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Wear mechanism of a novel AlSiMgAl2O4Al2O3 composite used in the low vessel of an RH secondary refining furnace
Ceramics International 45 (2019) 11275–11280
Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Wear mechanism of a novel AleSieMgAl2O4eAl2O3 composite used in the low vessel of an RH secondary refining furnace
T
Chenhong Maa, Yong Lia,∗, Lixin Zhangb, Yang Suna, Jialin Suna a b
School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing, 100083, China Sinosteel Refractory Co., Ltd., Luoyang, Henan Province, 471039, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Al–SieMgAl2O4eAl2O3 composite RH degasser γ-AlON 21ReSiAlON
A novel AleSieMgAl2O4eAl2O3 composite brick was prepared and evaluated in the low vessel of an RH (the initials of Ruhrstahl and Hereaeus) secondary refining furnace; it was characterized by X-ray diffraction, scanning electron microscopy, and energy-dispersive spectroscopy. The results show that after use, the AleSieMgAl2O4eAl2O3 composite has a functional gradient with an erosion zone–reinforced zone–original zone phase distribution, in which the phases in the erosion zone (0–1.8 cm) are a Mg-hercynite spinel solid solution, α-Al2O3, and minor amount of Al3Fe5O12. Furthermore, the phases in the reinforced zone (1.8–5.0 cm) are γAlON, 21ReSiAlON, SiC, Mg0.388Al2.408O4, and α-Al2O3; i.e., the Al and Si in the composite are completely converted into non-oxide reinforced phases. Finally, the phases in the original zone (> 5.0 cm) show no change. The reaction mechanism is as follows. During operation, a Mg-hercynite spinel solid solution is formed in the erosion zone due to a reaction between MgAl2O4 and FeO from a refinery operation. Therefore, the slag erosion of the material is improved. The Al and Si metals undergo active oxidation, and 21ReSiAlON flakes are subsequently formed from the products of the metastable Al2O(g), SiO(g), and N2(g) in the ambient. The γ-AlON is formed by a carbothermal reduction nitridation of the α-Al2O3 and residual active carbon from the resin binder. The 21ReSiAlON and γ-AlON reinforce the composite brick and improve its high temperature performance accordingly. Its service life is 110% that of the magnesia-chrome bricks used in the same period. The reaction model was also established.
1. Introduction The main functions of secondary steelmaking refining are removing impurities and inclusions in the steel, degassing, adjusting the chemical composition and homogenization. RH (the initials of Ruhrstahl and Hereaeus) refining, which is a vacuum degassing method, is one of the most important and widely used methods in secondary steelmaking refining [1]. It has the advantages of high efficiency, good suitability for batch processing, use of existing equipment, and easy operation [2,3]. However, the requirements for the equipment lining materials are very demanding due to the versatility of RH vacuum refining technology. Magnesia–chrome brick has good high temperature performance, superior thermal shock stability, and good resistance to slag corrosion. Consequently, magnesia-chrome is the main material used in the low vessel and snorkels of the RH degasser at present [4–7]. However, the service life of magnesia-chrome bricks will significantly decrease due to the highly basic slag when they are used in the production of silicon steel. In addition, the trivalent chromium in the
∗
magnesia–chrome brick will convert to hexavalent chromium under operational conditions, which are harmful to both human beings and the environment. Therefore, it is vital and meaningful to develop new environmentally friendly chrome-free materials. Oxide refractories, oxide–carbon refractories, and non-oxide–oxide refractories represent three milestones in the development of refractories. Non-oxide–oxide refractories have a substantially superior high-temperature strength and better oxidation resistance than oxide–carbon refractories, better thermal shock resistance than pure oxide refractories, and good corrosion resistance to metallurgical slag and alkali environments [8–10]. Therefore, environmentally friendly nonoxide–oxide refractories are currently the most promising material for high-performance steel production, and related studies have been investigated [11–22]. AleAl2O3 refractory has attained satisfactory trial results when used for a sliding gate [23,24] and RH degasser. The transient plastic phase process is an effective way to prepare non-oxide–oxide composites [23,25,26], where metal phases convert to high-performance non-oxide phases in situ at high temperatures and in
Corresponding author. E-mail address: [email protected] (Y. Li).
