Study on properties of CaF2–CaO–Al2O3–MgO–B2O3 electroslag remelting slag for rack plate steel

Yong-jiao Zhang; Ling-zhong Kong; Xi-min Zang; Shi-sen Li

doi:10.1515/htmp-2022-0316

Article Open Access

Study on properties of CaF₂–CaO–Al₂O₃–MgO–B₂O₃ electroslag remelting slag for rack plate steel

Yong-jiao Zhang , Ling-zhong Kong , Xi-min Zang and Shi-sen Li

Published/Copyright: February 22, 2024

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From the journal High Temperature Materials and Processes Volume 43 Issue 1

Abstract

The loss of boron (B) in the rack plate steel during the electroslag remelting process has a significant impact on its mechanical properties. Therefore, it is necessary to design appropriate slags to suppress the loss of B. CaF₂–CaO–Al₂O₃ phase diagram was calculated to determine the basic slag. The influence of MgO and B₂O₃ 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% CaF₂–24.81 mass% CaO–24.71 mass% Al₂O₃–2 mass% MgO-y mass% B₂O₃ (y was 1–10) slag were studied to examine the variation in B content in the steel. Results indicate the presence of a temperature-qualified eutectic point in the CaF₂–CaO–Al₂O₃ 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, B₂O₃ reduces the melting point of the slag and increases the equilibrium B content in the steel. The 50.48 mass% CaF₂–24.81 mass% CaO–24.71 mass% Al₂O₃–2 mass% MgO-y mass% B₂O₃ (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.

Keywords: rack plate steel; electroslag remelting; slag; melting temperature; viscosity; activity

1 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 Al₂O₃ 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 time-consuming. 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 B₂O₃ 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.

2 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.

Table 1

Chemical compositions of rack plate steel (mass%)

	C	Si	Mn	P	S	Ni	Cr	Mo	Ti	Cu	V	Al	N	B
Max.	0.12	0.25	1.40	≤0.01	≤0.003	2.90	0.55	0.65	0.013	0.35	0.07	0.08	≤0.004	0.0015
Min.	0.09	0.19	1.10	≤0.01	≤0.003	2.50	0.45	0.50	0.007	0.15	0.05	0.04	≤0.004	0.001
Goal	0.1	0.22	1.2	0.01	0.001	2.7	0.5	0.55	0.01	0.25	0.06	0.055	0.003	0.0012

Designing a suitable melting slag with the aid of phase diagrams is a widely accepted approach [19,20,21,22]. In this study, CaF₂, CaO, and Al₂O₃ were chosen as the fundamental constituents of the ESR slag. The CaF₂–CaO–Al₂O₃ 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% CaF₂–24.81 mass% CaO–24.71 mass% Al₂O₃ (denoted as the red dot) was selected as the basic slag.

Figure 1

Phase diagram of CaF₂–CaO–Al₂O₃.

2.2 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 B₂O₃ to the slag can suppress the oxidation of B in the alloy. The impact of MgO and B₂O₃ 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) 2 [ B ] + 3 [ O ] = (B 2 O 3 ),

(2) η = η 0 ( 1 − a f ) − n ,

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].

2.2.2 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.

(3) κ = 100 exp ( 1.911 − 1.38 x X − 5.69 x X 2 ) + 0.39 ( T − 1,973 ) ,

(4) x X = x ( Al 2 O 3 ) + 0.2 x ( CaO ) + 0.8 x ( MgO ) + 0.75 x ( SiO 2 ) + 0.5 [ x ( TiO 2 ) + x ( ZrO 2 ) ] ,

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(Al₂O₃) = 0–0.5, x(CaO) = 0–0.65, x(MgO) = 0–0.1, x(SiO₂) = 0–0.17, x(TiO₂) = 0–0.18, x(ZrO₂) = 0–0.15.

Ogino [32] obtained the electrical conductivity values of 60 mass% CaF₂–20 mass% CaO–20 mass% Al₂O₃ slag and 40 mass% CaF₂–30 mass% CaO–30 mass% Al₂O₃ slag at 1,600°C, which were 352 and 217 S·m⁻¹, respectively. According to production experience, the electrical conductivity of 60 mass% CaF₂–20 mass% CaO–20 mass% Al₂O₃ slag resulted in energy waste, while the low electrical conductivity of 40 mass% CaF₂–30 mass% CaO–30 mass% Al₂O₃ 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.

3 Slag-steel equilibration experiments

The equilibrium reactions between 50.48 mass% CaF₂–24.81 mass% CaO–24.71 mass% Al₂O₃-x mass% MgO-y mass% B₂O₃ and rack steel (containing 0.0012 mass% B) were calculated using the equilib mode of Factsage 8.2. The influence of B₂O₃ 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 slag-steel 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 MoSi₂ furnace. The pre-melt slag was then removed for air cooling and crushed.

