Study on properties of CaF 2 – CaO – Al 2 O 3 – MgO – B 2 O 3 electroslag remelting slag for rack plate steel

: The loss of boron (B) in the rack plate steel during the electroslag remelting process has a signi ﬁ cant impact on its mechanical properties. Therefore, it is necessary to design appropriate slags to suppress the loss of B. CaF 2 – CaO – Al 2 O 3 phase diagram was calculated to determine the basic slag. The in ﬂ uence of MgO and B 2 O 3 on the performance of the basic slag was studied to determine the optimal amount of MgO addition. The equilibrium reactions between rack plate steel and the 50.48 mass% CaF 2 – 24.81 mass% CaO – 24.71 mass% Al 2 O 3 – 2 mass% MgO-y mass% B 2 O 3 ( y was 1 – 10) slag were studied to examine the variation in B content in the steel. Results indicate the presence of a tem-perature-quali ﬁ ed eutectic point in the CaF 2 – CaO – Al 2 O 3 phase diagram. With an increase in MgO content in the slag, the melting temperature of the basic slag initially decreases and then increases, while the electrical conductivity decreases. On the other hand, B 2 O 3 reduces the melting point of the slag and increases the equilibrium B content in the steel. The 50.48 mass% CaF 2 – 24.81 mass% CaO – 24.71 mass% Al 2 O 3 – 2 mass% MgO-y mass% B 2 O 3 ( y was 5 – 7) slag can achieve the target range of B content in the steel, and its viscosity meets the requirements of electric slag remelting.


Introduction
The growing energy crisis has promoted nations worldwide to focus on the development and utilization of marine resources.Among these resources, deep-sea oil and gas exploration has garnered significant attention as a pivotal sector for energy security.Self-elevating drilling platforms, serving as iconic facilities for deep-sea oil and gas operations, require thick-grade rack plate steel plates in their spud legs to withstand complex loads and multiple corrosive conditions.These steel plates necessitate high strength, high toughness, fatigue resistance, corrosion resistance, low-temperature toughness, resistance to lamellar tearing, and excellent weldability [1][2][3][4][5][6][7].Therefore, the development of rack plate steel appears to be particularly significant.
As the terminal smelting process of high-end special alloy steel, electroslag remelting (ESR) is favoured by metallurgical workers due to its advantages of dense microstructure, uniform composition and structure, and smooth surface of the ingots [8].The rack plate steel is therefore usually produced with ESR.However, in the process of remelts B-containing rack plate steel, the loss of boron (B) in the alloy exhibits a serious adverse effect on the properties of the alloy [9].As we all know, the composition of slag has a considerable influence on the oxidation loss of alloy elements in steel.Shi [10] believed that the oxidation during ESR is affected by many factors such as alloy composition, remelting atmosphere, and slag composition.Chang et al. [11] found that reducing the content of Al 2 O 3 in slag can effectively reduce oxygen content.Duan et al. [12] found that adding CaO into the slag can inhibit the loss of Al in the alloy.The loss of alloy element in steel can also be inhibited by addition of oxide of the element into slag [13,14].However, so far, there has been little literature on the suppression of B element oxidation in B-containing rack plate steel during ESR.It is therefore necessary to design an appropriate slag for smelting B-containing rack plate steel.
In the past, metallurgists used a trial and error approach to design slag systems [15,16], which involved adjusting the composition of the slag based on traditional slag or existing slag to test the feasibility of the modified slag.However, the trial and error method often resulted in a wide range of potential slag systems, leading to instability in the actual production process.Moreover, this method is costly and timeconsuming.In contrast, the use of Factsage thermodynamic software can effectively reduce the research cost of metallurgical problems and narrow down the range of designed slags.In this study, Factsage software was employed to investigate the effects of different MgO and B 2 O 3 contents in the slag on the performance of the basic slag, as well as the influence of B content in the alloy.The suitable slag systems for remelting boron-containing rack plate steel were designed, which provides a reference for the design of slag used in electric slag remelting processes.

