Corrosion mechanism and protection of BOF refractory for high silicon hot metal steelmaking process

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Original Article

Corrosion mechanism and protection of BOF refractory for high silicon hot metal steelmaking process Yuxiang Dai, Jing Li ∗ , Wei Yan, Chengbin Shi State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing (USTB), Beijing 100083, PR China

a r t i c l e

i n f o

a b s t r a c t

Article history:

Corex process can produce hot metal without using coke and has some advantages in reduc-

Received 14 December 2019

ing pollution. However, the silicon content of hot metal produced by Corex furnace is 0.6–1.5

Accepted 14 February 2020

mass%, which results in the serious corrosion of lining. In order to study the mechanism

Available online xxx

of lining corrosion, the sample of corrosion lining was taken, and the morphology of the corrosion region was analyzed by SEM. The optimum composition of slag for splashing was

Keywords:

determined through a combination of plant experiments and theoretical calculation. The

Lining corrosion

results show that the chemical reaction and the dissolution of (MgO) lead to the lining cor-

Refractory dissolution

rosion of BOF. Concentration gradient of Mg and Fe were found between slag and refractory

Slag attack

brick, which promote the mass exchange and reaction between slag and refractory brick.

High Si hot metal

The low basicity of slag leads to low melting point and less formation of solid phase at the same temperature, which result in the scouring down of slag at early stage of steelmaking in BOF. In addition, the low basicity lead to high saturated solubility of (MgO), which is beneficial to the dissolution of (MgO) of refractory lining. The constant reaction and dissolution of (MgO) between lining and slag lead to the lining corrosion. The basicity of slag should be 1.0–1.2. The content of (FeO) should be controlled less than 5 mass%, and the content of (MgO) should be 5–7 mass%. © 2020 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

1.

Introduction

Refractories are widely used in steelmaking industry because of its high corrosion resistance, low thermal expansion and good thermal conductivity [1]. During steelmaking process, the refractory is used as the lining material which contacts with molten steel and slag. The lining corrosion is one of

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the major issues for low cost steelmaking production [2]. In modern steelmaking process, higher temperature and techniques such as bottom blowing greatly increase the flow and fluids within the lining. It also accelerates the degradation rate of refractory lining [3]. More frequent replacement of the refractory increases the cost of production. Recently, with the increase of demands for advanced refractory, more attention is focused on the corrosion of refractory lining. The interaction between steel and refractory is mainly caused by chemical reaction and molten metal penetration [4,5,6]. The chemical reaction and penetration effect the microstructure evolution of the contact area of refectory lin-

Corresponding author. E-mail: [email protected] (J. Li). https://doi.org/10.1016/j.jmrt.2020.02.055 2238-7854/© 2020 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http:// creativecommons.org/licenses/by-nc-nd/4.0/). Please cite this article in press as: Dai Y, et al. Corrosion mechanism and protection of BOF refractory for high silicon hot metal steelmaking process. J Mater Res Technol. 2020. https://doi.org/10.1016/j.jmrt.2020.02.055

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Fig. 1 – Schematic diagram of two-converter steelmaking process.

ing and molten steel. Previous studies [7–11,12,13] indicated that a series of physical and chemical reactions, as well as mechanical impact lead to the destruction of furnace lining. Mechanical abrasion mainly occurs in the process of scrap charging. Meanwhile, the oscillation of bath and high-speed oxygen jets induce the corrosion of refractory lining. Previous studies [14–17,18] have been done to analyse the relationship between MgO dissolution of refractory and different conditions. However, the interaction phenomenon of molten steel, slag and refractory is still clear, especially when the chemical composition of hot metal or slag is abnormal. In this paper, the corrosion mechanism of refractory lining during the steelmaking process of high silicon hot metal is studied. These high silicon hot metal is produced by Corex furnace. Corex process can replace blast furnace to produce hot metal without using coke which has some advantages in reducing pollution [19,20]. However, the average silicon content of hot metal is 0.6–1.5 mass%. The silicon content is even 2.0–5.0 mass% within the first week of reopening of Corex furnace. The high silicon hot metal brings many difficulties to steelmaking process. For current converter steelmaking method (such as single slag method, double slag method and duplex process), the Si content of hot metal is below 1.0 mass%. When Si content of hot metal is higher than 1 mass%, more lime should be added to modify the basicity of slag. In order to maintain the basicity about 3, 100 kg/t lime should be added. The formation slag is huge (about 150 kg/t) which tends to lead slag splashing. Double slag method can adopt for high silicon hot metal, but have to pour slag many times to avoid excessive slag spills. It is worth noting that the viscosity of slag increases with the increase of SiO2 content [21–23]. And the separation of slag and iron is influenced by the viscosity of slag. When silicon content of hot metal is higher than 1 mass% the content of silica in the slag increase rapidly during converter refining process which raise the viscosity of slag. It is difficult to separate slag and iron because of the high viscosity of slag. Many times of slag pouring-out reduces the production efficiency and lead to more iron into the slag, resulting in a huge waste of iron materials. Therefore, two-converter steelmaking process was employed to treat such hot metal. The new method is more efficient and reliable in high silicon hot metal smelting but it also brings some problems. One of the problems is the corrosion lining by high silicon hot metal. The steelmaking process of high silicon hot metal aggravates the lining corrosion. Therefore, in this presented paper, the influence of low basicity (basicity is 1.0–1.5) slag on lining thickness was studied. The microstructural investigations of refractory