Table 2

Composition of the designed slags (mass%)

	CaF₂	CaO	Al₂O₃	MgO	B₂O₃
1#	48.97	24.07	23.96	2	1.00
2#	48.46	23.82	23.72	2	2.00
3#	47.96	23.57	23.47	2	3.00
4#	46.95	23.07	22.98	2	5.00
5#	45.94	22.58	22.48	2	7.00

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 MoSi₂ 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.

Figure 2

Schematic diagram of the equilibration experiment device.

4 Results and discussions

4.1 Effect of MgO on the properties of CaF₂–CaO–Al₂O₃ slag

4.1.1 Effect of MgO on the liquid phase zone

The polythermal liquidus projections of CaF₂–CaO–Al₂O₃-x mass% MgO (x ≤ 7) slag at 1,200–1,600°C are shown in Figure 3. Liquid phase zone at 1,250°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 Al₂O₃ direction, whereas liquid phase zone at 1,400°C tends to expand towards CaF₂ 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.

Figure 3

Polythermal liquidus projection of CaF₂–CaO–Al₂O₃-x mass% MgO (x ≤ 6) slag at 1,200–1,600°C: (a) 1 mass% MgO; (b) 2 mass% MgO; (c) 3 mass% MgO; (d) 4 mass% MgO; (e) 5 mass% MgO; (f) 6 mass% MgO; and (g) 7 mass% MgO.

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 CaF₂–CaO–Al₂O₃ 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.

Figure 4

Liquid phase area of CaF₂–Al₂O₃–CaO slag with different w(MgO) at 1,350°C.

The phase diagrams of the CaF₂-Al₂O₃-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 CaF₂-HT (high temperature CaF₂), Ca₁₂Al₁₄F₂O₃₂, and Ca₄Al₆F₂O₁₂. When MgO content rises to 2 mass%, monoxide#2 (the main components are MgO) forms in the slag. Spinel appears first in CaF₂-Al₂O₃-CaO-3 mass% MgO slag. When the MgO reaches 4 mass%, Ca₄Al₆F₂O₁₂ 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·Al₂O₃ inclusion [33], thus affecting the cleanliness of the ingot.

Figure 5

Phase diagrams of CaF₂–CaO–Al₂O₃-x mass% MgO slag: (a) 1 mass% MgO; (b) 2 mass% MgO; (c) 3 mass% MgO; (d) 4 mass% MgO; (e) 5 mass% MgO; (f) 6 mass% MgO; and (g) 7 mass% MgO. (The area circled by red lines is liquid area at 1,350°C).

4.1.2 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.

Figure 6

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

4.1.3 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 284.3 S·m⁻¹ which was very close to the median value (284.5 S·m⁻¹). In addition, Zhang et al. [38] found that B₂O₃ can enhance the electrical conductivity of aluminosilicate slag. Therefore, the MgO content should be less than 6%.

Figure 7

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

The addition of 3 mass% MgO to CaF₂–CaO–Al₂O₃ 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.

4.2 Effect of B₂O₃ on the performance of CaF₂–CaO–Al₂O₃-4 mass% MgO slag

4.2.1 Effect of B₂O₃ on the liquid phase zone

The polythermal liquidus projections of CaF₂–CaO–Al₂O₃-4 mass% MgO-y mass% B₂O₃ slag was calculated and presented in Figure 8. The addition of B₂O₃ evidently increases the liquid phase region of the slag, with liquid phase region appearing at 1,200°C when 3 mass% B₂O₃ is added to the slag. The most pronounced changes occur at 1,350°C, where another liquid phase region is generated on the CaF₂-Al₂O₃ boundary when 1 mass%B₂O₃ is added in the slag. Similarly, when the B₂O₃ content increases to 2 mass%, both liquid phase regions expand and merge to form a larger liquid phase region. In summary, B₂O₃ can significantly expand the liquid phase region of CaF₂–CaO–Al₂O₃ slag and promote the rapid melting of the slag.

Figure 8

Polythermal liquidus projections of CaF₂–CaO–Al₂O₃-4 mass% MgO-y mass% B₂O₃ (y ≤ 5) slag at 1,200–1,600°C: (a) 1 mass% B₂O₃; (b) 2 mass% B₂O₃; (c) 3 mass% B₂O₃; (d) 4 mass% B₂O₃; and (e) 5 mass% B₂O₃.