https://doi.org/10.1016/j.ceramint.2019.02.202 Received 21 November 2018; Received in revised form 18 February 2019; Accepted 27 February 2019 Available online 04 March 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Ceramics International 45 (2019) 11275–11280
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a reducing atmosphere. Correspondingly, the temperature during an RH refining operation is relatively high, approximately 1650–1700 °C, the oxygen partial pressure is very low, and the conditions present a gradient change for the low vessel brick from the hot face to the interior. Therefore, it is considered that a high-performance functional gradient non-oxide–oxide material is obtained during an RH refining operation by using metal Al and Si as raw materials. The Al and Si in the hot zone convert to non-oxide reinforced phases, whereas the Al and Si in the interior zone remain as free form and act as metal plastic phases. Consequently, in this work, an AleSieMgAl2O4eAl2O3 composite brick was prepared and evaluated in the low vessel of RH degasser. The phase composition and microstructure of the AleSieMgAl2O4eAl2O3 composite after use was analysed by X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive spectroscopy (EDS). The reaction model was established. 2. Experiments Fused corundum (5–3 mm, 15 wt%; 3–1 mm, 15 wt%; < 0.074 mm, 17 wt%), tabular alumina (3–1 mm, 15 wt%; 1–0 mm, 20 wt%), fused magnesia-alumina spinel (3–1 mm, 5 wt%; 1–0 mm, 3 wt %; < 0.074 mm, 5 wt%), metallic aluminium powders (4 wt%), and silicon powders (1 wt%) were used as raw materials, and thermosetting phenolic resin (PF5323, 3.5 wt% relative to the total weight) was added as a binding agent. The powders were grounded and shaped under a 630t friction presser. After natural dehydration for 24 h, the AleSieMgAl2O4eAl2O3 composite bricks were dried in a tunnel kiln for 12 h at a temperature of 200 °C. The chemical compositions of the different raw materials are shown in Table 1. The prepared AleSieMgAl2O4eAl2O3 composite bricks were applied to the low vessel of the RH degasser. As shown in Fig. 1, the erosion zone and reaction zone (numbered 1 and 2, respectively) of the brick after use were ball-milled into powders for XRD measurements. The patterns were collected on a Rigaku instrument using Cu Kα radiation (wavelength: 1.5415 nm). A scan speed of 5°/min in continuous mode was applied, and the data were collected in a 2θ range of 10–90°. The microstructure was observed by an environmental scanning electron microscope (SEM) (Quanta FEG450, FEI) equipped with an energydispersive spectroscope (EDS) to analyse the elements in specific areas. A gold coating was used to make the sample electrically conductive enough for SEM analysis after being polished. 3. Results and discussion 3.1. Phase evolution The phases of the AleSieMgAl2O4eAl2O3 composite brick after use in the RH degasser show a functional gradient distribution with distance. In particular, the major phases in the erosion zone are Al15.44Fe6.16Mg2.32O32.00 (Mg-hercynite spinel solid solution), α-Al2O3, and a minor amount of Al3Fe5O12 and SiO2 are also detected. The phases in the reaction zone are α-Al2O3, Mg0.388Al2.408O4 spinel, γAlON, 21ReSiAlON (SiAl6O2N6), a minor amount of SiC, and C, which means that Al and Si in the reaction zone transformed to 21ReSiAlON and SiC during the RH refining operation. The phases in the interior are close to those in the original AleSieMgAl2O4eAl2O3 composite. The
Fig. 1. Samples for XRD and SEM characterization.
XRD patterns of the erosion zone and reaction zone are shown in Fig. 2. The XRD diffraction results show that after use, the original AleSieMgAl2O4eAl2O3 composite was successfully transformed into a gradient material with an anti-erosion phase–reinforced phase–plastic phase distribution.
3.2. Microstructure analysis Corresponding to the results of the phase composition, the microstructure of the AleSieMgAl2O4eAl2O3 composite brick after use also varies with the distance from the working face. According to the evolution of the microstructure, three typical zones are generally present: an erosion zone (0–1.8 cm), reaction zone (1.8–5.0 cm), and original zone (> 5.0 cm). The microstructure of the erosion zone and the reaction zone is shown in Figs. 3 and 4, respectively. Fig. 3 shows the microstructure of the erosion zone (0–1.8 cm). In this zone, the overall structure is relatively loose at room temperature. It is mainly comprised of bright white octahedral particles with a particle size of approximately 50 μm [Fig. 3(c)] and greyish white matter [Fig. 3(d)]. The results of the EDS analysis are listed in Table 2. Combining the results of XRD, the bright white octahedral particles containing Al, Fe, Mg, and O in Fig. 3(c) are Al15.44Fe6.16Mg2.32O32.00 (Mghercynite spinel solid solution), the white substance marked as “2” in Fig. 3(f) is Al3Fe5O12, while the grey substance marked as “3” is Al2O3. Considering the results of XRD, SEM, and EDS, it can be seen that the Fe from the RH operation exists in this zone. The microstructure changes 1.8 cm away from the working surface. Fig. 4 shows the microstructure of the reaction zone (1.8–5.0 cm). As shown in Fig. 4(a)–4(b), the large particles are tightly bound to the matrix. A large amount of flaky material containing Al, Si, O and N is formed in the matrix, which interpenetrate and overlap with each other, forming a network structure [Fig. 4(c) and (e)]. It is suggested that this zone is dominated by gas phase reactions. Combining the results of XRD, the flakes are 21ReSiAlON. As shown in Fig. 4(f), some 21ReSiAlON flakes fill in the pores to enhance the bridging effect. Furthermore, spinel structure materials with a particle size of approximately 2 μm formed and are pinned on the surface of the α-Al2O3 particles, and the EDS results show that they are mainly composed of Al, O, and N elements, i.e., γ-AlON. Fig. 5 shows the morphology of the AleSieMgAl2O4eAl2O3 composite brick after use in the RH degasser at a low magnification. Based on the above analysis, a model of the evolution of the structure of the AleSieMgAl2O4eAl2O3 composite brick before and after use is shown
Table 1 Chemical composition of experimental raw materials (wt.%).
