b). In the case of the slag line area (S9 zone), oxygen acts on the refractory material mainly in the form of metal oxides.

3.3. Consumption of Gunning Refractories during the Studied Campaign

During the investigated campaign, 17,669 heats were performed in the BOF with a total consumption of 2326.343 t of gunning refractory mixes, equaling a consumption of 0.7738 kg/t of molten crude steel.

Table 5. Refractory gunning mix consumption for the analyzed zones of the BOF MgO–C lining.

Field of the BOF Lining	Share of Total Consumption %	Total Consumption %
Trunnion ring area (S3 zone)	1.34	63.06
Trunnion ring area (S9 zone)	16.79
Slag line area (S3 zone)	44.83
Slag line area (S9 zone)	0.10
Remaining areas 1	36.94	36.94

¹ Tap pad, charge pad, tap hole and cone area.

shows the total consumption of gunning material for the particular areas of the MgO–C refractory lining in the BOF.

The greatest consumption of gunning refractory mixtures occurs in the slag line area (Zone S3), with the second most exposed location within the trunnion ring area (Zone S9). Based on the above results, it can be concluded that the greatest wear of the MgO–C refractory lining in the BOF during the investigated campaign occurs in the trunnion ring and slag line area, accounting for 63.06% of the total consumed refractory gunning material.

3.4. Technological Causes of Refractory Lining Wear in BOFs

Based on the results (Section 3.3), it can be concluded that the opposite areas of the zoned refractory lining (S3 zone vs. S9 zone) were not equally stressed in the trunnion ring area and slag line area, as evidenced by the significantly different consumption of gunning refractories in the particular areas. Technological reasons have a significant influence on this fact.

3.4.1. Trunnion Ring Area (S9 Zone)

The significant difference in the trunnion ring area refractory wear can be explained by the corrosive effect of the oxygen blown onto the refractory lining. Due to the design of the 5-hole oxygen lance tip, the orientation of the nozzles to the MgO–C refractory; thus, the impact area of the blowing oxygen can be such that no excessive wear occurs in zone S3 (90°), while in the opposite zone, S9 (270°) the refractory is subjected to excessive oxidation by the blowing oxygen (Figure 18). This is also evidenced by the excessive consumption of gunning refractories in the respective area and zone (Table 5).

3.4.2. Slag Line Area (S3 Zone)

Since the chemical composition of the converter slag affecting the MgO–C lining in the slag line area in zones S3 and S9 is approximately the same for the duration of the steelmaking process, it is likely that technological and operational factors also contribute to such significant differences in refractory wear (Table 5). Due to the technical and architectural layout of the oxygen converter on which the analysis was performed, slag-forming fluxes were added from above the furnace through a chute from the slag-forming additives storage tank (lime and dolomitic lime). These fluxes fall into the BOF at approximately the S10 zone (about 300°) and directly impact the investigated slag line area within the S3 zone (Figure 19a). The charged and dolomitic lime strike the refractory with full force each time they are added, mechanically damaging the MgO–C refractory (Figure 19b).