Fig. 2 – Photo of refractory brick.

lining were carried out to analyze the mechanism of lining corrosion. The physical properties of slag with different composition were analyzed by FactSage 7.2 software, and the optimum slag composition for splashing was determined through a combination of plant experiments and theoretical calculation.

2.

Experiments

Within first week of reopening of Corex furnace, the silicon content of hot metal is about 2.0–5.0 mass%. Two-converter steelmaking process was employed to treat such hot metal (doi is https://doi.org/10.1080/03019233.2019.1601467). The schematic diagram is shown in Fig. 1. Two converters are used in the operation process. High silicon hot metal first enters BOF 1 for desiliconization. The chemical composition of hot metal and semi steel are shown in Table 1. After refining in BOF 1, the silicon content of the semi-steel is 0.5–1.0 mass%. In BOF 1, the oxidation of a large amount of silicon increase the silica content of slag, which is about 45 mass%. The chemical composition of slag in BOF 1 is shown in Table 2. The low basicity leads to high viscosity of the slag, and it is difficult to separate the slag from molten iron when inclining the converter. Therefore, the semi-steel is tapped through tap hole and then transported to BOF 2 by ladle. The process of decarburization and dephosphorization in BOF 2 is similar to single-slag operation except for composition difference between semi-steel and normal hot metal.

Please cite this article in press as: Dai Y, et al. Corrosion mechanism and protection of BOF refractory for high silicon hot metal steelmaking process. J Mater Res Technol. 2020. https://doi.org/10.1016/j.jmrt.2020.02.055

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Fig. 3 – Lining thickness (before being BOF 1 for desilicication, 1908th heats).

Table 1 – Condition of hot metal and semi steel. T/◦ C Hot metal Semi steel

1350–1400 1400–1600

C/mass %

Si/mass %

Mn/mass %

4.0 2.0–3.0

3.0–4.0 0.5–1.0

0.85–1.10 0.3–0.5

P/mass %

S/mass %

0.070–0.150 0.070–0.150

0.030–0.060 0.025–0.060

Table 2 – Chemical composition of slag of BOF 1. CaO/mass %

SiO2 /mass %

MgO/mass %

T.Fe/mass %

P2 O5 /mass %

MnO/mass %

35.0–40.0

40.0–45.0

4.0–6.0

5.0–8.0

0.05–0.20

3.0–5.0

When the new process is adopted, the lining corrosion of BOF 1 is more serious. Therefore, it is necessary to study the corrosion mechanism in order to protect the lining of BOF 1. The lining thickness of BOF 1 was detected by laser contouring system. Meanwhile, the sample of corrosion lining of BOF 1 was taken when changing the converter lining. The refractory brick is shown in Fig. 2. The microstructure of corrosion refractory was analysed by scanning electron microscope (MLA250).

3.

Results and discussion

3.1.

Thickness change of lining

The new method is used in first week after reopening of Corex furnace. The thickness change of lining of BOF 1 are shown in Fig. 3 to Fig. 7. As shown in Fig. 3, before being applied as BOF 1 for desilicication, the lining thickness of slag-line section was 500–700 mm.

As shown in Figs. 4, 5 and 6, after been using as BOF 1 for desilicication, the lining thickness of slag-section reduced to 450–550 mm, and the lining thickness of front side also reduced. As shown in Fig. 7, after ending of two-converter process, the lining corrosion of BOF 1 was alleviated. The area whose lining thickness less than 550.0 mm reduced. The statistical results of thickness of converter are shown in Fig. 8. As shown in Fig. 8, the desilicication process had a great damage on the lining thickness of converter. When operation heats was 1997th, the corrosion of slag-line section was relative serious, and area whose lining thickness less than 400 mm increased from 2.01 m2 to 3.65 m2 . At the fifth day (2103th heats) of the operation, the lining thickness of front side area became thinner, and the area whose lining thickness less than 400 mm was 1.72 m2 . When the normal process resumed, the area less than 400 mm was still 2.03 m2 . The lining corrosion reduced, but the thickness of the lining at the slag-line section was still thin.