In order to analyse the influence of B₂O₃ on the liquid phase region more intuitively, the calculation of the lowest eutectic temperature and liquid phase area at 1,350°C of CaF₂–CaO–Al₂O₃-4 mass% MgO-y mass% B₂O₃ (y ≤ 5) slag was performed, and the results are shown in Figure 9. With an increase in the B₂O₃ 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%.

Figure 9

The low eutectic temperature and liquid phase area at 1,350°C of CaF₂–CaO–Al₂O₃-4 mass% MgO-y mass% B₂O₃ slag.

The phase diagrams of CaF₂–CaO–Al₂O₃-4 mass% MgO-y mass% B₂O₃ (y ≤ 5) slag are shown in Figure 10. The main phases in slags are CaF₂-HT, monoxide (the main components are CaO and MgO), Ca₁₂Al₁₄F₂O₃₂, and spinel (the main component is MgAl₂O₄). With the increase in B₂O₃ content in the slag, the area of CaF₂-HT gradually increases, but monoxide and spinel decreases. This leads to a decrease in the melting temperature of the slag and an increase in the liquid phase area. In summary, B₂O₃ can facilitate the melting of the slag, thereby contributing to the attainment of a smooth surface of the steel ingot.

Figure 10

Phase diagrams of CaF₂–CaO–Al₂O₃-4 mass% MgO-y mass% B₂O₃ slag: (a) 1 mass% B₂O₃; (b) 2 mass% B₂O₃; (c) 3 mass% B₂O₃; (d) 4 mass% B₂O₃; and (e) 5 mass% B₂O₃ (The area circled by red lines is liquid area at 1,350°C).

4.2.2 Effect of B₂O₃ on the melting temperature

The melting temperature of the slag with different w(B₂O₃) 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 B₂O₃ 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 B₂O₃ content. This indicates that B₂O₃ could decrease the melting temperature of the slag. When the B₂O₃ 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 B₂O₃ 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.

Figure 11

Melting temperature of the basic slag with different w(MgO) and w(B₂O₃).

4.3 Effect of B₂O₃ 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) + 1.5O₂ = B₂O₃(l), B(s) = [B], and 1/2O₂ = [O]. The ∆G ^θ of these equations are standard molar Gibbs free energies.

(5) lg K B = lg a ( B 2 O 3 ) f [ B ] 2 w [ B ] 2 ⋅ f [ O ] 3 w [ O ] 3 = 39 , 004 T − 13.673 ,

where K _B represents the equilibrium constant for equation (1), a _(B2O3) denotes the activity of B₂O₃, f _[B] denotes the interaction coefficient of B, and T represents the temperature in Kelvin, K.

The effect of B₂O₃ addition in the 50.48 mass% CaF₂–24.81 mass% CaO–24.71 mass% Al₂O₃–2 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 × 10⁻¹⁰atm. In Figure 12, the blue and red lines represent the upper and lower limits of target B content, respectively. The increase in B₂O₃ 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 B₂O₃ in the slag, the greater the increase in B content in the alloy. From the thermodynamic calculation results, it can be observed that adding 4.7–6.0 mass% B₂O₃ to CaF₂–CaO–Al₂O₃–2 mass% MgO slag enables the equilibrium B content in the rack steel to fall within the target range when P O 2 was 10⁻¹⁰ atm.

Figure 12

The effect of varied w(MgO) and w(B₂O₃) in the basic slag on equilibrium content of B.

Based on the calculation results of the melting temperature and electrical conductivity of the CaF₂–CaO–Al₂O₃-x mass% MgO-y mass% B₂O₃ 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 4.7–6.0 mass% B₂O₃ 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% B₂O₃ in the slag to investigate the influence of B₂O₃ 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(B₂O₃), 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(B₂O₃) is 5 mass%, the B content in the steel just reaches the lower limit of the target range. However, when w(B₂O₃) 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% B₂O₃ 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 CaF₂ in the slag volatilizes about 4 mass% after the test. There is no significant change in the content of CaO and Al₂O₃. However, the MgO content increases, possibly because the MgO crucible is eroded by the slag at high temperatures. The content of B₂O₃ in the slag 1#–4# increases indicating that the B in the steel is burned. However, the B₂O₃ content in slag 5# decreases, which indicates that the B in the steel is not oxidized. Therefore, 7 mass% B₂O₃ addition seems to completely suppress the loss of B in the rack steel. The occurrence of SiO₂ 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.

