Deoxidation Mechanism of Al-Killed Steel during Industrial Refining Process

Abstract

Deoxidation is a key step for steel refining, and it would influence the cleanliness of final product. In present study, the measured and calculated oxygen activities in Al-killed bulk steel were compared according to industrial trails, and the Fe–O equilibrium at the interface between slag and steel was also concerned by thermodynamic analysis. Finally, the deoxidation mechanism of Al-killed during industrial refining was proposed. It is found that the activity of oxygen in bulk steel is controlled by Al content, since the activity of oxygen in bulk steel is very close to the equilibrium value as the activity of Al₂O₃ is treated as unity. In order to keep low oxygen in molten steel, a certain Al content needs to be guaranteed. At slag-steel interface, the activity of oxygen is controlled by FeO, which is much larger than that of bulk steel, so the oxygen would transfer from slag into bulk steel. Slagging therefore presents its importance for deoxidation to reduce the oxygen transfer. Besides, the essence of SiO₂ supplying oxygen is Al consumption in bulk steel, which gives rise to the increase of dissolved oxygen. So, stable refining slag is also very important to obtain low oxygen steel, and the slag basicity needs to be no smaller than 3 to reduce the Al consumption caused by SiO₂ reduction. In present study, the basicity of 3–4 is recommended to refine low oxygen Al-killed steel.

1. Introduction

It is well known that oxides are the main non-metallic inclusions in steel, and the cleanliness of steel is usually reflected by total oxygen content. Many studies support the quality of steel is closely related to its total oxygen content, therefore reducing oxygen content becomes one of the main tasks for steelmaking.

During secondary steelmaking, aluminum or aluminum alloy is a widely used deoxidizer due to its strong reductibility and economic efficiency, and there are a lot of research achievements about Al deoxidation in the past decades. In general, most scholars believed that the refining slag would equilibrate with liquid steel,^1,2) and the slag components would influence the activities of deoxidation products. So, controlling slag composition to reduce the activity of deoxidation product can improve the deoxidation efficiency, which is called as slag deoxidation.³⁾ Based on this rule, many scholars^1,2,3,4,5,6) investigated the refining slag to obtain lower activity of deoxidation product, including Al₂O₃ which is the main deoxidation product in Al-killed steel. However, Suito et al.¹⁾ was also aware of that the calculated oxygen activity was smaller than industrial measured data, when the activity of Al₂O₃ in slag was used to calculate the dissolved oxygen activity in Al-killed bulk steel. Besides, Ekengård⁷⁾ and Björklund et al.⁸⁾ also pointed out the calculated values were always lower than measured data. In fact, most of researcher studied the Al deoxidation in laboratory under the ideal experimental conditions, which would be different from the real industrial production. Therefore, the deoxidation mechanism of Al-killed steel during industrial refining needs to be studied further.

In present study, the measured and calculated oxygen activities in Al-killed bulk steel were compared according to thermodynamic calculation and industrial measurements, and the Fe–O equilibrium at slag-steel interface was also concerned by thermodynamic analysis. Based on the results, the deoxidation mechanism of Al-killed during industrial refining was proposed. Besides, combined with the proposed mechanism, the consideration of refining slag were also discussed for choosing suitable refining slag.

2. Deoxidation Reactions

For Al-killed steel, the dissolved oxygen activity is controlled by Eq. (1). Equation (2) expresses the equilibrium constant K of Eq. (1). It is well known that K is a constant related to temperature, so if the temperature is certain, K would not have any change. According to Eq. (2), it is easy to know that the decrease of Al₂O₃ would drive the reaction in Eq. (1) to the right direction.   

$$\text{2[Al]}+\text{3[O]}={\text{(Al}}_{\text{2}}{\text{O}}_{\text{3}}\text{)}$$(1)

  

$$K_1=\frac{a_{{\text{Al}}_{\text{2}}{\text{O}}_{\text{3}}}}{a_{[\text{Al}]}^2\cdot a_{[\text{O}]}^3}$$(2)

There were a lot of studies on Al–O equilibrium, and different data have been cited by many researchers. The frequently used thermodynamic data are listed as shown in Eqs. (3), (4), (5), (6).   