Fused corundum Fused magnesia-alumina spinel Tabular alumina Al powders Si powders
Al2O3
MgO
Fe2O3
SiO2
CaO
Al
Si
Ca
Fe
99.21 76.25 99.20 / /
/ 22.18 / / /
0.04 0.17 / / /
0.03 0.62 / / /
0.27 0.38 / / /
/ / / 99.50 /
/ / / 0.20 98.00
/ / / / 0.30
/ / / 0.15 0.60
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Fig. 2. XRD patterns of the erosion zone and the reaction zone (No. 1 and 2, respectively) of the AleSieMgAl2O4eAl2O3 composite brick after use in the RH degasser.
in Fig. 6. As shown in Fig. 6, a protective layer consisting of a Mghercynite spinel solid solution is formed in the erosion zone, a 21RSiAlONe and γ-AlON-bonded Al2O3 composite is formed in the reaction zone, and the inner zone is close to the original AleSieMgAl2O4eAl2O3 composition, forming an “anti-erosion zone–reinforced zone–plastic phase zone” functional gradient composite.
FeO(l) + MgAl 2 O4 (s) → Mg − hercynite spinel solid solution (Al15.44 Fe6.16 Mg 2.32 O32.00)(s) 2Fe(l)+ O2 (g ) = 2FeO(l) =
4. Reaction mechanism of the AleSieMgAl2O4eAl2O3 composite during RH refining operation During the refining process, Ar is blowing through the snorkel inlet so that high-temperature molten steel generates a high-speed circulating flow. FeO from the refining operation has a low melting point and low viscosity and easily penetrates into the brick. At the RH refining temperature (approximately 1650 °C), FeO(l) reacts with MgAl2O4 spinel spontaneously; therefore, a Mg-hercynite spinel solid solution (Al15.44Fe6.16Mg2.32O32.00) is formed, per reaction (1). The Mghercynite spinel solid solution has a high melting point and good erosion resistance, and it provides a protective layer that inhibits further penetration of the slag, which effectively improves the slag erosion resistance of the material as shown in Fig. 6:
ΔGΘ
ΔGΘ = −459400 + 87.45T
(1)
ΔG
(a )2 + RT ln FeO Θ PO2/P
(2)
The added Al(s) and Si(s) in the brick likely play an important role in the formation of the flaky 21ReSiAlON. The possible reaction pathway is listed as follows:
Al(s) = Al(l)
Al(l) = Al(g)
ΔGΘ = 10795 + 11.55T
ΔGΘ = 304640 − 109.50T
(3) (4)
Si(s) = Si(l)
ΔGΘ = 50540 − 30.0T
(5)
Si(l) = Si(g)
ΔGΘ = 395400 − 111.38T
(6)
2Al(l) +
Si(l) +
1 O2 (g ) = Al2O (g ) 2
1 O2 (g ) = SiO(g ) 2
ΔGΘ = −170700 − 49.37T
ΔGΘ = −155230 − 47.28T
(7) (8)
Fig. 3. Microstructure of the erosion zone (0–1.8 cm) in the composite AleSieMgAl2O4eAl2O3 brick after use in the RH degasser: (a), (c) and (e) are the SEM photographs for Mg-hercynite; (b), (d) and (f) are the SEM photographs for Al2O3 and Fe-rich phase.
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Fig. 4. Microstructure of the reaction zone (1.8–5.0 cm) in the composite AleSieMgAl2O4eAl2O3 brick after use in the RH degasser: (a), (b) and (c) are the low magnification photographs; (e), (d) and (f) are the SEM photographs for 21ReSiAlON and γ-AlON.
Significant quantities of metastable Al2O(g) and SiO(g) are formed by the active oxidation of Al and Si. With the formation of the Mghercynite spinel solid solution by FeO(l) and MgAl2O4(s) in the erosion zone, a “sealing wall” is formed. Therefore, the metastable Al2O(g) and SiO(g) are enclosed in the system, and the gas pressure increases. Driven by high temperature, the metastable Al2O(g) and SiO(g) diffuse along the pores and further react with the N2 and C that came from phenolic resin, per reaction (9); therefore, 21ReSiAlON flakes are formed, which interpenetrate and overlap with each other, forming a network reinforcement structure.
