ISIJ International
Online ISSN : 1347-5460
Print ISSN : 0915-1559
ISSN-L : 0915-1559
Regular Article
Thermodynamic Model for Prediction of Slag-Steel-Inclusion Reactions of 304 Stainless Steels
Ying RenLifeng Zhang
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2017 Volume 57 Issue 1 Pages 68-75

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Abstract

A thermodynamic model was developed to predict slag–steel–inclusion reactions of 304 stainless steels. The dissolved aluminum in the steel, the sulfur distribution ratio and the composition of inclusions equilibrated with varying slags were predicted using the current model. The model can also be widely used to predict the liquid fraction of inclusions. For Al-killed 304 stainless steels, the optimized composition of CaO–Al2O3–SiO2–MgO slags is proposed to modify solid Al2O3 inclusions to liquid CaO–Al2O3–MgO at 1873 K. For Si-Mn-killed 304 stainless steels, the optimized composition of CaO-SiO2-MgO-Al2O3-20%CaF2 slags is suggested to suppress the formation of Al2O3–MgO in SiO2–CaO–MnO–MgO–Al2O3 inclusions and lower their melting temperatures.

1. Introduction

As demand for high performance steels increases every year, improving steel cleanliness is a main task of steelmakers.1,2) Non-metallic inclusions in steel have a detrimental effect on the performance of steels, such as their strength, toughness, fatigability, cleanliness, surface finishing, etc.3) A wide range of operating approaches including deoxidation,4,5) calcium treatment,6,7,8,9,10,11) slag refining,12,13,14,15,16,17) and prevention against reoxidation18,19,20,21) are applied throughout the steelmaking processes of stainless steels. Inclusion composition is greatly influenced by the composition of the top slag and the refractory material.22) The precise control of the composition of inclusions in stainless steels can be achieved by choosing the proper top slag composition.12,13,14,15,16,17,23,24)

A large number of thermodynamic models based on thermodynamic and phase equilibrium data for a given system has been developed,25) which can be used to predict the relationship between compositions of slag, steel, and inclusions in relatively simple systems, such as Al–O,26,27,28) Al–Mg–O,29,30,31) Al–Ca–O,32,33,34) Al–Ti–O,35,36,37) Al–Si–Ca–O,38,39) Al–Si–Mn–O,40,41,42) etc. However, the thermodynamic models can hardly be applied to a multicomponent system due to complicated calculations and lack of thermodynamic data. With the application of the thermodynamic software, all available thermodynamic and phase equilibrium data for a system are critically evaluated simultaneously in order to obtain one self-consistent set of model equations for the Gibbs energies which best reproduce the data for all phases as functions of temperature and composition. Meanwhile, the models and optimized model parameters for low-order (binary and ternary) sub-systems can be used to provide good estimates for the lacking data for a multicomponent system.43) Thus, the formation of inclusions at different temperatures and varying steel compositions in a multicomponent system can be apace and accurately predicted by means of thermodynamic software. Holappa et al.44) developed a thermodynamic model using ChemSage to calculate equilibrium oxide and sulfide inclusions in steel and predict the “liquid window” for the formation of liquid calcium aluminates. Jung et al. predicted the evolution of liquid and solid inclusions during the ladle furnace process using Factsage.45) Kang and Lee46) predicted the slag/steel and steel/inclusions relationships in Si-Mn-killed steel using Factsage. The sulfide capacity of Al2O3–CaO–FeO–Fe2O3–MgO–MnO–SiO2–TiO2–Ti2O3 multicomponent slags can be evaluated using Factsage.47)

Most of the reported models using thermodynamic software are used to predict the slag/steel reactions, or steel/inclusion reactions. Whereas, there was few model developed to thermodynamically predict a large number of slag/steel/inclusion reactions. In the current study, a thermodynamic model based on Factsage was developed to achieve the control of steel chemistry and the composition of inclusions through optimizing the composition of refining slag of 304 stainless steels.