3.5. Analysis of the Influence of Slag Viscosity on the Wear of the BOF MgO–C Refractory Lining

Viscosity, a vital converter slag parameter, influences clogging in pores and cracks, impacting MgO–C refractory. It is determined by chemical composition. Understanding component influence is crucial for controlling slag composition during steelmaking and the entire BOF campaign. The average proportion of CaO in the BOF slag during the campaign was 46.12%, which means that the CaO compound also has the largest representation in the BOF slag; therefore, the variance of the CaO values also has the largest effect on the dynamic viscosity of the BOF slag. The large variance of the CaO values (Figure 20a) significantly affected the slag viscosity evolution during the campaign. The average MgO content of the BOF slag during the campaign was 8.27%. The dispersion rate of the MgO content (Figure 20b) was in the average range; hence, the variation of the MgO content during the campaign did not have a major influence on the BOF slag viscosity. The evolution of the MgO content is mainly related to the control of the slag basicity value during the smelting process and, therefore, throughout the campaign. It can be seen from Figure 21a that significant fluctuation occurred in the FeO content of the BOF slag during the campaign. Chemically, this resulted in significant variation in the dynamic viscosity value of the BOF slag, while the average FeO content of the BOF slag during the campaign was 12.75%; thus, the overall effect on the dynamic viscosity was moderate. In the case of MnO (Figure 21b), much less variation in MnO content can be seen during the campaign. The average MnO content in the BOF slag was only 3.81%; thus, MnO contributed less to the evolution of the dynamic viscosity of the BOF slag than other compounds. During the campaign, the average SiO₂ content in the BOF slag was 11.37%. Therefore, together with the variance of the FeO content in the BOF slag (Figure 22a), SiO₂ content had the greatest chemical influence on the dynamic viscosity value during the campaign. However, due to its average content in the BOF slag, the overall influence on the final slag viscosity was suppressed by the CaO content. In the case of P₂O₅ as a component of the BOF slag (Figure 22b), the magnitude of the influence of P₂O₅ on the viscosity of the slag during the campaign was small; likewise, the average P₂O₅ content of the BOF slag during the campaign had a very low value of 0.94%.

3.6. Refractory Lining Wear Mechanism in Basic Oxygen Furnace

3.6.1. Thermodynamic Modeling of Compounds Causing Wear of BOF Refractory Lining

The average values of the chemical composition of the BOF slag (

Table 6. Average composition of analyzed BOF slag.

Component	CaO	Fegeneral	SiO2	MgO	MnO	P2O5	Al2O3	S
Content %	46.12	19.13	11.37	8.27	12.75	0.94	0.99	0.06

), composition of the MgO–C refractory lining in the wear area (

Table 7. Average composition of analyzed MgO–C refractory lining.

Component	MgO	SiO2	CaO	Fe2O3
Content %	98	0.4	1.2	0.5

), and composition of the applied gunning refractory material (

Table 8. Average composition of analyzed gunning material.

Component	MgO	CaO	P2O5	SiO2	Al2O3	Fe2O3
Content %	87	7.7	2.7	1.3	0.4	0.1

) are given below.

Thermodynamic modeling revealed that the predominant chemical reaction products within the BOF slag originated from the prevalent chemical elements in the converter slag, including Ca, Fe, Si, Mg, and O. The chemical products of these elements are listed in

Table 9. List of the most probable products within the input components in BOF slag.

Name	Chemical Formula	ΔG kJ.mol−1		Melting Point °C
		1300 °C	1700 °C
Merwinite	Ca3MgSi2O8	−274.424	−285.121	–
Akermanite	Ca2MgSi2O7	−146.355	−126.345	1454
Diopside	CaMgSi2O6	–139.683	−166.519	1391
Monticellite	CaMgSiO4	−101.513	−101.158	1503
Wolastonite	CaSiO3	−90.774	−91.510	1190
–	Ca2Fe2O5	−76.285	−106.286	1477
Cyclowollastonite	CaSiO3(C)	−66.134	−50.025	1540
Forsterite	Mg2SiO4	−58.338	−58.020	–
–	CaFe2O4	−43.149	−74.190	1240
–	MgSiO3	−40.568	−46.535	1577

. Using the chemical compositions provided in Table 8 for the gunning material and specific temperatures, the most probable chemical compounds were modeled (

Table 10. List of the most expected reaction products within the gunning material.

Name	Chemical Formula	ΔG kJ.mol−1		Melting Point °C
		1300 °C	1700 °C
Gehlenite	Ca2Al2SiO7	−1278.827	−1911.329	1584
–	Ca2P2O7	−520.103	−529.981	1353
–	Mg3(PO4)2	−419.220	−430.862	1353
Dicalciumferrite 1	Ca2Fe2O5	−342.090	−290.176	1477
Akermanite	Ca2MgSi2O7	−146.355	−126.345	1454
Diopside	CaMgSi2O6	−139.683	−166.519	1391
Anorthite	CaAl2Si2O8	−138.618	−147.361	1550
Monticellite	CaMgSiO4	−101.513	−101.158	1503
Cordierite	Mg2Al4Si5O18	−98.826	−155.127	1467
Dicalciumferrite 2	Ca2Fe2O5	−76.285	−106.286	1477
Fayalit	Fe2SiO4	−21.446	−31.835	1217

¹ based on the reaction: 4 CaO + 4 FeO + O_2(g) = 2 Ca₂Fe₂O₅. ² based on the reaction: 2 CaO + Fe₂O₃ = Ca₂Fe₂O_5.