Table 2 EDS results for the areas in Figs. 3 and 4. Marked areas
Fig. 3
Fig. 4
Area Area Area Area Area Area
Atomic mole percentage/%
1 2 3 1 2 3
N
O
Mg
Al
Fe
Si
03.20 / / 24.06 08.91 25.20
54.83 54.56 52.55 15.69 50.20 23.69
04.22 / / 02.48 03.05 02.01
23.96 13.09 42.57 49.27 37.84 39.16
13.79 32.35 4.88
/ / / 8.50 / 9.93
/ /
Al2O3 (s) + C(s) + N2 (g) → γ− AlON(s ) + CO(g ) SiO(g) + Al2O(g )+ N2 (g ) + C(s) → 21 R− Sialon(s ) + CO(g )
(9)
The melting points of metallic Al and Si are 660 °C and 1412 °C, respectively. Therefore, at the RH refining temperature, Al(s) and Si(s) will convert to Al(l, g) and Si(l, g), Reactions (3)–(6). Parameters PAl and PSi are calculated as a function of temperature, as shown in Fig. 7. Moreover, Al2O(g) and SiO(g) can also form from reactions (7)–(8) depending on the oxygen partial pressure. During the RH refining operation, the minimum PO2 in the RH degasser is estimated by Equation (2). Because the content of FeO in the refining slag is low, the PO2 in the RH degasser is relatively low. The PO2 – aFeO curve is plotted at 1923 K, as shown in Fig. 8, where lgPO2/PΘ is −14 ∼ −10 when aFeO is 0.001–0.1. Based on this, PAl2O and PSiO are calculated as a function of lgPO2/PΘ, as shown in Fig. 9. From Fig. 9, during RH refining, the PSiO and PAl2O inside the AleSieMgAl2O4eAl2O3 composite brick would probably be in the range of 0.03–10 MPa; that is, they would be 2 to 4 and 4 to 7 orders of magnitude higher than PAl and PSi, respectively.
(10)
Except for the Al, Si, and MgAl2O4 raw materials, there is approximately 45–65% residual carbon from the thermosetting phenolic resin binding agent (3.5 wt%), which is evenly dispersed on the surface of raw material particles. Compared with that for graphite and carbon black, the residual carbon from a resin is much more reactive so that it can easily react with Al2O3 [27,28]. Therefore, with the continuous oxygen consumption by Al and Si, the PO2 in the system continuously decreases; when the partial pressure of oxygen decreases to a certain value, the carbothermal reduction and nitridation reaction between αAl2O3 and residual carbon occurs, and the γ-AlON spinel is formed, per reaction (10). The model for the formation mechanism of γ-AlON and 21ReSiAlON is shown in Fig. 10. 5. Conclusions A novel AleSieMgAl2O4eAl2O3 composite brick was prepared and
Fig. 5. Morphology of AleSieMgAl2O4eAl2O3 composite brick after use in the RH degasser at low magnification. 11278
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Fig. 6. Simplified model of AleSieMgAl2O4eAl2O3 composite brick before and after use in the RH degasser.
Fig. 7. Diagram of PAl(g)/Si(g) – T for reactions (4) and (6).
Fig. 9. Diagram of log (PAl2O(g)/SiO(g)/PO2) – log (PO2/PΘ) at 1923 K for reactions (7) and (8).
(i) In the erosion zone (0–1.8 cm), a Mg-hercynite spinel solid solution protective layer is formed by reaction between MgAl2O4 and FeO, which improves the slag erosion resistance of the material. (ii) In the reaction zone (1.8–5.0 cm), the Al and Si occur an active oxidation, resulting in significant quantities of metastable Al2O(g) and SiO(g). Additionally, 21ReSiAlON flakes form, resulting in a connected network reinforcement structure. The γ-AlON forms by a carbothermal reduction nitridation reaction between the α-Al2O3 and reactive residual carbon from the phenolic resin. The γ-AlON and 21ReSiAlON act as reinforcement phases that contribute to the high temperature performance of the material. (iii) In the original zone (> 5.0 cm), metal Al and Si stay in their original form and act as plastic phases to improve the toughness of the material.
Acknowledgements Fig. 8. Diagram of PO2
–
aFeO at 1923 K for Equation (2).
used in the low vessel of an RH degasser. The brick after use shows a functional gradient with an anti-erosion zone–non-oxide reinforced zone–plastic zone distribution. It achieved good results with a service life that was 110% that of the magnesia-chrome bricks used over the same period.
This work was supported by the National Natural Science Foundation of China under Grant No. 51872023.
Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.ceramint.2019.02.202.
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Fig. 10. Model of the formation mechanism of γ-AlON and 21ReSiAlON in the reaction zone: zone 1, 2 and 3 represent the erosion zone, reaction zone and original zone, respectively.