2. Thermodynamic Model

There are many reactions involved during the slag refining process, as shown in Fig. 1. In the schematic representation, each reaction represents the following: R1: Slag/Steel reaction, R2: Steel/Inclusion reaction. To simplify the model and calculation, the following assumptions are made in the current thermodynamic model: (1) Slag/steel reaction and steel/inclusion reaction are assumed to reach the equilibrium; (2) The temperature can be maintained at a curtained fixed temperature; (3) The removal of inclusions to slag, Refractory/Slag reaction and Refractory/Steel reaction are not considered in the current model.

Fig. 1.

Schematic diagram of reactions during the slag refining process. (Online version in color.)

In the FactSage software, the Equilib program can be called by the embedded coding called ‘‘Macro Processing.’’ All the input conditions and output can be stored and passed to the different equilibrium calculations or externally to a simple text file or Microsoft ExcelTM worksheets using this macro processing code. A small program can be written using this macro processing code for equilibrium calculations.45) Thus, the calculation process of the current model is as following: (1) The initial conditions (compositions and amounts of slag, steel and inclusions, and temperature) are set up using the Microsoft ExcelTM as an interface. (2) The initial slag reacts with the initial steel; (3) The reacted steel reacts with the initial inclusions; (4) The equilibrated conditions are output to the Microsoft ExcelTM. In the calculation, the reactions are calculated one by one in the order of reaction number and the outputs of the previous equilibrium reaction can be added as stream inputs to further calculations to keep track of reaction heats. The thermodynamic phase equilibria of all reactions were calculated adiabatically using FactSage with FactPS, FToxid and FTmisc databases.43)

3. Verification of Thermodynamic Model

To verify the accuracy of the developed model, experimental data reported in the previous studies is referenced and compared with the calculated results, as shown in Fig. 2. Reported data of the initial slag, steel, and inclusions of 304 stainless is given in Table 1.4,12,23) Figure 2(a) shows the comparison of the predicted dissolved aluminum ([Al]s) and measured [Al]s in 304 stainless steels. The predicted result shows a good agreement with the measured result in the range of [Al]s from several ppm to above 200 ppm. The calculated sulfur distribution ratio (Ls) roughly agrees with the measured results, as shown in Fig. 2(b). The agreement between the calculated and measured concentrations of [Al]s and S in stainless steels demonstrates that the current thermodynamic model can be used to predict the effect of slags on steel chemistry. In Figs. 2(c) and 2(d), the calculated Al2O3 and MgO in inclusions are basically in accordance with experimental values, indicating that the composition of inclusions influenced by slags and steels can be successfully predicted using the current thermodynamic model.

Fig. 2.

Comparison of predicted and measured data of 304 stainless steels.

Table 1. Reported data of the initial slag, steel and inclusions of 304 stainless steels.
AuthorsSlagSteelMass ratio of slag/steelInclusionsTemperature (K)Ref.
BasicityAl2O3 (%)MgO (%)[Al]s (%)TypeT.O. (ppm)
Mizuno et al.1.7–5.02.8–8.56.7–8.70.001–0.241/5Cr2O3351823 K4, 23)
The previous work1.0–2.30.7–202.2–200.00182/25CaO–SiO2–Al2O3–MnO–MgO–TiO2501873 K12)

4. Prediction of Slag–Steel–Inclusion Reactions of 304 Stainless Steels

To optimize the composition of refining slag and improve the cleanliness of 304 stainless steels, a large number of calculations were apace carried out using FactSage macro processing. Based on the analysis of a typical slab of 304 stainless steels, the initial compositions of steel and inclusions in Si-Mn-killed 304 stainless steels are listed in Tables 2 and 3. Since CaO–SiO2–Al2O3–MgO–CaF2 slag is widely used during the slag refining process of Si-Mn-killed 304 stainless steels, the composition of the initial slag is given in Table 4. The contents of CaF2 and MgO are fixed to 20% and 5%, respectively. The CaO, SiO2, and Al2O3 in slags were in a wide range from 0 to 75%. Four hundred calculations were carried out with a 3.75% increment of each component in the CaO–SiO2–Al2O3 ternary phase diagram. The mass ratio of slag/steel is fixed at 1/50.