Table 3

Composition of final slags after experiments (mass%)

	CaF₂	CaO	Al₂O₃	MgO	B₂O₃	SiO₂	FeO
1#	44.85	24.48	23.18	4.56	1.62	1.23	0.08
2#	44.03	23.92	22.78	4.91	2.93	1.31	0.12
3#	43.88	23.37	22.39	5.17	3.54	1.37	0.28
4#	42.94	22.37	22.74	5.13	5.11	1.42	0.29
5#	42.19	22.25	23.01	4.67	6.13	1.43	0.32

4.4 Effect of B₂O₃ 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 B₂O₃ at high temperature (more than 1,320°C). Geng et al. [39] and Huang et al. [40] found B₂O₃ 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.

Figure 13

The viscosity–temperature curve of slags.

5 Conclusion

The eutectic temperature of the CaF₂–CaO–Al₂O₃ slag is determined to be 1344.57°C, which is suitable for the remelting of rack steel. A slag composition containing 50.48 mass% CaF₂, 24.81 mass% CaO, and 24.71 mass% Al₂O₃ closely approximates the eutectic composition, making it more suitable for electro-slag remelting of rack steel.
With an increase in MgO content, the liquid phase area at 1,350°C of CaF₂–CaO–Al₂O₃-x mass% MgO (x ≤ 7) increases first and then decreases, and melting temperature first decreases and then increases. With the increase in B₂O₃ content, the liquid phase area at 1,350°C of CaF₂–CaO–Al₂O₃-4 mass% MgO-y mass% B₂O₃ (y ≤ 5) slag increases, and the melting temperature decreases. The CaF₂–CaO–Al₂O₃–2 mass% MgO slag exhibits suitable characteristics such as melting point, electrical conductivity, and viscosity for the remelting of rack steel.
The results of the slag–steel equilibrium reaction show that 50.48 mass% CaF₂–24.81 mass% CaO–24.71 mass% Al₂O₃–2 mass% MgO-y mass% B₂O₃ (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.

Acknowledgements

This paper is supported by National Natural Science Foundation of China (NSFC) under grant Nos 52174317 and 52274337.

Funding information: This project is supported by the National Natural Science Foundation of China (Grant Nos 52174317 and 52274337).
Author contributions: Yong-jiao Zhang: Conceptualization, Software, Investigation, Formal Analysis, Writing – Original Draft; Ling-zhong Kong: Conceptualization, Funding Acquisition, Resources, Supervision, Writing – Review & Editing. Xi-min Zang: Conceptualization, Funding Acquisition, Resources, Supervision, Writing – Review & Editing. Shi-sen Li: Conceptualization; Investigation; Supervision; Writing – Review & Editing.
Conflict of interest: Authors state no conflict of interest.
Data availability statement: The authors confirm that the data supporting the findings of this study are available within the article [and/or its supplementary materials].

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Received: 2023-09-21

Revised: 2024-01-16

Accepted: 2024-01-20

Published Online: 2024-02-22

This work is licensed under the Creative Commons Attribution 4.0 International License.

Articles in the same Issue

https://doi.org/10.1515/htmp-2022-0316

Keywords for this article

rack plate steel; electroslag remelting; slag; melting temperature; viscosity; activity

Creative Commons

BY 4.0

Study on properties of CaF2–CaO–Al2O3–MgO–B2O3 electroslag remelting slag for rack plate steel

Article

Abstract

1 Introduction

2 Calculation method

2.1 Determination of basic slag

2.2 Physical properties of the slag

2.2.1 Melting temperature and viscosity

2.2.2 Electrical conductivity

3 Slag-steel equilibration experiments

4 Results and discussions

4.1 Effect of MgO on the properties of CaF2–CaO–Al2O3 slag

4.1.1 Effect of MgO on the liquid phase zone

4.1.2 Effect of MgO on the melting temperature

4.1.3 Effect of MgO on the electrical conductivity and viscosity

4.2 Effect of B2O3 on the performance of CaF2–CaO–Al2O3-4 mass% MgO slag

4.2.1 Effect of B2O3 on the liquid phase zone

4.2.2 Effect of B2O3 on the melting temperature

4.3 Effect of B2O3 on the equilibrium B content in the steel

4.4 Effect of B2O3 on the viscosity

5 Conclusion

Acknowledgements

References

Articles in the same Issue

Articles in the same Issue

Articles in the same Issue

Study on properties of CaF₂–CaO–Al₂O₃–MgO–B₂O₃ electroslag remelting slag for rack plate steel

4.1 Effect of MgO on the properties of CaF₂–CaO–Al₂O₃ slag

4.2 Effect of B₂O₃ on the performance of CaF₂–CaO–Al₂O₃-4 mass% MgO slag

4.2.1 Effect of B₂O₃ on the liquid phase zone

4.2.2 Effect of B₂O₃ on the melting temperature

4.3 Effect of B₂O₃ on the equilibrium B content in the steel

4.4 Effect of B₂O₃ on the viscosity