).

3.6.2. Wear Mechanism of MgO–C Refractory in the Basic Oxygen Furnace

The wearing mechanism of MgO–C refractory in the BOF comprises the following steps:

• Decarburization of MgO–C refractory layer by oxygen: The decarburized refractory layer is created by saturation of free oxygen dissolved in metal, the oxide form of oxygen in molten slag, or blown oxygen into the upper refractory layer. This forms a decarburized layer saturated with slag oxides.
• Reaction of refractory and slag through the glass phase
• Partial dissolution of iron oxides in periclase: Partial dissolution of iron oxides in MgO creates a solid Fe₂MgO₄ solution. This process reduces the amount of iron oxides in BOF slag and inhibits the effects of refractory corrosion.
• Formation of magnesium silicate: BOF slag also contains a noticeable amount of SiO₂, which causes significant periclase corrosion. Based on the thermodynamic calculations, the most expected chemical products are given in

  Table 11. Thermodynamic parameters of incipient magnesium silicate.

  Name	Chemical Formula	ΔG kJ.mol−1		Melting Point °C
  		1300 °C	1700 °C
  Magnesium orthosilicate	Mg2SiO4	−60.411	−60.211	1898
  Magnesium metasilicate	MgSiO3	−40.568	−46.535	1577

  .

• Formation of dicalcium silicate coating: Dicalcium silicate is formed by the interactive reaction of slag and periclase (

  Table 12. Thermodynamic parameters of incipient dicalcium silicate.

  Name	Chemical Formula	ΔG kJ.mol−1		Melting Point °C
  		1300 °C	1700 °C
  Larnite	Ca2SiO4(L)	−141.138	−148.840	–

  ) and forms a coating above the refractory. This shield slows the corrosive action of the BOF slag into the refractory.
• Reaction of iron oxides with carbon: Carbon from the refractory material reduces iron oxides from slag. The most expected results of carbon and iron oxides are given in

  Table 13. The most expected carbon reactions from refractory and iron oxides.

  Iron Oxide	Reaction	ΔG kJ.mol−1
  		1300 °C	1700 °C
  Fe2O3	Fe2O3 + 3C = 2Fe + 3CO	−332.842	−534.580
  FeO	FeO + C = Fe + CO	−88.797	−144.791

  .
• Solid solution formation: Iron oxides react with carbon and periclase (MgO). The reaction of iron oxides with periclase produces solid solutions (melting point 1750 °C) according to the formula Fe₂O₃ + MgO = MgFe₂O₄.

• Reduction of MgO by carbon at high temperatures: The components of the refractory material—MgO (periclase) and C (carbon) interact at high temperatures. The reduction takes place through the following chemical reaction: MgO + C = Mg_(g) + CO_(g). The starting temperature of this reaction is 1846 °C.
• Formation and decomposition of spinel and magnetite: The formation of solid solutions, such as spinel (by reaction MgO + Al₂O₃ = MgAl₂O₄) and magnetite (by reaction 3Fe + 4O_(g) = Fe₃O₄), can contribute to the destruction of the refractory lining by volume change. Decomposition is the main cause of the volume change.

3.6.3. Protective Layer Formation Mechanism of Gunning Material on MgO–C Refractory Lining in BOFs

Gunning repair materials are characterized by easy sinterability, connectivity, and self-compacting ability due to the formation of dicalcium ferrite (Ca₂Fe₂O₅) (Equation (9)). The MgO–2CaO.SiO₂ system melts at 1800 °C. The melting point is rapidly reduced to 1308 °C by the presence of 2CaO.Fe₂O₃. The cumulative value of Fe₂O₃ causes an increase in melt quantity. If this state is not changed, the layer of gunning material is destroyed and washed away. A significant change is caused by the decomposition of dicalcium ferrite (Ca₂Fe₂O₅), which is unstable at low O₂ partial pressure, or by a reduction in the gaseous environment of CO.