$$log{K}_1=\frac{64\mathrm{\hspace{0.17em} }000}{T}-20.57$$(3) ⁹⁾

  

$$log{K}_1=\frac{45\mathrm{\hspace{0.17em} }300}{T}-11.62$$(4) ¹⁰⁾

  

$$log{K}_1=\frac{47\mathrm{\hspace{0.17em} }400}{T}-12.32$$(5) ¹¹⁾

  

$$log{K}_1=\frac{62\mathrm{\hspace{0.17em} }780}{T}-20.17$$(6) ¹²⁾

In order to compare these data, the relationship between temperature T and equilibrium constant K is shown in Fig. 1. It can be seen that at the steelmaking temperature (1500–1700°C), these data are very close to each other. That means the calculated values would not be very scatter.

Fig. 1.

Relationship between K₁ and T.

After BOF, some BOF slag would be poured into ladle during tapping. FeO is one of the main components in BOF slag, therefore at the beginning of deoxidation, the FeO content in refining slag would be very high. Usually, the equilibrium relationship between FeO and liquid steel can be expressed as Eq. (7). In fact, the liquid Fe activity can be considered as unity, so according to Eq. (8), the dissolved oxygen activity is only related to the FeO activity at a certain temperature. In industrial practice, some reductants such as Al particles are commonly added into ladle for slagging with the purpose of reducing FeO content and controlling the oxygen transferring from slag into liquid steel.   

$$\text{[O]}+[\text{Fe]}=\text{(FeO)}$$(7)

  

$$K_7=\frac{a_{\text{FeO}}}{a_{[\text{O}]}\cdot a_{\text{[Fe]}}}$$(8)

  

$$\mathrm{\Delta }{G}_7^{\mathrm{\Theta }}=-117\mathrm{\hspace{0.17em} }733.7+49.85T\hspace{1em}\text{(J/mol)}$$(9) ^13,14)

Based on the above thermodynamic equations, the deoxidation of Al-killed steel mainly includes two parts: One is deoxidation in bulk steel (bulk deoxidation) and the other one is deoxidation at slag (slagging). Therefore, these two sides need to be controlled in industrial practice.

Except for the unstable oxides such as FeO and MnO, SiO₂ is also considered as an oxygen source which would supply oxygen to liquid steel. The reaction is listed as shown in Eq. (10). If the SiO₂ is unstable in slag, the reaction would occur with formation of Al₂O₃. It is known from Eqs. (10), (11) that lower activity of SiO₂ would help to control the reaction. So, suitable slag composition is also very important for deoxidation.   

$${\text{3(SiO}}_{\text{2}}\text{)}+\text{4[Al]}={\text{2(Al}}_{\text{2}}{\text{O}}_{\text{3}}\text{)}+\text{3[Si]}$$(10)

  

$$K_{10}=\frac{a_{{\text{Al}}_{\text{2}}{\text{O}}_{\text{3}}}^2\cdot a_{[\text{Si}]}^3}{a_{{\text{SiO}}_{\text{2}}}^3\cdot a_{[\text{Al}]}^4}$$(11)

  

$$\mathrm{\Delta }{G}_{10}^{\mathrm{\Theta }}=-703\mathrm{\hspace{0.17em} }190+121.9T\hspace{1em}\text{(J/mol)}$$(12) ¹⁵⁾

3. Industrial Trails of Deoxidation

3.1. Steel Grades for Industrial Trails

Four steel grades were considered in present work, namely SCM435, GCr15, 55SiCrA and pipeline steel. These steel grades include low carbon steel (pipeline steel), middle carbon steel (SCM435) and high carbon steel (GCr15 and 55SiCrA), and their compositions are listed in Table 1. The data of pipeline steel were from Ref. 16).

Table 1. Chemical compositions of different steel grades (mass%).