Please cite this article in press as: Dai Y, et al. Corrosion mechanism and protection of BOF refractory for high silicon hot metal steelmaking process. J Mater Res Technol. 2020. https://doi.org/10.1016/j.jmrt.2020.02.055

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Fig. 4 – Lining thickness (first day as BOF 1 for desilicication, 1966th heats).

Fig. 5 – Lining thickness (second day as BOF 1 for desilicication, 1997th heats).

3.2.

Corrosion mechanism of refractory

In order to analyse the corrosion mechanism of refractory, the sample of refractory brick was taken and polished

with sandpaper. The microstructure of corrosion refractory was analysed by scanning electron microscope. The photo of refractory brick under 50 times is shown in Fig. 9.

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Fig. 6 – Lining thickness (fifth day as BOF 1 for desilicication, 2103th heats).

Fig. 7 – Lining thickness (first day after ending of two-converter process, 2126th heat).

As shown in Fig. 9, the brighter area on the left side is slag, and black area on the right side is refractory brick. The contact area between slag and refractory brick is porous. The porous hole is resulted from the oxidation of carbon in refractory bricks by slag.

The microstructure of slag is shown in Fig. 10. The chemical composition of region 1 and 2 are shown in Table 3. As shown in Fig. 10 and Table 3, the region 1 are RO phase. The main composition of region 1 are Fe, Mn, O and Mg. The content of Fe is 69.78 mass% which is higher than those in

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Fig. 8 – Effect of desilicication on lining thickness.

Fig. 9 – Corrosion area of refractory brick (50 X). Table 3 – Chemical composition of region 1 and 2 of slag/mass %.

1 2

Fe

O

C

Mg

Mn

Ca

Si

P

69.78 4.91

15.68 26.34

5.89 5.91

4.54 0.29

3.49 0.10

0.46 49.89

0.08 11.33

0.08 1.23

normal RO phase. Region 2 is calcium silicate phase, and some phosphorus also exists in it.

The microstructure of interface between slag and refractory brick is shown in Fig. 11. The chemical composition of region 3 and 4 are shown in Table 4. As shown in Fig. 11a, it is the RO phase that contact with the refractory brick. The chemical composition of RO phase (region 4) which is far away from the refractory are basically the same as that of the RO phase in the slag. However, the chemical composition of RO phase (region 3) which is adja-

Fig. 10 – The microstructure of slag (2000 X). Please cite this article in press as: Dai Y, et al. Corrosion mechanism and protection of BOF refractory for high silicon hot metal steelmaking process. J Mater Res Technol. 2020. https://doi.org/10.1016/j.jmrt.2020.02.055

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Fig. 11 – The microstructure between slag and refractory brick.

Table 4 – Chemical composition of region 3 and 4/mass %.

3 4

Fe

O

C

Mg

Mn

Ca

Si

P

52.11 57.80

3.48 25.17

11.20 6.96

2.26 7.86

6.28 1.84

0.43 0.28

0.24 0.07

0.00 0.03

Fig. 12 – The interface of slag and refractory brick (500 X).

cent to refractory are different with the composition of RO phase of slag. The carbon content of region 3 is higher than RO phase of slag, and Fe content of region 3 is less. It indicates that the chemical reaction happen in the area between slag and refractory. The FeO in slag is reduced to Fe by carbon in the refractory. The white bright particle in the black area is the iron grain (shown in Fig. 11b). The interface of slag and refractory brick under 500 times is shown in Fig. 12. The chemical composition of region 5 and 6 are shown in Table 5. As shown in Fig. 12, it is RO phase in the interface between refractory and slag. The Mg content of region 5 is 19.43 mass% which is higher than that of RO phase of other region. It indi-

Table 5 – Chemical composition of region 5 and 6/mass %.