Calculation method 2.1 Determination of basic slag
The melting temperature of the slag is crucial for the entire process of electric slag remelting.Therefore, the melting temperature is the primary consideration when designing slag.Generally, the melting temperature of the slag used in ESR should be lower than that of the alloy steel by 100-200°C [17].Table 1 shows the composition of the boron-containing rack plate steel, and melting temperature of rack plate steel was calculated using an empirical formula summarized by Chen [18], resulting in a calculated melting point of 1,509°C.Therefore, the melting point range of suitable slag for remelting rack plate steel is within the range of 1,309-1,409°C.
Designing a suitable melting slag with the aid of phase diagrams is a widely accepted approach [19][20][21][22].In this study, CaF 2 , CaO, and Al 2 O 3 were chosen as the fundamental constituents of the ESR slag.The CaF 2 -CaO-Al 2 O 3 ternary phase diagram was calculated using FactSage 8.2, and is shown in Figure 1.The black dot is a eutectic point with a eutectic temperature of 1344.57°C.When selecting the slag composition, it is desirable to choose components close to the eutectic point to suppress the liquid chromatography during solidification.On the other hand, the slag composition should be away from the eutectic point to promote selective crystallization [9].Additionally, the slag composition should be designed within a relatively stable temperature range to prevent significant variation in the slag melting temperature caused by changes in composition.Based on the above considerations, a composition of 50.48 mass% CaF 2 -24.81 mass% CaO-24.71mass% Al 2 O 3 (denoted as the red dot) was selected as the basic slag.

Physical properties of the slag 2.2.1 Melting temperature and viscosity
The addition of MgO to the slag can reduce gas permeability of slag [9].Meanwhile, according to equation (1), the addition of B 2 O 3 to the slag can suppress the oxidation of B in the alloy.The impact of MgO and B 2 O 3 on the performance of slag was investigated to determine the optimal amounts.The liquid phase region of the slag system was explored using the Phase Diagram mode in Factsage 8.2.The melting temperature of the slag system was calculated using the Equilib mode.Considering that solid particles will be precipitated in the slag at low temperatures, and these solid particles have a great influence on the viscosity of the slags, therefore, the viscosity of the slag system was calculated by the Viscosity mode of Factsage 8.2 and was corrected by equation (2).Equation (2) has been applied in literature [23,24]. (1) where η and η 0 are the viscosity of the solid-containing and the solid-free melt, respectively; f is the volume fraction of solid particles in the melt, which can be replaced with the mass fraction; and for spherical particles of a uniform size, a and n are 1.35 and 2.5, respectively [25].

Electrical conductivity
Ogino et al. [26] summarized an electrical conductivity calculation formula for multicomponent slag systems with an error within 10%, but the influence of MgO was not considered.Dong et al. [27] made a modification to the formula and obtained equations ( 3) and ( 4).The modified equation has been applied in in literature [28][29][30][31].By comparing the conductivity measurement value of the ternary slags, it is found that the average error of equation ( 3) was 6.71%.Therefore, it is reliable to use equations ( 3) and ( 4) to calculate the conductivity of the slag.
where κ is the electrical conductivity of the slag, S•m −1 ; T is the temperature, K, and the applicable range is 1,823-2,053 K; x(i) is the mole fraction of i, and the applicable range is x Ogino [32] obtained the electrical conductivity values of 60 mass% CaF 2 -20 mass% CaO-20 mass% Al 2 O 3 slag and 40 mass% CaF 2 -30 mass% CaO-30 mass% Al 2 O 3 slag at 1,600°C, which were 352 and 217 S•m −1 , respectively.According to production experience, the electrical conductivity of 60 mass% CaF 2 -20 mass% CaO-20 mass% Al 2 O 3 slag resulted in energy waste, while the low electrical conductivity of 40 mass% CaF 2 -30 mass% CaO-30 mass% Al 2 O 3 slag resulted in insufficient reaction.Therefore, the appropriate conductivity should be within the range of 217-352 S•m −1 , and the median value (284.5 S•m −1 ) may be the best choice.