4 CaO + 4 FeO + O_2(g) = 2 Ca₂Fe₂O₅

(9)

In the strong reduction environment, Ca₂Fe₂O₅ begins to reduce from 900 °C to form finely diffused CaO, FeO, and Fe. If there is sufficient MgO in the system and the temperature exceeds 1200 °C, FeO enters into solid solution in periclase, and CaO is excluded as a solid phase beyond solubility. The process occurs by Equation (10):

2 Ca₂Fe₂O₅ + x MgO = (x + 4) (Mg, Fe)O + 4 CaO + O₂; x > 9.3

(10)

The condition for the layer of gunning mixture to be formed and solidified is that it is covered with metal or the partial pressure of O₂ is sufficiently low, and CO is present in significant concentrations. An analysis of the mechanism by which the gunning repair mix detaches from the refractory lining indicates that the FeO content of the converter slag, as well as the FeO content of the gunning material itself, has the greatest effect on the process by which the gunning mixture detaches from the BOF refractory lining. Since the gunning repair mixes are only chemically bonded to the converter-shaped refractory lining, there is a synergy with BOF converter slag infiltration into the gunning material to detach the gunning mixture from the converter-working refractory lining. This leads to an increased consumption of gunning repair mixture, as documented in Figure 23.

4. Discussion

Figure 2a,b demonstrates distinct trends in refractory thickness evolution over time in the studied areas. Figure 3 illustrates these changes in the BOF cross-section. Significant skull formation on the MgO–C lining occurred predominantly within the initial third of the campaign, up to approximately melt 6,000. After this period, skull development decreased. A resurgence in large skulls occurred around heat 12,000, coinciding with increased gunning refractory consumption. The most substantial loss in working lining thickness was observed between heats 14,739 and 16,400. Subsequently, the residual thickness of the gunning mix only increased to its final level due to the extensive use of gunning refractory.

The wear of the MgO–C refractory lining in both trunnion ring zones (S3 and S9) is primarily influenced by slag additive consumption and overall oxygen consumption levels. Flux additives, like lime and dolomitic lime, were initially thought not to impact the refractory lining within trunnion ring wear because, based on [18,19,20], the durability of the MgO–C lining is positively affected by the addition of lime and dolomitic lime. However, statistical analysis revealed that excessive skulls on the refractory lining, caused by reduced slag viscosity due to lime and dolomitic lime, necessitated their removal with a 20-hole oxygen nozzle. This method, though effective, led to oxidation damage of the BOF lining. Oxygen had a similar statistical impact in both zones but differed operationally. In the S3 zone, wear occurred during skull removal, while in the S9 zone, oxygen from the blowing lance significantly wore down the lining.

From a statistical point of view, the total oxygen consumption has a major influence on the wear of the BOF refractory lining in the slag line area (S3 zone); this fact is consistent with the literature [21,24,26]. However, the effect of oxygen in the form of metal oxides in the slag is more pronounced than that of blowing or free oxygen in the steel. From this, it can be concluded that the greater the amount of oxygen acting on the system, the more unfavorable it is for the development of the MgO–C lining thickness. In the S3 zone of the slag line area, dolomitic lime is of particular importance. In addition to its chemical influence, it has a mechanical effect on the refractory material. Dolomitic lime causes impact damage to the lining walls and, therefore, has a particular effect on the MgO–C in this area. The amount of dolomitic lime and lime added significantly affects the basicity of the slag and decreases the BOF slag viscosity. The greater the amount of dolomitic lime and lime added, the higher the slag basicity, which protects MgO–C lining, and lower the viscosity, which causes skull formation on the lining. However, the skull on the lining must be removed, causing excessive oxidation of the BOF lining. The higher flux consumption also results in a higher mechanical wear rate of the MgO–C liner within the S3 zone, resulting in higher gunning refractory consumption. Thus, more flux additives result in a higher abrasive effect.