5 6

Fe

O

C

Mg

Mn

Ca

Si

P

48.07 0.35

22.38 29.74

7.39 9.04

19.43 60.35

2.43 0.12

0.23 0.18

0.06 0.08

0.00 0.00

Fig. 13 – The interface of slag and refractory brick under 1500 times magnified (1500 X).

cates that MgO and carbon in refractory brick exchange in slag at the same time. FeO in RO phase constantly react with the carbon of brick, and RO phase have high solubility of MgO which results in the constant transfer of MgO from brick to slag. When there is crack in brick, the slag will enter into crack, and the structure of brick will be destroyed from the inside. The microstructure of the interface between slag and refractory under 1500 times magnified is shown in Fig. 13. As shown in Fig. 13, there is an obvious layer formation between slag and refractory brick. Therefore, there is a reaction between the refractory brick and slag at the interface, accompanied by the formation of the phase. The chemical

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Fig. 14 – Chemical composition along yellow line in Fig. 13.

Table 6 – Chemical composition of region 7, 8 and 9/mass %.

7 8 9

Fig. 15 – New phase between slag and refractory brick 10,000 X.

composition along the yellow line in Fig. 13 was analysed by EDS, and the result is shown in Fig. 14. As shown in Fig. 14, there are obvious concentration gradient of Mg and Fe exist between slag and refractory brick, which promote the mass transfer and reaction between slag and refractory brick.

Fe

O

C

Mg

Mn

Ca

Si

51.03 5.14 38.60

20.79 22.24 22.89

5.62 9.57 5.65

8.06 39.02 13.99

1.90 0.12 1.38

0.66 0.05 0.12

0.13 0.01 0.01

The microstructure of new formation phase at interface between slag and refractory brick is shown in Fig. 15. The chemical composition of region 7, 8 and 9 are shown in Table 6. As shown in Fig. 15 and Table 6, there is no significant difference of chemical composition between the white particle and RO phase. The content of Fe and Mg of white particle are between those of slag and refractory. The precipitation of RO phase from refractory can also be considered as a process in which the refractory is dissolved and eroded by liquid slag. Previous studies [14–16] indicated that the formation of these layers were controlled by mass transfer. Layer thickness depended on the growth of MgO particle at the interface between liquid slag and refractory. Other studies [18,24] of MgO dissolution also reported that the solid solution layer either formed at a distance from the solid surface, or initially formed there and then became detached and moved out into the liquid slag. Under these conditions, this layer could be easily removed by forced liquid flow. Therefore, the principle of lining corrosion by slag can be divided into the following steps, as shown in Fig. 16: (1) At early stage of desiliconization in BOF 1, current boundary between refractory and liquid slag is solid slag

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Fig. 16 – Mechanism of lining corrosion by slag.

splashed by last heat (shown in Fig. 16a). The solid slag layer prevents direct contact between liquid slag and refractory and protects the lining from being eroded. (2) The concentration gradient between the initial interfaces area and the liquid slag results in the mass transfer between solid slag and liquid slag, so that the solid layer melts gradually after reaching a certain temperature. The oscillation of bath and high-speed oxygen jets also induce the loss of the solid slag layer (shown in Fig. 16b). When the solid slag layer disappears, the liquid slag begins to contact directly with the lining (shown in Fig. 16c). (3) The concentration gradient between the liquid slag and refractory results in a constant mass transfer between them. MgO particles begin to precipitate in the lining brick at the interface and diffuses into liquid slag gradually (shown in Fig. 16d and e). (4) The liquid slag flows and takes away the MgO particles formed in the refractory brick. The MgO particles melt in the liquid slag gradually. After a period, the composition of the interface area is consistent with that of the liquid slag. The interface area between slag and lime shrinks into the refractory, and the refractory is eroded from outside to inside (Fig. 6f). In the end, the solid slag and the interfacial layer disappear completely, and the refractory is dissolved into the liquid slag until the end of steelmaking process.

3.3.

Composition optimization of BOF 1 slag

The results in the previous section show that the new RO phase appears in refractory near the boundary between slag and refractory. The transfer of Mg from the refractory to slag promotes the formation of MgO particles and leads to the corrosion of the lining. If the slag and the lining can remain relatively static, the corrosion could stop after reaching a certain extent, because the concentration gradient has kept basically