Table 2. Composition of the initial stainless steel (wt%).
DeoxidationCSiMnSPCrNiNTiCaT.O.[Al]s
Si–Mn0.0480.481.060.0030.02418.118.000.0370.0030.000330.00500.0018
Al0.00200.0500
Table 3. Composition of the initial inclusions (wt%).
DeoxidationAl2O3MgOSiO2CaOMnOTiO2
Si–Mn20.041.6539.108.3726.044.79
Al10000000
Table 4. Composition of the initial slag (wt%).
CaF2MgOCaOSiO2Al2O3
2050–750–750–75

The predicted effects of the initial slag composition on inclusion characteristics and steel chemistry of 304 stainless steels are given in Fig. 3. Figure 3(a) shows the predicted [Al]s in 304 stainless steels equilibrated with various slags. With the requirement of the liquid slag during the refining process, the liquidus of the slags at 1873 K is illustrated as the outer contour line of colour zone in the ternary phase diagram. Different colours mean various [Al]s in stainless steels. The [Al]s in stainless steels dramatically declines with decreasing slag basicity since [Al]s in stainless steels is expected to react with the unstable SiO2 in the low basicity (CaO/SiO2) slag as given in Eq. (1). With the slag basicity lower than 2.0, the addition of Al2O3 in slag enhances [Al]s. With the slag basicity higher than 2.0, there is lower [Al]s in steel equilibrated with the high Al2O3 slag. It can be inferred that low [Al]s in 304 stainless steels can be achieved by using the low basicity CaO-SiO2-based slag. The high basicity CaO-Al2O3-based slag can increase [Al]s in 304 stainless steels.   

4[Al]+3 (Si O 2 ) slag =3[Si]+2 (A l 2 O 3 ) slag (1)
Fig. 3.

Effects of the initial slag composition on inclusion characteristic and steel chemistry of 304 stainless steels. (a) [Al]s in 304 stainless steels, (b) (S)/[S] of 304 stainless steels, (c) Al2O3 in inclusions in 304 stainless steels, (d) MgO in inclusions in 304 stainless steels, (e) Liquid fraction of inclusions. (Online version in color.)

The calculated sulfur distribution ratio, (S)/[S], of 304 stainless steels equilibrated with various slags are given in Fig. 3(b). The desulfurization reaction is given in Eq. (2). Increasing the slag basicity in slag can significantly enhance the (S)/[S] of 304 stainless steels. The (S)/[S] increases with decreasing SiO2 in slag. Therefore, the CaO-Al2O3-based slag is recommended to promote the desulfurization of 304 stainless steels.   

[O]+ ( S 2- ) slag =[S]+ ( O 2- ) slag (2)

The Al2O3-rich inclusions are harmful to both the quality of steel products and the castability of stainless steels due to the high melting point and hardness.12) The effect of slag composition on Al2O3 in inclusions in 304 stainless steels is shown in Fig. 3(c). The unstable oxides in inclusions are reduced by [Al]s in 304 stainless steels. The main reactions of the formation of Al2O3 in inclusions are given in Eqs. (3) and (4). The Al2O3 in inclusions distinctly rises up with increasing slag basicity. It is mainly because high basicity slag increases [Al]s in 304 stainless steels, promoting the formation of Al2O3 in inclusions. It can be deduced that the low basicity CaO-SiO2-based slag is beneficial to lower Al2O3 in inclusions.   

4[Al]+3 (Si O 2 ) inclusion =3[Si]+2 (A l 2 O 3 ) inclusion (3)
  
2[Al]+3 (MnO) inclusion =3[Mn]+ (A l 2 O 3 ) inclusion (4)

Figure 3(d) shows the relationship between slag composition and MgO in inclusions in 304 stainless steels. It can be seen that MgO in inclusions obviously goes up with increasing slag basicity. It should be noted that the evolution of MgO in inclusions has a similar tendency with that of the predicted [Al]s in 304 stainless steels in Fig. 3(a) due to the reduction of MgO in slag by [Al]s in 304 stainless steels.48) With the main reduction reactions between inclusions and the [Mg] in Eqs. (5) and (6), MgO in inclusions can be lowered by using the low basicity CaO-SiO2-based slag.   