Slag-steel equilibration experiments
The equilibrium reactions between 50.48 mass% CaF 2 -24.81 mass% CaO-24.71mass% Al 2 O 3 -x mass% MgO-y mass% B 2 O 3 and rack steel (containing 0.0012 mass% B) were calculated using the equilib mode of Factsage 8.2.The influence of B 2 O 3 and MgO on the B content in the rack steel was investigated.The reaction temperature was set at 1,600°C, and the oxygen partial pressure was set to 1 × 10 −10 atm.Additionally, a slagsteel mass ratio of 6:1 was used to simulate the actual refining process.Based on the physical properties of the slag and the calculated results of the slag-steel equilibrium reactions, suitable slags were selected.Subsequently, slag-  steel equilibrium experiments were conducted to test the smelting performance of the designed slags.Before the slag was pre-melted, the analytical-grade reagents were calcined in a muffle furnace at 1,273 K for 8 h to remove hydroxide and moisture.The slag made by Table 2 was put into a graphite crucible lined with a 0.2 mm thick molybdenum film and held at 1,600°C for 1 h in a MoSi 2 furnace.The pre-melt slag was then removed for air cooling and crushed.150 g rack steel (32 mm diameter and 27 mm height) and 20 g pre-melt slag were placed into a MgO crucible (35 mm inner diameter and 50 mm height) with a 0.2 mm thick molybdenum film.The molybdenum film was fixed above the steel by a corundum rod, so it would not fall into the steel liquid.A graphite crucible (60 mm inner diameter and 130 mm height) containing the MgO crucible was then placed in MoSi 2 furnace at 1,600°C, as shown in Figure 2.After 1 h of reaction, it was removed from the furnace.Argon gas was blown in the furnace at a flow rate of 2,000 mL•min −1 during the experiments.The Optical Emission Spectrometer was used to determine the B content in the steel sample after the reaction.Based on the calculation results presented in Section 2.1, the melting point of the slag should be in the range from 1,309 to 1,409°C, with a possible optimal value around 1,350°C.The liquid phase zone of CaF 2 -CaO-Al 2 O 3 slag with different w(MgO) at 1,350°C were calculated, and the results are shown in Figure 4.With the increase in MgO, the liquid phase area initially expands and then starts to decrease at 3 mass% MgO.When MgO reaches 7 mass%, the liquid phase area accounts for only 1.3%.A small liquid phase area indicates that ESR process is difficult to conduct.
The phase diagrams of the CaF 2 -Al 2 O 3 -CaO-x mass% MgO slag (x ≤ 7) with liquid phase zone at 1,350°C are shown in Figure 5.The main primary phases in the slags are CaF 2 -HT (high temperature CaF 2 ), Ca 12 Al 14 F 2 O 32 , and Ca 4 Al 6 F 2 O 12 .When MgO content rises to 2 mass%, monoxide#2 (the main components are MgO) forms in the slag.Spinel appears first in CaF 2 -Al 2 O 3 -CaO-3 mass% MgO slag.When the MgO reaches 4 mass%, Ca 4 Al 6 F 2 O 12 disappears, but spinel and monoxide#2 continued to increase.At the same time, the liquid phase area at 1,350°C decreases.In summary, the addition of MgO may elevate the content of MgO•Al 2 O 3 inclusion [33], thus affecting the cleanliness of the ingot.

Effect of MgO on the melting temperature
The effect of MgO on the melting temperature of the basic slag is shown in Figure 6.The melting point first decreases from 1,351 to 1281.5°C when MgO content increases from 0 mass% to 5 mass%.However, the melting point of the slag containing 7 mass% MgO is 1411.4°C.The significant change in melting temperature emphasizes the importance of strict control over the MgO content.