stable and the driving force of material transport has become less. However, in actual production of BOF, the slag covered on the lining is constantly scoured, and recovered by splashing after tapping. A material exchange also exist between the slag and the solidified slag on the lining. It accelerate the dissolution of the solid slag on the lining at the slag line, resulting in direct contact between the lining and slag, and accelerate corrosion. If more slag can be retained on the lining during steelmaking process in BOF, the lining can be protected better, especially at the slag-line section. In addition to the operation of slag splashing process, the slag composition is an impact factor of slag splashing. When the melting point of slag is low, it will melt earlier and lose its protective effect in the smelting process. The basicity of slag splashing of normal steelmaking process in converter is about 3.5. A certain amount of silicate in the slag. C3 S (3CaO·SiO2 ) and C2 MS2 (2CaO·MgO·2SiO2 ) cannot coexist and react as following equation: 2CaO · MgO · 2SiO2 + 2(3CaO · SiO2 ) = 4(2CaO · SiO2 ) + MgO (1) The melting temperature of slag is increased by the formation of 2CaO·SiO2 . Therefore, the converter lining corrosion is less. For BOF 1 of new process, the basicity of slag is less than 1.5, and the MgO content of slag is relative low (about 5 mass%). When slag splashing process is adopted, it is difficult to adjust the low basicity slag to an optimized composition in a short time. The efficiency of slag splashing can be improved if the slag can be adjusted to the optimized composition for slag splashing during desiliconization process. In order to obtain optimized slag composition, it is necessary to analyse the influence of slag composition on its physical and chemical properties and lining corrosion. (1) The influence of basicity on the melting temperature and phase of slag

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Fig. 17 – Influence of basicity on melting temperature of slag.

The influence of basicity on melting temperature of slag was analysed by FactSage 7.2, and the result is shown in Fig. 17. As shown in Fig. 17, when the basicity of slag is 0.9, the completely melting temperature of slag is low. When the basicity of slag is higher than 0.9, the melting temperature of slag is increased with the increase of basicity. The melting temperature of different basicity slag is effected by its formation phase. The results obtained from the theoretical calculations are consistent with previous experimental studies [25]. The formation phase of different basicity slag were analysed by FactSage 7.2, and the results are shown in Fig. 18. As shown in Fig. 18, when basicity is 0.6, the main phase are liquid slag, clinopyroxene phase, wollastonite phase. For 0.9 basicity slag, when temperature is less than 1350 ◦ C, the

formation amount of liquid slag is more than that of 0.6 basicity slag. For 1.2 basicity slag, the proportion of merwinite phase increases, and the proportion of liquid phase decreases. In addition, the completely melting temperature of slag increases greatly due to formation of 2CaO·SiO2 phase. When basicity increase from 1.2 to 1.5, the proportion of 2CaO·SiO2 phase increases, and the formation of liquid slag decreases under same temperature.When the temperature is higher than 1250 ◦ C, the ratio of liquid phase increases with the increase of basicity. When the basicity is less than 1, slag begins to melt between 1200 ◦ C and 1300 ◦ C. Therefore, the basicity of slag effect the corresponding melting point because it can influence the structure of slag. In general, silicate structure changes from the three dimensional network to discrete anionic groups containing simples chain and rings according to an increase in CaO content, If the content of CaO greater than a specific composition increases, the silicate network will be sufficiently dissociated and the melting temperature of slag will be increased [26]. For desilication process in BOF 1, low basicity slag is beneficial to formation of liquid slag. However, during slag splashing process, low basicity slag takes a long time to splash because of its low melting temperature. Even it is splashed on the furnace wall, it melt early in the next heat, which is difficult to protect the lining. At the later stage of normal steelmaking process, the ratio of residual solid slag on lining is between 60 mass% and 80 mass%. The steelmaking temperature of desilication in BOF 1 is between 1400 ◦ C and 1500 ◦ C. As shown in Fig. 18, the basicity should be at least more than 1.0 to meet the requirements of protecting lining. Considering the lining protection, the basicity of splashing slag should be controlled 1.0–1.2, because higher basicity is not conducive to the kinetic conditions of the reaction. In addition, if the target basicity

Fig. 18 – Phase of slag with different basicity. Please cite this article in press as: Dai Y, et al. Corrosion mechanism and protection of BOF refractory for high silicon hot metal steelmaking process. J Mater Res Technol. 2020. https://doi.org/10.1016/j.jmrt.2020.02.055

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Fig. 19 – Influence of (FeO) content on melting temperature of slag.

of slag is too high, more slagging materials should be added which cannot be melted completely because of the limited smelting time (smelting time is about 5–10 min) of desilication process in BOF 1. (2) The influence of (FeO) content on the melting temperature and phase of slag The influence of (FeO) content on melting temperature of slag was analysed by FactSage 7.2, and results is shown in Fig. 19. As shown in Fig. 19, with the increase of (FeO) content of slag, the beginning melting temperature of slag rise. FeO) content of slag has little effect on the completely melting temperature. The result indicates that the increase of (FeO)