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[14] W.M. James, N.D. Corbin, Phase relations and reaction sintering of transparent cubic aluminum oxynitride spinel (AlON), J. Eur. Ceram. Soc. 62 (9–10) (2010) 476–479. [15] N.D. Corbin, Aluminum oxynitride spinel: a review, J. Eur. Ceram. Soc. 5 (3) (1989) 143–154. [16] W.M. James, P. Parimal, C. Mingwei, et al., AlON: a brief history of its emergence and evolution, J. Eur. Ceram. Soc. 29 (2) (2009) 223–236. [17] T. Ekström, M. Nygren, SiAlON ceramics, J. Am. Ceram. Soc. 75 (2) (1992) 259–276. [18] K.H. Jack, Sialons and related nitrogen ceramics, J. Mater. Sci. 11 (6) (1976) 1135–1158. [19] V.A. Izhevskiy, L.A. Genova, J.C. Bressiani, et al., Progress in SiAlON ceramics, J. Eur. Ceram. Soc. 20 (13) (2000) 2275–2295. [20] K.M. Awadesh, C.A. Nurcan, K. Ferhat, et al., A comparative study of SiAlON ceramics, Ceram. Int. 38 (2012) 5757–5767. [21] D.A. Gunn, A theoretical evaluation of the stability of sialon-bonded silicon carbide in the blast furnace environment, J. Eur. Ceram. Soc. 11 (1) (1993) 35–41. [22] X. Deng, X. Li, B. Zhu, et al., In-situ synthesis mechanism of plate-shaped β-Sialon and its effect on Al2O3–C refractory properties, Ceram. Int. 41 (10) (2015) 14376–14382. [23] Y. Li, J. Zhang, J. Li, et al., Preparation of Al-Al2O3 composite sliding gate by transient plastic phase process, J. Chin. Ceram. Soc. 41 (3) (2013) 416–421. [24] S. Tong, Y. Li, J. Zhao, et al., Effect of Al addition on creep resistance of MgO-Al2O3 composite for sliding plate at 1400°C, Ceram. Int. 43 (2017) 11610–11615. [25] M.W. Barsoum, B. Houng, Transient plastic phase processing of titanium-boroncarbon composites, J. Am. Ceram. Soc. 76 (6) (1993) 1445–1451. [26] M.W. Barsoum, A. Zavaliangos, S.R. Kalidindi, et al., The transient plastic phase processing of ceramic-ceramic composites, JOM 47 (11) (1995) 52–55. [27] H. Qin, Y. Li, M. Long, et al., In situ synthesis mechanism of 15R–SiAlON reinforced Al2O3 refractories by Fe–Si liquid phase sintering, J. Am. Ceram. Soc. 101 (5) (2018) 1870–1879. [28] H. Qin, Y. Li, P. Jiang, et al., In-situ synthesis of AlON reinforcing phases in resin bonded Al2O3 composite materials, J. Alloy. Comp. 711 (2017) 1–7.
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Wear mechanism of a novel AleSieMgAl2O4eAl2O3 composite used in the low vessel of an RH secondary refining furnace
T
Chenhong Maa, Yong Lia,∗, Lixin Zhangb, Yang Suna, Jialin Suna a b
School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing, 100083, China Sinosteel Refractory Co., Ltd., Luoyang, Henan Province, 471039, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Al–SieMgAl2O4eAl2O3 composite RH degasser γ-AlON 21ReSiAlON
A novel AleSieMgAl2O4eAl2O3 composite brick was prepared and evaluated in the low vessel of an RH (the initials of Ruhrstahl and Hereaeus) secondary refining furnace; it was characterized by X-ray diffraction, scanning electron microscopy, and energy-dispersive spectroscopy. The results show that after use, the AleSieMgAl2O4eAl2O3 composite has a functional gradient with an erosion zone–reinforced zone–original zone phase distribution, in which the phases in the erosion zone (0–1.8 cm) are a Mg-hercynite spinel solid solution, α-Al2O3, and minor amount of Al3Fe5O12. Furthermore, the phases in the reinforced zone (1.8–5.0 cm) are γAlON, 21ReSiAlON, SiC, Mg0.388Al2.408O4, and α-Al2O3; i.e., the Al and Si in the composite are completely converted into non-oxide reinforced phases. Finally, the phases in the original zone (> 5.0 cm) show no change. The reaction mechanism is as follows. During operation, a Mg-hercynite spinel solid solution is formed in the erosion zone due to a reaction between MgAl2O4 and FeO from a refinery operation. Therefore, the slag erosion of the material is improved. The Al and Si metals undergo active oxidation, and 21ReSiAlON flakes are subsequently formed from the products of the metastable Al2O(g), SiO(g), and N2(g) in the ambient. The γ-AlON is formed by a carbothermal reduction nitridation of the α-Al2O3 and residual active carbon from the resin binder. The 21ReSiAlON and γ-AlON reinforce the composite brick and improve its high temperature performance accordingly. Its service life is 110% that of the magnesia-chrome bricks used in the same period. The reaction model was also established.
1. Introduction The main functions of secondary steelmaking refining are removing impurities and inclusions in the steel, degassing, adjusting the chemical composition and homogenization. RH (the initials of Ruhrstahl and Hereaeus) refining, which is a vacuum degassing method, is one of the most important and widely used methods in secondary steelmaking refining [1]. It has the advantages of high efficiency, good suitability for batch processing, use of existing equipment, and easy operation [2,3]. However, the requirements for the equipment lining materials are very demanding due to the versatility of RH vacuum refining technology. Magnesia–chrome brick has good high temperature performance, superior thermal shock stability, and good resistance to slag corrosion. Consequently, magnesia-chrome is the main material used in the low vessel and snorkels of the RH degasser at present [4–7]. However, the service life of magnesia-chrome bricks will significantly decrease due to the highly basic slag when they are used in the production of silicon steel. In addition, the trivalent chromium in the
∗
magnesia–chrome brick will convert to hexavalent chromium under operational conditions, which are harmful to both human beings and the environment. Therefore, it is vital and meaningful to develop new environmentally friendly chrome-free materials. Oxide refractories, oxide–carbon refractories, and non-oxide–oxide refractories represent three milestones in the development of refractories. Non-oxide–oxide refractories have a substantially superior high-temperature strength and better oxidation resistance than oxide–carbon refractories, better thermal shock resistance than pure oxide refractories, and good corrosion resistance to metallurgical slag and alkali environments [8–10]. Therefore, environmentally friendly nonoxide–oxide refractories are currently the most promising material for high-performance steel production, and related studies have been investigated [11–22]. AleAl2O3 refractory has attained satisfactory trial results when used for a sliding gate [23,24] and RH degasser. The transient plastic phase process is an effective way to prepare non-oxide–oxide composites [23,25,26], where metal phases convert to high-performance non-oxide phases in situ at high temperatures and in
Corresponding author. E-mail address: [email protected] (Y. Li).