Steel grade	C	Si	Mn	Cr	Mo	Al
SCM435	0.35	0.19	0.74	1.01	0.19	0.032
GCr15	1.00	0.25	0.30	1.47	–	0.011
55SiCrA	0.56	1.42	0.67	0.67	–	0.006
Pipeline steel16)	0.04	0.14	1.83	–	–	0.037

3.2. Description of Industrial Trails

The trail of pipeline steel was described in Ref. 16). Except for the pipeline steel, the other industrial trails were carried out in a steel plant, and these steel grades were produced by the process of hot metal pre-desulfurization → Basic Oxygen Furnace (BOF) → Ladle Furnace (LF) → RH → Continuous Casting (CC). During the tapping of BOF, most alloys, refining fluxes and Al blocks were added into the ladle for early slag formation. During LF refining, some fluxes and Al particles were also added for slagging, and argon bottom blowing was used to stir liquid steel. In RH refining, no alloy and flux were added. The time for LF refining was about 40 min, while the RH treat time was about 25 min. After RH, the dissolved oxygen activity was measured by oxygen probe. These refining slags are shown in Table 2.

Table 2. Slag compositions of different steel grades after refining (mass%).

Steel grades	CaO	SiO2	Al2O3	MgO	FeO	Basicity
SCM435	46–55	8–15	24–30	5–10	0.4–0.9	3–5
GCr15	47–55	10–16	20–26	6–11	0.4–0.9	3–5
55SiCrA	45–52	11–18	21–29	5–10	0.5–1.0	3–4
Pipeline steel16)	48–60	5–9	24–34	9–12	0.5–0.9	7–10

4. Results and Discussion

4.1. Measured and Calculated Oxygen Activity in Bulk Steel

Before calculation, the activity of Al₂O₃ in slag needs to be confirmed at first. Because different steel grades are refined by different slags, and each heat is also different even though they are the same steel grades, it is not convenient to obtain the activities of Al₂O₃ for each heat. In order to have a evident comparison, the different slags were considered as CaO-Al₂O₃-SiO₂-x%MgO system, and the the compositions were marked in the phase diagram of CaO-Al₂O₃-SiO₂-x%MgO system as shown in Fig. 2(a). Meanwhile, some data from industrial plants and references were also marked in Fig. 2(a). It can be seen from Fig. 2(a) that in the pseudo-ternary CaO–Al₂O₃–SiO₂ system, the compositions of refining slags mainly distributed in the range of CaO 50–70%, SiO₂ 0–20% and Al₂O₃ 20–40%, namely the yellow region in Fig. 2(a). Based on this region and combined the real contents of MgO in refining slags, CaO-Al₂O₃-SiO₂-5%MgO system was chosen to calculate the iso-activity lines of Al₂O₃ according to the thermodynamic software FactSage 6.4.¹⁷⁾

Fig. 2.

Measured data of refining slags from different steel plants.

The iso-activity lines of Al₂O₃ are shown in Fig. 2(b). The composition region of industrial refining slags is also shown in Fig. 2(b). It is easy to know that the activity of Al₂O₃ is very low in this region, and the largest value is about 0.05. Although the MgO content is considered as 5%, MgO is a basic oxide and would react with Al₂O₃ to generate other complex oxides, indicating that more MgO content would decrease the activity of Al₂O₃. Therefore, the activity of Al₂O₃ in real refining slags would be much less than 0.05.

If slag-steel reaction reaches equilibrium, the reaction of Eq. (1) would also reach equilibrium and the activity of Al₂O₃ would be the same as the activity in slag. The activity of Al in bulk steel can be calculated by Eq. (13). In the calculation, the interaction coefficients are listed in Table 3.   

$$lg{a}_{\text{Al}}=\sum e_{\text{Al}}^j[\text{mass}\%j]+lg[\text{mass}\%\text{Al}]$$(13)

Table 3. Interaction coefficients used in thermodynamic calculations.²⁰⁾

j	C	Si	Mn	Cr	Mo	Al	O
Al	0.091	0.056	0.035	0.0096	–	0.045	–1.98

The activity of Al₂O₃ was assumed as 0.05 here, then the equilibrium lines for different steel grades were calculated as shown in Fig. 3. Especially, all the thermodynamic data in Eqs. (3), (4), (5), (6) were used in order to decrease the deviation caused by non-suitable thermodynamic data. At the same time, more than 400 sets of measured data were obtained and marked in Fig. 3.

Fig. 3.