Effect of MgO on the electrical conductivity and viscosity
The electrical conductivity (at 1,600°C) and viscosity (at 1,800°C) of the basic slag are illustrated in Figure 7.
When MgO content changes from 0 mass% to 7 mass%, the viscosity of slag increases from 0.0113 to 0.0125 Pa•s.In contrast, the conductivity of the slag decreases with the increase in the MgO content, which is consistent with the findings of Presoly et al. [34].The conductivity and viscosity of the slag exhibit opposite trends, as described in previous literature [35,36].Yan et al. [37] attributed this phenomenon to an increase in melt polymerization degree and the cation consumption.The increase in viscosity of the slag hampers the movement of charged ions within the slag, resulting in a decrease in electrical conductivity of the slag.The electrical conductivity of the slag containing 1 mass% MgO is 284.3S•m −1 which was very close to the median value (284.5 S•m −1 ).In addition, Zhang et al. [38] found that B 2 O 3 can enhance the electrical conductivity of aluminosilicate slag.Therefore, the MgO content should be less than 6%.The addition of 3 mass% MgO to CaF 2 -CaO-Al 2 O 3 slag results in a maximum liquid phase region at 1,350°C.The addition of 5 mass% MgO makes the lowest melting temperature of the slag, which is favourable for rapid melting of the ESR slag.These findings suggest that the melting performance of the slag containing 4 mass% MgO is also satisfactory.Furthermore, the electrical conductivity and viscosity of the slag with 4 mass% MgO content are also appropriate.This implies that the slag containing 4 mass% MgO seems to be optimal.decrease in the melting temperature of the slag and an increase in the liquid phase area.In summary, B 2 O 3 can facilitate the melting of the slag, thereby contributing to the attainment of a smooth surface of the steel ingot.

Effect of B 2 O 3 on the melting temperature
The melting temperature of the slag with different w(B 2 O 3 ) are shown in Figure 11.When MgO is less than 5 mass%, the melting temperature of the slag slightly decreases within the range of 1,330-1,270°C with the increase in the B 2 O 3 content.When MgO is more than 5 mass%, the melting temperature of the slag initially decreased sharply and then gradually decreased with the increase in the B 2 O 3 content.This indicates that B 2 O 3 could decrease the melting temperature of the slag.When the B 2 O 3 content is constant, the melting temperature of the slag decreases initially and then increases with the increase in MgO, which is consistent with the results described in Section 3.1.2.The effect of B 2 O 3 in reducing the melting temperature is stronger than that of MgO.The red dotted line represents the upper limit of the melting temperature required (1,409°C), and the blue dotted line represents the lower limit of the melting temperature required (1,309°C).The slag with 4 mass% MgO has a melting point lower than 1,309°C, whereas the slag containing 1 mass% and 2 mass% MgO has a more suitable melting point.Considering that MgO has the effect of inhibiting the transfer of oxygen to the slag, it was decided to add 2 mass% MgO instead of 1 mass% MgO to the studied slag.