https://doi.org/10.1016/j.ceramint.2019.02.202 Received 21 November 2018; Received in revised form 18 February 2019; Accepted 27 February 2019 Available online 04 March 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
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a reducing atmosphere. Correspondingly, the temperature during an RH refining operation is relatively high, approximately 1650–1700 °C, the oxygen partial pressure is very low, and the conditions present a gradient change for the low vessel brick from the hot face to the interior. Therefore, it is considered that a high-performance functional gradient non-oxide–oxide material is obtained during an RH refining operation by using metal Al and Si as raw materials. The Al and Si in the hot zone convert to non-oxide reinforced phases, whereas the Al and Si in the interior zone remain as free form and act as metal plastic phases. Consequently, in this work, an AleSieMgAl2O4eAl2O3 composite brick was prepared and evaluated in the low vessel of RH degasser. The phase composition and microstructure of the AleSieMgAl2O4eAl2O3 composite after use was analysed by X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive spectroscopy (EDS). The reaction model was established. 2. Experiments Fused corundum (5–3 mm, 15 wt%; 3–1 mm, 15 wt%; < 0.074 mm, 17 wt%), tabular alumina (3–1 mm, 15 wt%; 1–0 mm, 20 wt%), fused magnesia-alumina spinel (3–1 mm, 5 wt%; 1–0 mm, 3 wt %; < 0.074 mm, 5 wt%), metallic aluminium powders (4 wt%), and silicon powders (1 wt%) were used as raw materials, and thermosetting phenolic resin (PF5323, 3.5 wt% relative to the total weight) was added as a binding agent. The powders were grounded and shaped under a 630t friction presser. After natural dehydration for 24 h, the AleSieMgAl2O4eAl2O3 composite bricks were dried in a tunnel kiln for 12 h at a temperature of 200 °C. The chemical compositions of the different raw materials are shown in Table 1. The prepared AleSieMgAl2O4eAl2O3 composite bricks were applied to the low vessel of the RH degasser. As shown in Fig. 1, the erosion zone and reaction zone (numbered 1 and 2, respectively) of the brick after use were ball-milled into powders for XRD measurements. The patterns were collected on a Rigaku instrument using Cu Kα radiation (wavelength: 1.5415 nm). A scan speed of 5°/min in continuous mode was applied, and the data were collected in a 2θ range of 10–90°. The microstructure was observed by an environmental scanning electron microscope (SEM) (Quanta FEG450, FEI) equipped with an energydispersive spectroscope (EDS) to analyse the elements in specific areas. A gold coating was used to make the sample electrically conductive enough for SEM analysis after being polished. 3. Results and discussion 3.1. Phase evolution The phases of the AleSieMgAl2O4eAl2O3 composite brick after use in the RH degasser show a functional gradient distribution with distance. In particular, the major phases in the erosion zone are Al15.44Fe6.16Mg2.32O32.00 (Mg-hercynite spinel solid solution), α-Al2O3, and a minor amount of Al3Fe5O12 and SiO2 are also detected. The phases in the reaction zone are α-Al2O3, Mg0.388Al2.408O4 spinel, γAlON, 21ReSiAlON (SiAl6O2N6), a minor amount of SiC, and C, which means that Al and Si in the reaction zone transformed to 21ReSiAlON and SiC during the RH refining operation. The phases in the interior are close to those in the original AleSieMgAl2O4eAl2O3 composite. The
Fig. 1. Samples for XRD and SEM characterization.
XRD patterns of the erosion zone and reaction zone are shown in Fig. 2. The XRD diffraction results show that after use, the original AleSieMgAl2O4eAl2O3 composite was successfully transformed into a gradient material with an anti-erosion phase–reinforced phase–plastic phase distribution.