Comparison between measured and calculated oxygen activity ( $a_{{\text{Al}}_{\text{2}}{\text{O}}_{\text{3}}}$ = 0.05).

It can be seen from Fig. 3 that the calculated lines from different equations of equilibrium constant have a small deviation. In fact, this deviation is acceptable due to the different thermodynamic data, and these lines can be seen as approximate lines. Most of all, the measured data are much larger than the calculated values, although the activity of Al₂O₃ was assumed as the largest value in slag. In Fig. 3(b), some of data seem to meet the calculated values, but most data are far from the calculated values. Moreover, the real activity of Al₂O₃ is much smaller than 0.05, so the measured data would be also larger than the calculated values. It implies that the reaction at steel-slag interface would not reach equilibrium.

Suito et al.¹⁾ also believed that the oxygen activity in bulk steel is controlled by Eq. (1) when steel-slag reaction got equilibrium. Therefore, the activity of Al₂O₃ in slag was still used when the dissolved oxygen activity was calculated, and they found that the calculated oxygen activity was less than 1 mass ppm, which was much smaller than industrial data. Ekengård⁷⁾ and Björklund et al.⁸⁾ investigated the difference between the calculated and measured values, and also pointed out that the calculated values were always lower than measured data during Al-killed steel refining. It means that during Al-killed steel refining process, such phenomenon is not a chance.

Besides, the equilibrium lines were also calculated when the activity of Al₂O₃ was considered as unity, which are shown in Fig. 4, and the measured data were marked as well. It is interesting that the measured data meet the equilibrium lines very well, even though the equilibrium lines have deviation from different thermodynamic data. This phenomenon indicates that the activity of Al₂O₃ which equilibrates with Al and dissolved oxygen is very close to unity, and the activity of Al₂O₃ in slag could not change the Al–O equilibrium in bulk steel.

Fig. 4.

Comparison between measured and calculated oxygen activity ( $a_{{\text{Al}}_{\text{2}}{\text{O}}_{\text{3}}}$ = 1).

Based on the comparison between calculated and measured values, it is obvious that the steel-slag reaction can not reach equilibrium, therefore the traditional thermodynamic equilibrium theory can not be used to analyse deoxidation directly for Al-killed steel. In fact, there were still some studies on deoxidation based on the theory of slag-steel equilibrium.^1,2,21)

4.2. Fe–O Equilibrium at Slag-Steel Interface

As above description, there would be some FeO in refining slag, and the Fe–O equilibrium is mainly controlled by the activity of FeO in slag. Based on the thermodynamic data of Eqs. (7), (8), (9), the relationship between FeO activity and oxygen activity can be calculated as shown in Fig. 5. It is obvious that the oxygen activity would increase with the increase of FeO activity linearly.

Fig. 5.

Relationship between activities of FeO and oxygen.

Taniguchi et al.²²⁾ supported that the activity of FeO obeys Henry’s law. As shown in Fig. 6, the activity of FeO were in proportion to the mole of FeO values at a low FeO concentration and presented positive deviation from ideality. It means that the activity coefficient γ _FeO would be larger than 1. Based on the slag compositions shown in Table 2, the total mole number of per 100 g slag is about 1.6±0.05,²³⁾ for which, in terms of mass concentration:   

$$x_{\text{FeO}}=\frac{\text{[mass}\%\mathrm{\hspace{0.17em} }\text{FeO]}}{72\times 1.6}=0.0087\times [\text{mass\%}\mathrm{\hspace{0.17em} }\mathrm{\hspace{0.17em} }\text{FeO]}$$(14)

Fig. 6.

Relationship between activity and mole fraction of FeO.²²⁾

In present work, the mass percentage of FeO in slag is in the range of 0.4–1.0 mass%, which is very common in secondary refining slags. According to Eq. (14), the mole fraction of FeO is around 0.0035–0.0087. Related to Fig. 6, the activity of FeO for that mole fraction is in the range of 0.01–0.03. In fact, many scholars^{4,16,21,24,25,26)} have investigated the activity of FeO in slag, and most activity values were in the range of 0.01–0.03.