2[Mg]+ (Si O 2 ) inclusion =[Si]+2 (MgO) inclusion (5)
  
[Mg]+ (MnO) inclusion =[Mn]+ (MgO) inclusion (6)

Since the mass of the formed species in inclusions can be output, the liquid fraction of inclusions at 1873 K can be calculated, as shown in Fig. 3(e). It can be seen that the generated inclusions are almost fully liquid with slag basicity below 1.0. There is more solid phase formed in inclusions with increasing basicity and Al2O3 in slag. The change of the liquid fraction of inclusions has a similar tendency with that of Al2O3 in inclusions, indicating that the melting temperature of inclusions is significantly influenced by Al2O3 in inclusions.

5. Optimization of Slag Composition to Control Inclusions in 304 Stainless Steels

5.1. Optimization of the CaO–Al2O3–SiO2–MgO Slag for Al-killed 304 Stainless Steels

There are two typical process routes to control inclusions in 304 stainless steels. The first route is to lower the amount of oxide inclusions by Al deoxidation. Meanwhile, the slag refining or calcium treatment is commonly used to modify solid alumina inclusions to the liquid calcium aluminate and avoid the nozzle clogging during the continuous casting process. It has been reported that the CaO–Al2O3–SiO2–MgO slag has excellent refining properties, such as low oxidation potential, low melting temperature and low viscosity at steelmaking temperature.15,16) Therefore, the compositions of CaO–Al2O3–SiO2–MgO slags were optimized using the current thermodynamic model to increase the CaO/Al2O3 in inclusions at 1873 K and lower the melting temperature of inclusions in Al-killed 304 stainless steels. The compositions of the initial Al-killed 304 stainless steel and inclusions in the calculations are listed in Tables 2 and 3. The mass ratio of slag and steel is 1/50.

Figure 4 shows the predicted effect of the CaO–Al2O3–SiO2–MgO slag on the CaO/Al2O3 in inclusions in Al-killed 304 stainless steels. It can be seen from Fig. 4(a), with the CaO/Al2O3 in the CaO–Al2O3–SiO2 slag increasing from 0.5 to 1.5, the CaO/Al2O3 in inclusions obviously goes up. It should be noted that the CaO/Al2O3 in inclusions remains unchanged with the CaO/Al2O3 in slag reaching roughly 1.5–2.0 since CaO in the molten slag is oversaturated and the solid CaO phase is generated in the molten slag at 1873 K. To increase the CaO/Al2O3 in inclusions and avoid the formation of solid phases in the molten slag, the CaO/Al2O3 in slag at inflection points of the curves are recommended. It can be seen that the addition of SiO2 in slag can dramatically decrease the CaO/Al2O3 in inclusions. Figure 4(b) shows the relationship between the CaO/Al2O3 in inclusions and the CaO/Al2O3 in slags with varying MgO contents. Increasing MgO in slag from 0 to 10% can slightly enhance the CaO/Al2O3 in inclusions.

Fig. 4.

Effect of the CaO–Al2O3–SiO2–MgO slag on the CaO/Al2O3 in inclusions in Al-killed 304 stainless steels. (Online version in color.)

Since Si is alloyed in the molten steel, there is a small amount of SiO2 formed in slag. Typically, the predicted effect of a CaO-Al2O3-4%SiO2-2%MgO slag on the composition of inclusions at 1873 K is shown in Fig. 5(a). The decrease of CaO/Al2O3 in the CaO-Al2O3-4%SiO2-2%MgO slag dramatically promotes the formation of solid Ca2Mg2Al28O45, CaMg2Al16O27, CaAl4O7, CaAl12O19 and Al2O3 inclusions. The inclusions can be fully modified to liquid inclusions by increasing the CaO/Al2O3 in slag to above 1.4 at 1873 K. The calculated liquidus and solidus of formed oxide inclusions are shown in Fig. 5(b). The solidus of inclusions is roughly unchanged with the CaO/Al2O3 in the CaO-Al2O3-4%SiO2-2%MgO slag higher than 1.3. The liquidus of inclusions significantly declines with the increase of the CaO/Al2O3 in slag. The liquidus of inclusions can be lowered to 1479 K with a CaO/Al2O3 ratio of 1.6 in the CaO-Al2O3-4%SiO2-2%MgO slag, which agrees with the recommended ratios of the CaO/Al2O3 in the CaO–Al2O3–SiO2–MgO slag to increase the CaO/Al2O3 in inclusions.