3.2. Microstructure analysis Corresponding to the results of the phase composition, the microstructure of the AleSieMgAl2O4eAl2O3 composite brick after use also varies with the distance from the working face. According to the evolution of the microstructure, three typical zones are generally present: an erosion zone (0–1.8 cm), reaction zone (1.8–5.0 cm), and original zone (> 5.0 cm). The microstructure of the erosion zone and the reaction zone is shown in Figs. 3 and 4, respectively. Fig. 3 shows the microstructure of the erosion zone (0–1.8 cm). In this zone, the overall structure is relatively loose at room temperature. It is mainly comprised of bright white octahedral particles with a particle size of approximately 50 μm [Fig. 3(c)] and greyish white matter [Fig. 3(d)]. The results of the EDS analysis are listed in Table 2. Combining the results of XRD, the bright white octahedral particles containing Al, Fe, Mg, and O in Fig. 3(c) are Al15.44Fe6.16Mg2.32O32.00 (Mghercynite spinel solid solution), the white substance marked as “2” in Fig. 3(f) is Al3Fe5O12, while the grey substance marked as “3” is Al2O3. Considering the results of XRD, SEM, and EDS, it can be seen that the Fe from the RH operation exists in this zone. The microstructure changes 1.8 cm away from the working surface. Fig. 4 shows the microstructure of the reaction zone (1.8–5.0 cm). As shown in Fig. 4(a)–4(b), the large particles are tightly bound to the matrix. A large amount of flaky material containing Al, Si, O and N is formed in the matrix, which interpenetrate and overlap with each other, forming a network structure [Fig. 4(c) and (e)]. It is suggested that this zone is dominated by gas phase reactions. Combining the results of XRD, the flakes are 21ReSiAlON. As shown in Fig. 4(f), some 21ReSiAlON flakes fill in the pores to enhance the bridging effect. Furthermore, spinel structure materials with a particle size of approximately 2 μm formed and are pinned on the surface of the α-Al2O3 particles, and the EDS results show that they are mainly composed of Al, O, and N elements, i.e., γ-AlON. Fig. 5 shows the morphology of the AleSieMgAl2O4eAl2O3 composite brick after use in the RH degasser at a low magnification. Based on the above analysis, a model of the evolution of the structure of the AleSieMgAl2O4eAl2O3 composite brick before and after use is shown
Table 1 Chemical composition of experimental raw materials (wt.%).
Fused corundum Fused magnesia-alumina spinel Tabular alumina Al powders Si powders
Al2O3
MgO
Fe2O3
SiO2
CaO
Al
Si
Ca
Fe
99.21 76.25 99.20 / /
/ 22.18 / / /
0.04 0.17 / / /
0.03 0.62 / / /
0.27 0.38 / / /
/ / / 99.50 /
/ / / 0.20 98.00
/ / / / 0.30
/ / / 0.15 0.60
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Fig. 2. XRD patterns of the erosion zone and the reaction zone (No. 1 and 2, respectively) of the AleSieMgAl2O4eAl2O3 composite brick after use in the RH degasser.
in Fig. 6. As shown in Fig. 6, a protective layer consisting of a Mghercynite spinel solid solution is formed in the erosion zone, a 21RSiAlONe and γ-AlON-bonded Al2O3 composite is formed in the reaction zone, and the inner zone is close to the original AleSieMgAl2O4eAl2O3 composition, forming an “anti-erosion zone–reinforced zone–plastic phase zone” functional gradient composite.
FeO(l) + MgAl 2 O4 (s) → Mg − hercynite spinel solid solution (Al15.44 Fe6.16 Mg 2.32 O32.00)(s) 2Fe(l)+ O2 (g ) = 2FeO(l) =
4. Reaction mechanism of the AleSieMgAl2O4eAl2O3 composite during RH refining operation During the refining process, Ar is blowing through the snorkel inlet so that high-temperature molten steel generates a high-speed circulating flow. FeO from the refining operation has a low melting point and low viscosity and easily penetrates into the brick. At the RH refining temperature (approximately 1650 °C), FeO(l) reacts with MgAl2O4 spinel spontaneously; therefore, a Mg-hercynite spinel solid solution (Al15.44Fe6.16Mg2.32O32.00) is formed, per reaction (1). The Mghercynite spinel solid solution has a high melting point and good erosion resistance, and it provides a protective layer that inhibits further penetration of the slag, which effectively improves the slag erosion resistance of the material as shown in Fig. 6:
ΔGΘ
ΔGΘ = −459400 + 87.45T
(1)
ΔG
(a )2 + RT ln FeO Θ PO2/P
(2)
The added Al(s) and Si(s) in the brick likely play an important role in the formation of the flaky 21ReSiAlON. The possible reaction pathway is listed as follows:
Al(s) = Al(l)
Al(l) = Al(g)
ΔGΘ = 10795 + 11.55T
ΔGΘ = 304640 − 109.50T
(3) (4)
Si(s) = Si(l)
ΔGΘ = 50540 − 30.0T
(5)
Si(l) = Si(g)
ΔGΘ = 395400 − 111.38T
(6)
2Al(l) +
Si(l) +
1 O2 (g ) = Al2O (g ) 2
1 O2 (g ) = SiO(g ) 2
ΔGΘ = −170700 − 49.37T
ΔGΘ = −155230 − 47.28T
(7) (8)
Fig. 3. Microstructure of the erosion zone (0–1.8 cm) in the composite AleSieMgAl2O4eAl2O3 brick after use in the RH degasser: (a), (c) and (e) are the SEM photographs for Mg-hercynite; (b), (d) and (f) are the SEM photographs for Al2O3 and Fe-rich phase.
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Fig. 4. Microstructure of the reaction zone (1.8–5.0 cm) in the composite AleSieMgAl2O4eAl2O3 brick after use in the RH degasser: (a), (b) and (c) are the low magnification photographs; (e), (d) and (f) are the SEM photographs for 21ReSiAlON and γ-AlON.