Combined with the activity of FeO in real refining slags, a shadow region was marked in Fig. 5. It can be seen from the shadow region that when the activity of FeO is in the region of 0.01–0.03, the activity of oxygen would be in the region of 21–63 ppm. From measured data shown in Figs. 3 and 4, the activity of dissolved oxygen in bulk steel was less than 10 ppm and even lower than 5 ppm, indicating that the oxygen controlled by Fe–O equilibrium is much larger than the oxygen in bulk steel. Therefore, the oxygen at steel-slag interface would transfer from slag into bulk steel. Although the content of FeO is less than 1%, the FeO would still be a supplying source of oxygen.

4.3. Deoxidation Mechanism of Al-Killed Steel

During refining process, the ladle system is a complex system, including refining slag, steel, inclusions and refractory. Besides, the ladle system would contain atmosphere as well. So, in the whole system, many complex reactions would occur, which may cause the differences between industrial trails and laboratory experiments.

According to the above results, the activity of Al₂O₃ for Al–O equilibrium in bulk steel is near unity. In fact, the inclusions in liquid steel would not be pure Al₂O₃ inclusions after refining,²⁰⁾ and most of them present globular calcium aluminates. Therefore, the activity of Al₂O₃ in inclusions should be less than 1. Besides, as calculated above, the activity of Al₂O₃ in slag is much smaller than 1. It seems that these activities are conflicting with activity of Al₂O₃ for Al–O equilibrium in bulk steel, and both the activities in slag and inclusions indicate that the dissolved oxygen activity in bulk steel is not controlled by slag and inclusion compositions.

Ekengård⁷⁾ mentioned that the sampling technique would be the reason to explain this conflict, and proposed oxygen would be drawn down into liquid steel and locally oxidize the dissolved aluminum when the oxygen probe was introduced into the steel bath. The oxygen activity would therefore equilibrate with solid alumina at the probe tip. In fact, after refining, dissolved Al, Ca and Mg are coexisted in steel bath,²⁰⁾ so the oxygen drawn down into steel will react with these dissolved Al, Ca and Mg together when the probe is introduced into liquid steel, and pure solid alumina would not be stable at that time. Therefore, the activity of the deoxidation product would also be smaller than 1. Furthermore, because the probe needs a certain bath depth to measure the activity, and the oxygen drawn down into steel would be little, it would have burnt out before the probe tip reaches the right position for measurement. That means the oxygen drawn down into steel would have little influence on the accuracy of measurement.

In steel-slag equilibrium consideration, refractory was usually ignored by most researchers. In fact, the contact area between refractory and liquid steel is very large, and much larger than that between slag/inclusions. So the effect of refractory on the equilibrium need to be concerned. As known, high alumina and magnesite (carbon-bearing MgO) refractories were widely used in secondary refining process. For high alumina refractory, it is easy to consider the activity of Al₂O₃ is unity. However, that is only suitable for new ladles, because ladle glaze is usually adhered on the ladle wall. Therefore, the reaction of glazed refractory with Al-killed steel needs more consideration. Son et al.^27,28) have investigated the reaction of glazed refractory with Al-killed and Al-killed\Ca-treated molten steel, and found that spinel and alumina phases were still presented in the refractory and ladle glaze. Figure 7 shows these phases and their mole percentages. It can be seen that the phases for Al-killed steel were very similar to those of Al-killed\Ca-treated steel. Compared with the total refractory, this Al₂O₃ phase contacting steel is in small amount, while compared with slag and inclusions, the present authors believe that this Al₂O₃ phase is enough to affect the equilibrium in bulk steel based on the large contact area. Besides, it can be calculated from Fig. 7 that the mole percentage of Al₂O₃ in spinel phase is about 55 mol%. From the activity measurement for Al₂O₃ in spinel as shown in Fig. 8,²⁹⁾ the activity of Al₂O₃ in spinel is larger than 0.8, which is also close to unity. Beskow et al.³⁰⁾ have studied the carbon-bearing MgO glazed refractory, and the also found that spinel phase presented in refractory. The existence of alumina and high Al₂O₃ bearing spinel phases are reasonable to explain Al₂O₃ activity of Al–O equilibrium in liquid steel is very close to unity.