Effect of B 2 O 3 on the equilibrium B content in the steel
During the ESR process of B-containing rack plate steel, the molten metal film located at the tip of the electrode reacts with the slag as equation ( 1), and the equilibrium constant of the reaction is expressed as equation (5).Equation ( 5) is calculated by reactions of 2B(s) The ΔG θ of these equations are standard molar Gibbs free energies. ( where K B represents the equilibrium constant for equation (1), a (B2O3) denotes the activity of B 2 O 3 , f [B] denotes the interaction coefficient of B, and T represents the temperature in Kelvin, K.The effect of B 2 O 3 addition in the 50.48 mass% CaF 2 -24.81 mass% CaO-24.71mass% Al 2 O 3 -2 mass% MgO slag on the equilibrium B content in the rack steel (containing 0.0012 mass% B) at 1,600°C was calculated.And the oxygen partial pressure was set to 1 × 10 −10 atm.In Figure 12, the blue and red lines represent the upper and lower limits of target B content, respectively.The increase in B 2 O 3 significantly increases the B content in the steel ingot which is consistent with the conclusions reported in literature [13,14].The greater the amount of B 2 O 3 in the slag, the greater the increase in B content in the alloy.From the thermodynamic calculation results, it can be observed that adding 4.7-6.0mass% B 2 O 3 to CaF 2 -CaO-Al 2 O 3 -2 mass % MgO slag enables the equilibrium B content in the rack steel to fall within the target range when P O2 was 10 −10 atm.
Based on the calculation results of the melting temperature and electrical conductivity of the CaF 2 -CaO-Al 2 O 3 -x mass% MgO-y mass% B 2 O 3 slag, it can be concluded that the addition of 2 mass% MgO to the slag is appropriate.The calculation results of the equilibrium reaction show that adding 4.7-6.0mass% B 2 O 3 to the slag can achieve the desired range of B content in the alloy.Due to the instability of the operational and reaction processes, the oxygen partial pressure may increase or decrease.Therefore, in this study, it is proposed to add 1 mass%, 2 mass%, 3 mass%, 5 mass%, and 7 mass% B 2 O 3 in the slag to investigate the influence of B 2 O 3 on the equilibrium B content in rack steel.The composition of the designed slag is shown in Table 2.
The experimental results of the B content in the steel after the equilibrium reaction between the designed slag and the rack steel are shown in Figure 12.The B content in the alloy increases with an increase in the w(B 2 O 3 ), which is consistent with the thermodynamic calculation results.It is worth noting that the experimental values of B content in the alloy are consistently lower than the calculated values.This may be attributed to the entry of atmospheric oxygen into the furnace tube during crucible placement, thereby disrupting the argon atmosphere in the experimental furnace.Therefore, these experimental results can be understood.Furthermore, incomplete progress of the slag-steel reaction can also lead to deviations in the experimental results.Although the experimental values are slightly lower than the calculated values, the overall trend of both are very similar, indicating that the experimental results are still reliable.When w(B 2 O 3 ) is 5 mass%, the B content in the steel just reaches the lower limit of the target range.However, when w(B 2 O 3 ) is 7 mass %, the B content in the steel exceeds the upper limit of the target range.This calculation result closely matches the experimental result.It is initially believed that adding 5 mass% to 7 mass% B 2 O 3 in the slag can meet the B content requirement in the rack steel.This provides important theoretical references for practical smelting, but the feasibility of the designed slag still requires further validation through ESR experiments.The composition of final slags after equilibrium experiments are as seen in Table 3.After normalization, it can be clearly seen that the CaF 2 in the slag volatilizes about 4 mass % after the test.There is no significant change in the content of CaO and Al 2 O 3 .However, the MgO content increases, possibly because the MgO crucible is eroded by the slag at high temperatures.The content of B 2 O 3 in the slag 1#-4# increases indicating that the B in the steel is burned.However, the B 2 O 3 content in slag 5# decreases, which indicates that the B in the steel is not oxidized.Therefore, 7 mass% B 2 O 3 addition seems to completely suppress the loss of B in the rack steel.The occurrence of SiO 2 in the slag is related to Si in rack steel.During the smelting process, a small amount of oxygen may enter the steel liquid, so FeO is detected.

Effect of B 2 O 3 on the viscosity
The viscosity of the designed slags is shown in Figure 13.The viscosity of the slags decreased with the increase in temperature, while increased with the increase in B 2 O 3 at high temperature (more than 1,320°C).Geng et al. [39] and Huang et al. [40] found B 2 O 3 as a network generating oxide, enhancing the degree of polymerization of the slag and strengthening and complicating its network structure at high temperatures.The viscosity of slag at 1,440°C varied in the range from 0.042 to 0.048 Pa•s and the viscosity of slag changed slightly with temperature, which would improve the stability of the ESR and obtain the ingots with great surface quality.