11

is beneficial to formation of liquid slag, and has little effect on the dissolution of high melting temperature phase. The formation phase of slag with different (FeO) content were analysed by FactSage 7.2, and the results are shown in Fig. 20. As shown in Fig. 20, when temperature is less than 1350 ◦ C, the formation phase of 6% (FeO) content slag is complex, including wollastonite phase, merwinite phase, akermanite (melilite) phase. When temperature is 1300 ◦ C, the liquid phase ratio is 70–80 mass%, and it increases with the increase of (FeO) content. It has been reported [27] that in CaO–FeO–SiO2 melts, FeO does not associate with the silicate groups in the low basicity region (CaO/SiO2 < 1.86) but supplies free Fe2− and O2− ions to the silicate melts and causes the mixing entropy of silicate melt to increase. Therefore, FeO lowers the melting temperature of slags because it can effect the Gibbs free energy of mixing of molten slag would be more significant than those of other components, resulting in the decrease of melting temperatures and slag viscosities with increasing the FeO content. Therefore, the content of (FeO) should be controlled about 5 mass% to enhance solid phase ratio. (3) The influence of (MgO) content on the melting temperature and phase of slag The influence of (MgO) content on melting temperature of slag was analysed by FactSage 7.2, and result is shown in Fig. 21. As shown in Fig. 21, higher (MgO) content is beneficial to the formation of high melting temperature phase. When (MgO) content is higher than 3 mass%, the completely melting temperature increases with the increase of (MgO) content. When the (MgO) content increases from 3 mass% to 7 mass%, the completely temperature increases from 1266.5 ◦ C to 1367.3 ◦ C,

Fig. 20 – Phase of slag with different (FeO) content. Please cite this article in press as: Dai Y, et al. Corrosion mechanism and protection of BOF refractory for high silicon hot metal steelmaking process. J Mater Res Technol. 2020. https://doi.org/10.1016/j.jmrt.2020.02.055

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Fig. 21 – Influence of (MgO) content on melting temperature of slag.

while the beginning melting temperature decrease from 1070.4 ◦ C to 1050.15 ◦ C. The formation phase of slag with different (MgO) content were analysed by FactSage 7.2, and the results are shown in Fig. 22. As shown in Fig. 22, when the temperature is less than 1300 ◦ C, the formation phase of 3 mass% (MgO) slag are wollastonite phase, merwinite phase, akemanite (melilite) phase, and when the temperature is higher than 1300 ◦ C, only liquid phase exists. The ratio of merwintie phase is increase with the increase of (MgO) content. When (MgO) content is 7%, the merwinite exists until temperature is higher than 1400 ◦ C, which

increase the melting temperature of slag. Considering the limited steelmaking time and costs of BOF 1, it is more rational to control the (MgO) content of slag between 5 mass% and 7 mass%. (4) The influence of basicity on (MgO) solubility of slag The previous study about corrosion mechanism has shown that the MgO dissolution leads to refractory corrosion. Therefore, the low MgO solubility of slag is beneficial to alleviate lining corrosion. In BOF 1, the basicity slag is about 1 which lead to high solublity of MgO and corrosion of refractory. The region of liquid phase of CaO–SiO2 –MgO–MnO (5 mass%)–FeO (10 mass%) is shown in Fig. 23. As shown in Fig. 23, under same temperature, the (MgO) solubility increases with the increase of temperature. The saturated (MgO) solubility of slag increases when the basicity increases. The influence of temperature and basicity on the saturated (MgO) solubility of slag is shown in Fig. 24. Each point in the Fig. 24 is the intersection point between different basicity slag and liquid phase region at different temperature. As shown in Fig. 24, the saturated (MgO) solubility of slag can be reduced effectively by increasing the slag basicity at the same temperature. When slag basicity is 1.2, the saturated (MgO) solubility is 6.3 g at 1300 ◦ C and 18.67 g at 1600 ◦ C. When basicity is 1, the saturated solubility is 0 g at 1300 ◦ C and 13.6 g at 1600 ◦ C. High slag basicity can reduce the saturated solubility of (MgO), and high content of (MgO) of slag can reduce (MgO) transferring from furnace lining. During the desilication process in BOF 1, the reasonable basicity is about 1.2, and temperature is about 1400 ◦ C. According to the effect of basic-

Fig. 22 – Phase of slag with different (MgO) content. Please cite this article in press as: Dai Y, et al. Corrosion mechanism and protection of BOF refractory for high silicon hot metal steelmaking process. J Mater Res Technol. 2020. https://doi.org/10.1016/j.jmrt.2020.02.055