Fig. 7.

Alumina and Spinel phases of high Al₂O₃–MgO refractory and ladle glaze (in mol%).

Fig. 8.

Activities of MgO and Al₂O₃ in the spinel solid solution at 1873 K.²⁹⁾

Based on the above analysis, it can be seen the effect of refractory on the Al–O equilibrium, so the activity of dissolved oxygen in bulk steel is mainly controlled by Al content, and on the condition of present study, the effect of common slag components (CaO, SiO₂, Al₂O₃ and MgO) on the activity of dissolved oxygen is not obvious in bulk steel. That means a certain Al content needs to be guaranteed in order to obtain low dissolved activity in bulk steel.

However, the slag composition would still affect the oxygen content of steel, which has been proved by many results. As described above, FeO in slag would be a main supplying source of oxygen, and the oxygen would transfer from slag into bulk steel.

Yang et al.¹⁶⁾ proposed that there was a high oxygen boundary layer at the steel-slag interface, and the oxygen activity at the interface was controlled by Fe–O equilibrium. In present work, the analysis of Fe–O equilibrium also proves there is a high oxygen phenomenon at slag-steel interface. Furthermore, the present authors believe that there is also an oxygen transfer phenomenon at the interface. According to the Two-Film Theory, the oxygen in the high oxygen boundary would not be unity in the whole boundary, while there would be a concentration gradient. Figure 9 was illustrated in order to describe this clearly. It can be seen that the oxygen in bulk steel is mainly controlled by Al–O equilibrium, while at the slag-steel interface the oxygen is mainly controlled by Fe–O equilibrium. The oxygen at slag-steel interface connects the oxygen in bulk steel according to the high oxygen boundary layer. Oxygen at slag-steel interface is much higher than that in bulk steel, and the concentration gradient of oxygen in the boundary would be large, so the high oxygen boundary layer would be very thin, which may be the reason to explain why it can be hardly found when the activity of oxygen is measured.

Fig. 9.

Sketch map of high oxygen boundary layer.

In the real industrial production process, the refining slag would contact with atmosphere, so the oxygen in air is an important oxygen source. It is well known that Fe₂O₃ is more stable than FeO, therefore at high temperature of steelmaking, FeO in slag would be easily oxidized by the oxygen in air. The reaction can be expressed as Eq. (15).   

$$\text{4(FeO)}+{\text{O}}_{\text{2}}=2{\text{(Fe}}_{\text{2}}{\text{O}}_{\text{3}}\text{)}$$(15)

When these Fe₂O₃ oxides contact with liquid steel, Fe₂O₃ can be transferred into FeO again. Equation (16) describes this reaction.   

$${\text{(Fe}}_{\text{2}}{\text{O}}_{\text{3}}\text{)}+[\text{Fe]}=3(\text{FeO)}$$(16)

According to Eqs. (15), (16) and (7), the oxygen transfers from atmosphere to bulk steel via refining slag. This oxygen transfer would be a continuous process. Because of the oxygen transferred from slag, the oxygen activity increases in bulk steel. Then the Al–O equilibrium would be broken and a new equilibrium would be built with dissolved Al consumption. In other words, because of the Al consumption caused by oxygen transfer, the dissolved oxygen would increase in bulk steel. That also meets with the view that dissolved oxygen activity at bulk is controlled by Al content.

As above analysis, deoxidation for Al-killed steel needs deoxidation not only in bulk steel but also in slag. The sketch map of deoxidation for Al-killed steel is illustrated in Fig. 10. As shown in Fig. 10, in slag deoxidation, some reductants (such as Al particles) need to be added into ladle for slagging, and reduce the content of FeO, while in bulk steel, a certain Al content needs to be guaranteed to keep low dissolved oxygen activity. In industrial practice, to keep certain Al content is relatively easy, therefore slagging presents its importance for deoxidation.

Fig. 10.

Sketch map of deoxidation for Al-killed steel.