4. 2
Effect of B 2 O 3 on the performance of CaF 2 -CaO-Al 2 O 3 -4 mass% MgO slag 4.2.1 Effect of B 2 O 3 on the liquid phase zone The polythermal liquidus projections of CaF 2 -CaO-Al 2 O 3 -4 mass% MgO-y mass% B 2 O 3 slag was calculated and presented in Figure 8.The addition of B 2 O 3 evidently increases the liquid phase region of the slag, with liquid phase region appearing at 1,200°C when 3 mass% B 2 O 3 is added to the slag.The most pronounced changes occur at 1,350°C, where another liquid phase region is generated on the CaF 2 -Al 2 O 3 boundary when 1 mass%B 2 O 3 is added in the slag.Similarly, when the B 2 O 3 content increases to 2 mass%, both liquid phase regions expand and merge to form a larger liquid phase region.In summary, B 2 O 3 can significantly expand the liquid phase region of CaF 2 -CaO-Al 2 O 3 slag and promote the rapid melting of the slag.In order to analyse the influence of B 2 O 3 on the liquid phase region more intuitively, the calculation of the lowest eutectic temperature and liquid phase area at 1,350°C of CaF 2 -CaO-Al 2 O 3 -4 mass% MgO-y mass% B 2 O 3 (y ≤ 5) slag was performed, and the results are shown in Figure9.With an increase in the B 2 O 3 content in the slag, the eutectic temperature decreases from 1247.34 to 1172.48°C.Correspondingly, the proportion of the liquid phase area at 1,350°C of the slag increases from 11 to 37%.The phase diagrams of CaF 2 -CaO-Al 2 O 3 -4 mass% MgOy mass% B 2 O 3 (y ≤ 5) slag are shown in Figure10.The main phases in slags are CaF 2 -HT, monoxide (the main components are CaO and MgO), Ca 12 Al 14 F 2 O 32 , and spinel (the main component is MgAl 2 O 4 ).With the increase in B 2 O 3 content in the slag, the area of CaF 2 -HT gradually increases, but monoxide and spinel decreases.This leads to a

Figure 6 :
Figure 6: Melting temperature of the basic slag with different w(MgO).

Figure 7 :
Figure 7: Electrical conductivity and viscosity of the basic slag with different w(MgO).

Figure 11 :
Figure 11: Melting temperature of the basic slag with different w(MgO) and w(B 2 O 3 ).

Figure 12 :
Figure 12: The effect of varied w(MgO) and w(B 2 O 3 ) in the basic slag on equilibrium content of B.

Table 1 :
Chemical compositions of rack plate steel (mass%)

Table 2 :
Composition of the designed slags (mass%) 250°C expands first and then contracts, and other liquid phase zones with different temperatures exhibit a similar trend as MgO content increases in the slag.It is worth noting that liquid phase zone at 1,350°C expands towards the Al 2 O 3 direction, whereas liquid phase zone at 1,400°C tends to expand towards CaF 2 when MgO content is less than 2 mass%.The liquid phase zone at 1,200, 1,250, and 1,300°C disappear upon the addition of 2 mass%, 4 mass%, and 7 mass% MgO, respectively.Correspondingly, the liquid phase regions at other temperatures progressively diminish as MgO content increases, which means that variations in MgO content lead to significant changes in the melting temperatures of slag.
4.1.1Effect of MgO on the liquid phase zoneThe polythermal liquidus projections of CaF 2 -CaO-Al 2 O 3 -x mass% MgO (x ≤ 7) slag at 1,200-1,600°C are shown in Figure3.Liquid phase zone at 1, 1.The eutectic temperature of the CaF 2 -CaO-Al 2 O 3 slag is determined to be 1344.57°C,which is suitable for the remelting of rack steel.A slag composition containing 50.48 mass% CaF 2 , 24.81 mass% CaO, and 24.71 mass% Al 2 O 3 closely approximates the eutectic composition, making it more suitable for electro-slag remelting of rack steel.2. With an increase in MgO content, the liquid phase area at 1,350°C of CaF 2 -CaO-Al 2 O 3 -x mass% MgO (x ≤ 7) increases first and then decreases, and melting temperature first decreases and then increases.With the increase in B 2 O 3 content, the liquid phase area at 1,350°C of CaF 2 -CaO-Al 2 O 3 -4 mass% MgO-y mass% B 2 O 3 (y ≤ 5) slag increases, and the melting temperature decreases.The CaF 2 -CaO-Al 2 O 3 -2 mass% MgO slag exhibits suitable characteristics such as melting point, electrical conductivity, and viscosity for the remelting of rack steel.3. The results of the slag-steel equilibrium reaction show that 50.48 mass% CaF 2 -24.81 mass% CaO-24.71mass% Al 2 O 3 -2 mass% MgO-y mass% B 2 O 3 (y was 5-7) slag can achieve the desired level of B content in rack steel.Furthermore, the viscosity of the designed slags meets the requirements for ESR.

Table 3 :
Composition of final slags after experiments (mass%) The viscosity-temperature curve of slags.Study on properties of electroslag remelting slag  11