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The change of lining thickness of BOF 1 during the production process of desilication when optimization slag is adopted are shown in Figs. 25–29. As shown in Figs. 25–29, the lining thickness of slag-line section is 550.0–700.0 mm, while the thickness decrease after the BOF is used for desilication. But the lining corrosion is reduced compared with those before optimization slag is adopted. The statistical results of thickness of converter after optimization slag is adopted are shown in Fig. 30. As shown in Fig. 30, the lining thinning trend slows down with the increase of the number of desilication heats. On the third day of operation, area whose lining thickness lower than 400 mm is 1.73 m2 , which is lower than those (3.653 m2 ) before the application of optimization slag. On the fifth day of operation, area whose lining thickness lower than 400 mm is 1.095 m2 , which is lower than those (1.721 m2 ) before the application of optimization slag. The front side corrosion degree is obviously reduced. The optimization slag reduces the lining corrosion during steelmaking process of high silicon hot metal to a certain extent.

Fig. 23 – Phase diagram of CaO–SiO2 –MgO–MnO (5 mass%)–FeO (10 mass%).

4.

Conclusions

The desiliconizaiton process of high silicon hot metal aggravates the lining corrosion of BOF 1. In this paper, the lining corrosion mechanism in BOF 1 was studied. The reasonable slag composition of splashing in BOF 1 was determined through a combination of plant experiments and theoretical calculation. The main conclusions are as follows:

Fig. 24 – Saturated solubility of slag with different basicity.

Table 7 – Chemical composition of optimization slag for splashing process/mass %. CaO

SiO2

MgO

MnO

FeO

CaO/SiO2

30–42

25–35

5–7

<5

<5

1.0–1.2

ity on the saturated (MgO) solubility, the (MgO) content of slag should be controlled 5–7 mass%.

3.4.

Application effect of optimization slag

According to the research above, the chemical composition optimization of slag splashing process is shown in Table. 7.

(1) The observation of microstructure of corrosion lining shows that, the chemical reaction and the dissolution of (MgO) lead to the lining corrosion. Concentration gradient of Mg and Fe was found between slag and refractory brick, which promotes the transfer of composition between slag and refractory brick. (2) The low basicity of slag leads to low melting point and less formation of solid phase at the same temperature which results in the scouring down of slag at early stage of steelmaking process in BOF 1. In addition, the low basicity lead to high saturated (MgO) solubility, which is beneficial to the (MgO) dissolution of lining. In the production of each heats, the surface layer of lining is constantly contact with the new low basicity slag. The constant reaction and dissolution of (MgO) between lining and slag lead to the lining corrosion. (3) The optimization slag is determined by theoretical calculation. The basicity of slag should be 1.0–1.2. The (FeO) content should be controlled less than 5 mass%, and (MgO) content of should be 5–7 mass%. The industrial experiments show that optimization slag reduces the corrosion of furnace lining to a certain extent.

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Fig. 25 – Lining thickness (before being BOF 1 for desilicication, 957th heats).

Fig. 26 – Lining thickness (second day as BOF 1 for desilicication, 992th heats).

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Fig. 27 – Lining thickness (third day as BOF 1 for desilicication, 1032th heats).

Fig. 28 – Lining thickness (fifth day as BOF 1 for desilicication, 1097th heats).

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Fig. 29 – Lining thickness (first day after ending of two-converter process, 1158th heat).

references

Fig. 30 – lining thickness of BOF 1 after optimization slag adopted.

Conflicts of interest The authors declare no conflicts of interest.

Acknowledgment The authors would like to acknowledge the financial support from National Key R&D Program of China, 2017YFB0304000.

[1] Garbers-Craig AM. Presidential address: how cool are refractory materials? J S Afr Inst Min Metall 2008;108:491–506. [2] Riley FL. The corrosion of ceramics: where do we go from here? Key Eng Mater 1996;113:1–14. [3] Krestan J, Pritula O, Smrcok L, Sajgalık P, Lences Z, Wannberg A, et al. Corrosion o ␤-sialon-based ceramics by molten steel. J Eur Ceram Soc 2007;27:2137–43. [4] Muhammad IUH, Khanna R, Koshy P, Sahajwalla V. High-temperature interactions of alumina–carbon refractories with molten iron. ISIJ Int 2010;50:804–12. [5] Hong L, Sahajwalla V. Investigation of in-situ chemical reactions of Al2 O3 -SiC-SiO2 -C refractory and its interaction with slag. ISIJ Int 2004;44:785–9. [6] Mukai K, Toguri JM, Stubina NM, Yoshitomi J. A mechanism for the local corrosion of immersion nozzles. ISIJ Int 1989;29:469–76. [7] Zhao XJ, Ru HQ, Zhang N, Wang XY, Chen DL. Corrosion of aluminum oxynitride based ceramics by molten steel. Ceram Int 2013;39:3049–54. [8] Shen P, Zhang L, Zou Y, Gu H, Fu L. Wettability between liquid iron and tundish lining refractory. Metall Res Technol 2016;113:503. [9] Huang A, Wang Y, Zou Y, Gu H, Fu L. Dynamic interaction of refractory and molten steel: effect of alumina-magnesia castables on alloy steel cleanness. Ceram Int 2018;44:14617–24. [10] Wei Y, Shao Y, Chen J, Li N. Influence of carbonaceous materials on the interactions among (Al2 O3 -C)/Fe system with temperature and soaking time. J Eur Ceram Soc 2018;38:313–22. [11] Frushstorfer J, Schottler L, Dudczig S, Schmidt G, Gehre P, Aneziris CG. Corrosion and corrosion of alumina refractory by ingot casting steels. J Eur Ceram Soc 2016;36:1299–306.