Significant quantities of metastable Al2O(g) and SiO(g) are formed by the active oxidation of Al and Si. With the formation of the Mghercynite spinel solid solution by FeO(l) and MgAl2O4(s) in the erosion zone, a “sealing wall” is formed. Therefore, the metastable Al2O(g) and SiO(g) are enclosed in the system, and the gas pressure increases. Driven by high temperature, the metastable Al2O(g) and SiO(g) diffuse along the pores and further react with the N2 and C that came from phenolic resin, per reaction (9); therefore, 21ReSiAlON flakes are formed, which interpenetrate and overlap with each other, forming a network reinforcement structure.
Table 2 EDS results for the areas in Figs. 3 and 4. Marked areas
Fig. 3
Fig. 4
Area Area Area Area Area Area
Atomic mole percentage/%
1 2 3 1 2 3
N
O
Mg
Al
Fe
Si
03.20 / / 24.06 08.91 25.20
54.83 54.56 52.55 15.69 50.20 23.69
04.22 / / 02.48 03.05 02.01
23.96 13.09 42.57 49.27 37.84 39.16
13.79 32.35 4.88
/ / / 8.50 / 9.93
/ /
Al2O3 (s) + C(s) + N2 (g) → γ− AlON(s ) + CO(g ) SiO(g) + Al2O(g )+ N2 (g ) + C(s) → 21 R− Sialon(s ) + CO(g )
(9)
The melting points of metallic Al and Si are 660 °C and 1412 °C, respectively. Therefore, at the RH refining temperature, Al(s) and Si(s) will convert to Al(l, g) and Si(l, g), Reactions (3)–(6). Parameters PAl and PSi are calculated as a function of temperature, as shown in Fig. 7. Moreover, Al2O(g) and SiO(g) can also form from reactions (7)–(8) depending on the oxygen partial pressure. During the RH refining operation, the minimum PO2 in the RH degasser is estimated by Equation (2). Because the content of FeO in the refining slag is low, the PO2 in the RH degasser is relatively low. The PO2 – aFeO curve is plotted at 1923 K, as shown in Fig. 8, where lgPO2/PΘ is −14 ∼ −10 when aFeO is 0.001–0.1. Based on this, PAl2O and PSiO are calculated as a function of lgPO2/PΘ, as shown in Fig. 9. From Fig. 9, during RH refining, the PSiO and PAl2O inside the AleSieMgAl2O4eAl2O3 composite brick would probably be in the range of 0.03–10 MPa; that is, they would be 2 to 4 and 4 to 7 orders of magnitude higher than PAl and PSi, respectively.
(10)
Except for the Al, Si, and MgAl2O4 raw materials, there is approximately 45–65% residual carbon from the thermosetting phenolic resin binding agent (3.5 wt%), which is evenly dispersed on the surface of raw material particles. Compared with that for graphite and carbon black, the residual carbon from a resin is much more reactive so that it can easily react with Al2O3 [27,28]. Therefore, with the continuous oxygen consumption by Al and Si, the PO2 in the system continuously decreases; when the partial pressure of oxygen decreases to a certain value, the carbothermal reduction and nitridation reaction between αAl2O3 and residual carbon occurs, and the γ-AlON spinel is formed, per reaction (10). The model for the formation mechanism of γ-AlON and 21ReSiAlON is shown in Fig. 10. 5. Conclusions A novel AleSieMgAl2O4eAl2O3 composite brick was prepared and
Fig. 5. Morphology of AleSieMgAl2O4eAl2O3 composite brick after use in the RH degasser at low magnification. 11278
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Fig. 6. Simplified model of AleSieMgAl2O4eAl2O3 composite brick before and after use in the RH degasser.
Fig. 7. Diagram of PAl(g)/Si(g) – T for reactions (4) and (6).
Fig. 9. Diagram of log (PAl2O(g)/SiO(g)/PO2) – log (PO2/PΘ) at 1923 K for reactions (7) and (8).
(i) In the erosion zone (0–1.8 cm), a Mg-hercynite spinel solid solution protective layer is formed by reaction between MgAl2O4 and FeO, which improves the slag erosion resistance of the material. (ii) In the reaction zone (1.8–5.0 cm), the Al and Si occur an active oxidation, resulting in significant quantities of metastable Al2O(g) and SiO(g). Additionally, 21ReSiAlON flakes form, resulting in a connected network reinforcement structure. The γ-AlON forms by a carbothermal reduction nitridation reaction between the α-Al2O3 and reactive residual carbon from the phenolic resin. The γ-AlON and 21ReSiAlON act as reinforcement phases that contribute to the high temperature performance of the material. (iii) In the original zone (> 5.0 cm), metal Al and Si stay in their original form and act as plastic phases to improve the toughness of the material.
Acknowledgements Fig. 8. Diagram of PO2
–
aFeO at 1923 K for Equation (2).
used in the low vessel of an RH degasser. The brick after use shows a functional gradient with an anti-erosion zone–non-oxide reinforced zone–plastic zone distribution. It achieved good results with a service life that was 110% that of the magnesia-chrome bricks used over the same period.
This work was supported by the National Natural Science Foundation of China under Grant No. 51872023.
Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.ceramint.2019.02.202.
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Fig. 10. Model of the formation mechanism of γ-AlON and 21ReSiAlON in the reaction zone: zone 1, 2 and 3 represent the erosion zone, reaction zone and original zone, respectively.
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