Fig. 5.

Effect of the CaO-Al2O3-4%SiO2-2%MgO slag on the composition and melting temperature of inclusions in Al-killed 304 stainless steels. (Online version in color.)

To optimize the composition of CaO–Al2O3–SiO2–MgO slag for Al-killed 304 stainless steels, a large number of calculations were apace carried out using FactSage macro processing. The ratios of CaO/Al2O3 in slags with varying SiO2 and MgO are recommended to increase the CaO/Al2O3 in inclusions, lower the melting temperature of inclusions and avoid the formation of solid phases in the molten slag, as shown in Fig. 6. It can be seen that the proposed CaO/Al2O3 in slags distinctly goes up from 1.5 to 2.0 with an increase of SiO2 in slag from 2% to 10% since the addition of SiO2 in slag decreases CaO/Al2O3 in inclusions in Fig. 5(a). Meanwhile, the proposed CaO/Al2O3 in slags can hardly be influenced by MgO in slag in the range from 4% to 10%, which is in accord with the results given in Fig. 5(b).

Fig. 6.

Proposed CaO/Al2O3 in the CaO–Al2O3–SiO2–MgO slag for Al-killed 304 stainless steels. (Online version in color.)

5.2. Optimization of the CaO–SiO2–Al2O3–MgO–CaF2 Slag for Si-Mn-killed 304 Stainless Steels

The Al2O3-MgO-rich inclusions have a poor deformability, causing defects on the surface of stainless steel products due to their poor deformability.3) It has been reported that lowering the Al2O3–MgO in Al2O3–SiO2–CaO–MnO inclusions can significantly decrease the melting temperature and improve deformability of inclusions.49) In order to avoid the generation of the nondeformable Al2O3-MgO-rich inclusions, the Si–Mn deoxidation was introduced. However, the aluminum in the added ferrosilicon alloy can significantly promote the formation of spinel inclusions in Si-Mn-killed stainless steels.5) Typically, the CaO–SiO2–Al2O3–MgO–CaF2 slag is used to lower [Al]s in steel and suppress the formation of the Al2O3–MgO in inclusions.12,17) In the current study, the CaO–SiO2–Al2O3–MgO–CaF2 slag was optimized using the current thermodynamic model to lower the Al2O3–MgO in inclusions at 1873 K and decrease the melting temperature of inclusions in Si-Mn-killed 304 stainless steels. The compositions of the initial Si-Mn-killed 304 stainless steel and inclusions in the calculations are given in Tables 2 and 3. The CaF2 in the initial slag is fixed at 20%. The mass ratio of slag and steel is 1/50.

Figure 7 shows the predicted the effect of the CaO–SiO2–Al2O3–MgO–CaF2 slag on Al2O3–MgO in inclusions in Si-Mn-killed 304 stainless steels. It can be seen from Fig. 7(a), the Al2O3–MgO in inclusions rises up with an increase of the CaO/SiO2 in the CaO–SiO2–Al2O3–MgO–CaF2 slag from 0.8 to 1.8. Meanwhile, adding Al2O3 in slag can promote the formation of the Al2O3–MgO in inclusions. As shown in Fig. 7(b), the addition of MgO in slag can increase the Al2O3–MgO in inclusions. It should be noted that the breakpoints of the relationship curves are mainly caused by the saturation of MgO in the molten slag at 1873 K. Since low basicity slag increases the erosion of MgO refractory,12) the slag can lower the Al2O3–MgO in inclusions to below 35%, which is suggested for Si-Mn-killed 304 stainless steels.

Fig. 7.

Effect of the CaO–SiO2–MgO–Al2O3–CaF2 slag on the Al2O3–MgO in inclusions in Si-Mn-killed 304 stainless steels. (Online version in color.)