Please cite this article in press as: Dai Y, et al. Corrosion mechanism and protection of BOF refractory for high silicon hot metal steelmaking process. J Mater Res Technol. 2020. https://doi.org/10.1016/j.jmrt.2020.02.055

JMRTEC-1420; No. of Pages 17

ARTICLE IN PRESS j m a t e r r e s t e c h n o l . 2 0 2 0;x x x(x x):xxx–xxx

[12] Lee M, Sun S, Wright S, Jahanshahi S. Effects of transition metals on the kinetics of slag refractory reactions. Metall Trans B 2001;32:25–9. [13] Sahajwalla V, Wu C, Khanna R, Chaudhury N, Spink J. Kinetic study of factors affecting in situ reduction of silica in carbon-silica mixtures for refractories. ISIJ Int 2003;43:1309–915. [14] Bygden J, Debroy T, Seetharaman S. Dissolution of MgO in stagnant CaO-FeO-SiO2 slags. Ironmak Steelmak 1994;21:318–23. [15] Nightingale SA, Monaghan BJ, Brooks GA. Degradation of MgO refractory in CaO-SiO2 -MgO-FeOx and CaO-SiO2 -Al2 O3 -MgO-FeOx slags under forced convection. Metall Trans B 2005;36:453–61. [16] Zhang P, Seetharaman S. Dissolution of MgO in CaO-“FeO”-CaF2 -SiO2 slags under static conditions. J Am Ceram Soc 1994;77:970–6. [17] Vadasz P, Molnar L. Interaction at the periclase-slag melt phase boundary. Ceram Silikaty 1992;36:199–204. [18] Valdez M, Prapakorn K, Cramb AW, Seetharaman S. A study of the dissolution of Al2 O3 , MgO and MgAl2 O4 particles in a CaO-Al2 O3 -SiO2 slag. Steel Res 2001;72:291–7. [19] Kumar PP, Carg LM, Gupta SS. Modelling of Corex process optimisation of operational parameters. Ironmak Steelmak 2006;33:29–33.

17

[20] Fan JF, Li WG, Zhou YS, Li ZY. Ironmak Steelmak 2007;42:17–20 (In Chinese). [21] Barella S, Cecca CD, Mapelli C, Gruttadauria A, Bondi E, Marinari A. Study of a new operating practice for refining high silicon hot metal in a BOF converter. Steel Res 2016;87:941–6. [22] Barella S, Gruttadauria A, Mapelli C, Mombelli D. Critical evaluation of role of viscosity and gas flowrate on slag foaming. Ironmak Steelmak 2012;39:463–9. [23] Ohno K, Kaimoto M, Maeda T, Nishioka K, Shimizu M. Effect of slag melting behavior on metal-slag separation temperature in powdery iron, slag and carbon mixture. ISIJ Int 2011;51:1279–84. [24] Tran T, Nightingale S, Brooks G. Corrosion of MgO by alumina-containing slags. J Aust Ceram Soc 1998;34:33–8. [25] Chuang H, Hwang W, Liu S. Effects of basicity and FeO content on the softening and melting temperatures of the CaO-SiO2 -MgO-Al2 O3 slag system. Mater Trans B 2009;50:1448–56. [26] Iida T, Guthrie RIL. The physical properties of liquid metals. Oxford: Clarendon Press; 1988. pp. 147. [27] Lee YS, Min DJ, Jung SM, Yi SH. Influence of basicity and FeO content on viscosity of blast furnace type slags containing FeO. ISIJ Int 2004;44:1283–90.

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