4.4. Consideration of Refining Slag

As described above, the refining slag would also affect the oxygen content of steel. Suitable refining slag is very important to obtain low oxygen steel. Although from the industrial measurements, the main components of refining slag would have little influence on dissolved oxygen activity of bulk steel, the unstable oxides such as FeO would also be a source of oxygen, which would influence the final cleanliness. Therefore, stable refining slag is still required for Al killed steel. Besides FeO, other oxygen supplying sources need to be controlled as well.

4.4.1. Basicity of Refining Slag

As shown in Eq. (10), SiO₂ needs to be stable in refining slags because it is an oxygen source. Based on the deoxidation mechanism, the essence of SiO₂ supplying oxygen described in Eq. (10) is Al consumption in bulk steel, causing the increase of dissolved oxygen. So, it is necessary to discuss the stability of SiO₂ in refining slag. Based on the calculation with FactSage software package, Fig. 11(a) presents the iso-activity lines of SiO₂ in CaO-Al₂O₃-SiO₂-5%MgO slag system, and the iso-basicity lines were also marked in Fig. 11(a). It is obvious that the activity of SiO₂ decreases with the increase of basicity. In order to make it more clear, three different Al₂O₃ contents (20%, 30% and 40%) were considered and Fig. 11(b) were obtained. It can be seen from Fig. 11(b) that the decrease tendencies of SiO₂ activity are the same, and the activities of SiO₂ are very close to zero when the basicity is larger than 2, indicating that SiO₂ is almost stable at this time. Similar results were also reported by Kang et al.³¹⁾

Fig. 11.

Effect of basicity on the activity of SiO₂.

According to Eq. (10), the reduction of SiO₂ is also related to the activity of Al₂O₃. Figure 12(a) was obtained when the iso-basicity lines were marked in Fig. 2(b). It can be seen from Fig. 12(a) that the activity of Al₂O₃ decreases with the increase of basicity in general. Three different Al₂O₃ contents (20%, 30% and 40%) were considered to draw Fig. 12(b) as well. It is clear that the effect would be different for different Al₂O₃ contents: it has a slight fluctuation when the Al₂O₃ content is 20% and the basicity is lower than 2; except for that , the general tendency is also a decrease with the increase of the basicity. When the basicity is larger than 3, its effect on activity of Al₂O₃ becomes slight, and the activity changes little. This also indicates that the Al₂O₃ would become stable on the basicity condition.

Fig. 12.

Effect of basicity on the activity of Al₂O₃.

Based on the expression of equilibrium constant K₁₀ shown in Eq. (11), another expression can be obtained as listed in Eq. (17). In Eq. (17), $(a_{{\text{Al}}_{\text{2}}{\text{O}}_{\text{3}}}^2/a_{{\text{SiO}}_{\text{2}}}^3)$ is not only a parameter related to the activities of SiO₂ and Al₂O₃, but also a key parameter to control the reaction of Eq. (10), because the larger value of $(a_{{\text{Al}}_{\text{2}}{\text{O}}_{\text{3}}}^2/a_{{\text{SiO}}_{\text{2}}}^3)$ would hinder Eq. (10) to proceed toward right direction.   

$$K_{10}=\left(\frac{a_{{\text{Al}}_{\text{2}}{\text{O}}_{\text{3}}}^2}{a_{{\text{SiO}}_{\text{2}}}^3}\right)\cdot \frac{a_{[\text{Si}]}^3}{a_{[\text{Al}]}^4}$$(17)

Based on the calculated activities of SiO₂ and Al₂O₃, the diagram shown in Fig. 13 presents the relationship between basicity and the value of $(a_{{\text{Al}}_{\text{2}}{\text{O}}_{\text{3}}}^2/a_{{\text{SiO}}_{\text{2}}}^3)$ . This figure shows that the value of $(a_{{\text{Al}}_{\text{2}}{\text{O}}_{\text{3}}}^2/a_{{\text{SiO}}_{\text{2}}}^3)$ increases with the rise of basicity generally. When the basicity is lower than 3, the value of $(a_{{\text{Al}}_{\text{2}}{\text{O}}_{\text{3}}}^2/a_{{\text{SiO}}_{\text{2}}}^3)$ climbs rapidly, while it changes slightly when the basicity exceeds the region. Therefore, the slag basicity needs to avoid this region to reduce the Al consumption caused by SiO₂ reduction.