The Al2O3 and MgO in the CaO–SiO2–Al2O3–MgO–CaF2 slag can hardly be completely avoided due to the oxidation of [Al]s in the molten steel, the removal of Al2O3-rich inclusions, and the erosion of MgO refractory. Typically, the predicted effect of a CaO-SiO2-5%MgO-2%Al2O3-20% CaF2 slag on the composition of inclusions at 1873 K is shown in Fig. 8(a). The formed inclusions are fully liquid with the CaO/SiO2 in slag ranging from 0.8 to 1.8 at 1873 K. It can be seen that the increasing rate of Al2O3–MgO in inclusions apparently rises up with the CaO/SiO2 in slag exceeding 1.4. In Fig. 8(b), the calculated liquidus and solidus of formed oxide inclusions show an increasing trend with the CaO/SiO2 in CaO-SiO2-5%MgO-2%Al2O3-20%CaF2 slag rising up from 0.8 to 1.8. Since the low basicity slag can increase the erosion of the MgO refractory,12) the CaO-SiO2-5%MgO-2%Al2O3-20%CaF2 slag with a CaO/SiO2 ratio of roughly 1.4 is proposed to lower the melting temperature of inclusions in Si-Mn-killed stainless steels.

Fig. 8.

Effect of the CaO-SiO2-5%MgO-2%Al2O3-20% CaF2 slag on the composition and melting temperature of inclusions in Si-Mn-killed 304 stainless steels. (Online version in color.)

To optimize the composition of CaO-SiO2-MgO-Al2O3-20%CaF2 slag for Si-Mn-killed 304 stainless steels, a large number of calculations were apace carried out using FactSage macro processing. The ratios of CaO/SiO2 in slags with varying Al2O3 and MgO are recommended to reduce the Al2O3–MgO in inclusions to below 35%, lower the melting temperature of inclusions, and decrease the erosion of MgO refractory, as shown in Fig. 9. It can be seen that the proposed CaO/SiO2 in slags significantly declined from 1.6 to 0.6 with the increases of Al2O3 and MgO in slag from 2% to 10% since the addition of Al2O3 and MgO in slag can promote the formation of Al2O3–MgO in inclusions as shown in Fig. 7.

Fig. 9.

Proposed CaO/SiO2 in the CaO-SiO2-Al2O3-MgO-20% CaF2 slag for Si-Mn-killed 304 stainless steels. (Online version in color.)

6. Conclusions

(1) A thermodynamic model was developed using Factsage macro processing to investigate the effect of slag composition on the cleanliness of 304 stainless steels. The calculated results show a good agreement with the experimental data from the literature.

(2) The CaO-Al2O3-based slag can modify inclusions to CaO–Al2O3–MgO inclusions in Al-killed 304 stainless steels. The addition of SiO2 in the CaO–Al2O3–SiO2–MgO slag decreases the CaO/Al2O3 in inclusions. The CaO/Al2O3 in inclusions can hardly be influenced by MgO in the CaO–Al2O3–SiO2–MgO slag. The composition of the CaO–Al2O3–SiO2–MgO slag is optimized to increase the CaO/Al2O3 in inclusions and lower the melting temperature of inclusions.

(3) The CaO-SiO2-CaF2-based can modify inclusions to Al2O3-MgO-poor inclusions in Si-Mn-killed 304 stainless steels. The addition of Al2O3 and MgO in the CaO-SiO2-MgO-Al2O3-20%CaF2 slag can promote the formation of the Al2O3–MgO in inclusions. The CaO/SiO2 in slags with varying Al2O3 and MgO in slag is proposed to lower the Al2O3–MgO in inclusions and the melting temperature of inclusions.

Acknowledgements

The authors are grateful for support from the National Science Foundation China (Grant No. 51274034, No. 51334002, and No. 51404019), Beijing Key Laboratory of Green Recycling and Extraction of Metals (GREM), the Laboratory of Green Process Metallurgy and Modeling (GPM2) and the High Quality steel Consortium (HQSC) at the School of Metallurgical and Ecological Engineering at University of Science and Technology Beijing (USTB), China.

References
 
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