In the S9 zone of the slag line area, the silicon in pig iron contributes to refractory wear by increasing BOF slag viscosity, thus clogging the refractory pores and cracks. This effect is temporary and can be mitigated by adding basic flux additives, eliminating the impact of silicon and its compounds. The largest consumption of gunning refractories occurred in the S3 zone of the slag line area. A total of 2326.343 tons of gunning mixes were used during the campaign, of which 44.83% was used in this area. The second most exposed area was the S9 zone in the trunnion ring area, where 16.79% of the total gunning mix was used for repair. The other monitored areas experienced minor wear during the campaign. In total, 63.06% of the final amount of gunning mix for the entire BOF was used for the analyzed trunnion ring and slag line areas. After calculating the consumption of gunning mixes, it was found that 0.7738 kg of gunning mixes were used per ton of molten steel.

Based on the identified technological causes of BOF lining wear in the most exposed zones, the following actions can be proposed to eliminate excessive MgO–C lining loss and reduce the consumption of gunning refractory mixtures. The technological actions for the trunnion ring area (zone S9) include rotating the nozzles of the new tip 36° from the original position every time the 5-hole copper tip of the oxygen lance is replaced (Figure 24a). This will result in more uniform wear of the MgO–C lining. The idea behind the technological solutions for the slag line area (S3 zone) is to adjust the flux chute so that the added slag-forming additives do not fall on the refractory lining in the affected area and zone but rather toward the center of the BOF vessel on the already loaded scrap steel and molten pig iron (Figure 24b).

In the case of the influence of the converter slag viscosity on the wear of the MgO–C lining during the studied campaign, the graphs presented in Section 3.5 show that the change in the viscosity of the BOF slag during the campaign was influenced primarily by changes in the FeO content, followed by changes in the SiO₂ and CaO contents. This also impacts the dynamic viscosity of the BOF slag over time, resulting in differential clogging of the slag in pores and cracks and, thus, uneven wear of the refractory in terms of slag viscosity over the campaign. The results for the temperature range 1300–1700 °C show that each analyzed component of the converter slag increases the viscosity of the slag with decreasing temperature, however, this influence depends on composition of the BOF slag. When analyzing the contents of the unitary components of the converter slag, it can be concluded that the viscosity of the slag decreases with increasing FeO and MgO contents. The viscosity increased with increasing CaO, MnO, SiO₂, and P₂O₅ contents. These conclusions are consistent with the literature [35,36,37,38].

Analysis of the mechanism by which the gunning mixture acts on the MgO–C lining shows that the FeO content of the converter slag, as well as the FeO content of the gunning material, has the greatest effect on the separation process of the gunning mixture from the basic MgO–C refractory lining.

5. Conclusions

This study investigates the factors affecting MgO–C refractory lining degradation in the BOF and analyzes the consumption of gunning refractory material in the investigated areas of MgO–C refractory lining, such as trunnion ring area and slag line area. This study also describes the wear mechanism of MgO–C refractory lining in the BOF. The influence of viscosity on MgO–C refractory lining wear in the BOF is also described. Based on the results of the particular analyses, the following practical recommendations can be made to increase the lifespan of MgO–C refractory lining in BOFs and reduce monetary costs:

• Ensure the compatibility and synchronization of the databases of residual MgO–C refractory lining thickness measurements with the database containing data on the chemical composition of pig iron, slag, temperature data, slag basicity, and consumption of gunning refractory mixtures,
• Rotate the new nozzle of the 5-hole copper tip of the oxygen lance 36° from its original position each time the tip is replaced. This will result in more uniform wear of the MgO–C lining within the trunnion ring area.
• Adjust the flux chute so that flux additives do not fall on the MgO–C refractory lining in the slag line area but rather toward the center of the BOF vessel on the already charged steel scrap and molten pig iron.