Fig. 13.

Effect of basicity on $(a_{{\text{Al}}_{\text{2}}{\text{O}}_{\text{3}}}^2/a_{{\text{SiO}}_{\text{2}}}^3)$ .

As known, high basicity refining slag usually has poor mobility due to its high melting point, and CaF₂ is commonly added in order to improve its mobility. However, the usage of CaF₂ is also confined for the environmental reason. It is known from Fig. 13 when the basicity is not lower than 3, the SiO₂ reduction would be very slight. It means that the basicity of 3–4 would not change the dissolved oxygen activity much as well, in other words, this basicity could get close to the maximum deoxidation capability. Therefore, the basicity of 3–4 is recommended to refine low oxygen Al-killed steel.

4.4.2. Industrial Verification

Table 4 gives the slag basicity and total oxygen content in some steel plants. It can be seen from Table 4 that low total oxygen content (5–8 ppm) was obtained in these steel plants with the basicity around 4. Meanwhile, the low total oxygen content proves the slag can achieve a good deoxidation result.

Table 4. Slag basicity and total oxygen content in Chinese steel plants.

Steel Plant	Steel grade	Steel category	Production process	Slag basicity	Ave. T.[O]/ppm
Plant 1	SCM435	Middle carbon steel	BOF→LF→RH→CC	3.5–4.5	7.5
Plant 4	LG700T	Middle carbon steel	BOF→LF→RH→CC	4.5	8.0
Plant 1	55SiCrA	High carbon steel	BOF→LF→RH→CC	3.5–4.5	5.9
Plant 1	GCr15	High carbon steel	BOF→LF→RH→CC	3.5–4.5	7.1
Plant 2	GCr15	High carbon steel	BOF→LF→RH→CC	3.5–4.5	5.5
Plant 3	GCr15	High carbon steel	BOF→LF→VD→CC	≥3.5	6.3

Some scholars^32,33) believed that higher basicity would decrease the activity coefficient of FeO, so they supported higher basicity would obtain better deoxidation results. In fact, besides the activity coefficient, the activity of FeO also related to the content of FeO in slag, higher basicity therefore could not guarantee the low activity of FeO easily. Meanwhile, the coefficient of FeO would not have a big change relatively in the region of high basicity (R>3). Considering the poor mobility of higher basicity refining slags, the present authors believe that the decrease of FeO content in slag by slagging would make much sense rather than the influence of basicity on the FeO activity. So the significance of slagging on deoxidation should be emphasized.

Above all, the three key points for Al-killed steel refining are certain Al content in bulk steel, low FeO content in slag and stable slag components.

5. Conclusions

The measured and calculated oxygen activities in bulk steel were compared according to industrial trails for Al-killed steel, and the Fe–O equilibrium at slag-steel interface was also concerned. Based on the results, the deoxidation mechanism of Al-killed during industrial refining was proposed, and the consideration on slag basicity was also discussed. The conclusions can be drawn as follows.

(1) The activity of oxygen in bulk steel is controlled by Al content, since the activity of oxygen in bulk steel is very close to the equilibrium value as the activity of Al₂O₃ is treated as unity. In order to keep low oxygen in molten steel, a certain Al content needs to be guaranteed.

(2) At slag-steel interface, the activity of oxygen is controlled by FeO, which is much larger than that of bulk steel, so the oxygen would transfer from slag into bulk steel. Slagging therefore presents its importance for deoxidation to reduce the oxygen transfer.

(3) The essence of SiO₂ supplying oxygen is Al consumption in bulk steel, which gives rise to the increase of dissolved oxygen. Stable refining slag is also very important to obtain low oxygen steel, and its basicity needs to be no smaller than 3 to reduce the Al consumption caused by SiO₂ reduction. In present study, the basicity of 3–4 is recommended to refine low oxygen Al-killed steel.

Acknowledgements

The authors gratefully express their appreciation to National Natural Science Foundation of China (51134009) and The Specialized Research Fund for the Doctoral Program of Higher Education of China (20110042110010) for supporting this work. Mr. Baojun ZHONG and Mr. Zhixun DENG are also appreciated for their help in industrial experiments.

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