Reaction Mechanism of Bottom Blowing CO₂ with Molten Steel in Converter

Abstract

In the current study, the reaction behavior between bottom blowing CO₂ and the molten steel at different converter stages was studied by the thermodynamic calculation. At the initial stage, CO₂ reacts with [C] before the turning point and then reacts with [C] and [Si] simultaneously after that. CO₂ completely reacts with [C] and twice volume of CO generates at the middle stage. The reaction ratio of CO₂ and [C] gradually decreases at the final stage due to the ratio of CO₂ reacting with [Fe] and the residual unreacted CO₂ increases. Then the effect of the bottom blowing gas type and intensity on the [C]·[O] value and T.Fe content in the slag were studied by industrial trials in a 300 t converter. When Ar is blown at the final stage, the CO partial pressure decreases with the increasing of the bottom blowing intensity, while the opposite result is obtained when blowing with CO₂. The CO₂ bottom blowing into the converter should be switched with Ar, and the blowing time of CO₂ should be controlled in the early 12 minutes to avoid increasing the [C]·[O] value and the T.Fe content in the slag. In order to improve the reaction rate of CO₂ and [C], it is necessary for the CO₂ to be blown during the high-speed decarburization period when the [C] content is between 0.7 mass% and 3.3 mass% and the bottom blowing time is between 5 and 12 minutes.

1. Introduction

The increase of CO₂ emissions in the atmosphere is the main reason for the global warming. CO₂ emissions from the iron and steel industry is an important proportion of the total carbon emission, especially in the plant with BF-BOF route with 1.7–2.2 t CO₂ emissions per ton steel.^1,2,3) More and more steelmaking plants are focusing on the reduction of the energy consumption and the CO₂ emissions to address the crucial subject of the climate change.⁴⁾ To achieve carbon neutrality in the steel industry, a large-scale reform is required to improve the steelmaking processes. Research on the resource utilization of CO₂ is of great significance to reduce CO₂ emissions to cope with the global warming and bring beneficial metallurgical effects.

The application technology of CO₂ in the steelmaking has been studied since the 1980s. CO₂ is used instead of N₂ and Ar for the BOF bottom-blowing^{5,6,7,8,9,10,11,12,13,14,15,16,17)} and for EAF bottom-blowing.^18,19) CO₂ mixed with O₂ is blown from the top of the converter to improve the metallurgical effect²⁰⁾ and reduce the dust generation,^21,22,23,24) CO₂ replaces Ar for the bottom-blowing to enhance the stirring energy during the LF process^25,26,27) and RH process,^28,29) and also is used as the protective gas of the molten steel during the continuous casting.³⁰⁾ During the converter process, CO₂ is widely used as the bottom blowing gas, which has the following characteristics: I) Bottom blowing CO₂ in the converter has excellent denitrification performance since the reaction of CO₂ with [C] is a volume increasing reaction.^{8,9,10,11,12)} II) The cooling capacity of CO₂ is greater than N₂ and Ar because of the endothermic reaction of CO₂ with [C], so CO₂ can become the cooling gas of bottom blowing in the converter, and even replace the hydrogen carbide to avoid [H] increasing in the molten steel.^13,14) III) CO₂ is a kind of weak oxidizing gas, which reacts with the molten steel near the bottom blowing tuyere to generate FeO and MnO, resulting in faster corrosion of refractory.^{8,9,10,15,16,17)} IV) As for the effect of bottom blowing CO₂ in the converter on the content of [O] in the molten steel and T.Fe content in the slag, some studies^8,9) show that the oxygen content of molten steel and the T.Fe content in the slag at the end of converter are significantly increased with the bottom blowing CO₂, while other studies^10,12) have obtained opposite results.

In summary, researches on CO₂ application in the steelmaking mainly focused on the bottom blowing in the converter, which has the advantages of lower cost, excellent denitrification performance, and good cooling capacity for bottom blowing tuyere. However, CO₂ accelerates the corrosion of refractory since it is a kind of weak oxidizing gas, and it is controversial about the effect of CO₂ on the oxygen content of molten steel and the T.Fe content in the slag. In this paper, the reaction behavior of CO₂ bottom blowing in the converter was studied. Firstly, the reaction mechanism of CO₂ with main elements in the molten steel was analyzed by the thermodynamic calculation. Then, the industrial trials of the bottom blowing CO₂ were carried out in a 300 t converter. The effect of CO₂ blowing time and intensity on the [C]·[O] value and T.Fe in the slag was discussed. The effect of bottom blowing CO₂ on the decarburization rate was studied by the exhaust gas analysis.

2. Experimental Aspects

2.1. Thermodynamic Calculation

Reaction between the CO₂ and the molten steel were calculated by the thermodynamic software Factsage7.1. The composition and temperature of the molten steel in three typical stages are given in Table 1. The initial stage, the middle stage, and the final stage represent the molten steel when the [C] is 4.3 mass%, 2.0 mass%, and 0.1 mass%, respectively. CO₂ was continuously blown into the molten steel by the step of 0.05 Nm³/t. The molten steel reacted with CO₂ in the previous step continues to iterate and then continues to react with CO₂ in the next step. A total of 200 steps were calculated and 10 Nm³/t CO₂ were blown into the molten steel for each stage.

Table 1. Temperature and elements content (mass, %) of molten steel at different stage.

Stage	Temperature (°C)	C	Si	Mn	P	S
Initial stage	1350	4.300	0.270	0.100	0.070	0.002
Middle stage	1500	2.000	0.010	0.010	0.030	0.002
Final stage	1620	0.100	0.010	0.010	0.008	0.002

2.2. Industrial Trials

Industrial trials were carried out in a 300 t converter. Parameters are listed in Table 2 and Fig. 1, respectively. The top blowing time and intensity of the pure oxygen in the converter were approximately 15 minutes and 3.4 Nm³/(t·min). In order to study the effect of the bottom blowing time and the intensity of CO₂ on the metallurgical effect of the converter, 6 groups of trials were designed. As a comparative trial, No. 1 was carried out by the bottom blowing N₂/Ar. Trials No. 2 to No. 6 were carried out by the bottom blowing CO₂ and Ar at different time. The bottom blowing time of CO₂ varied from 3 to 12 minutes respectively and the blowing gas switched to Ar in the rest time in the trials of No. 2 to No. 5. Trial No. 6 was conducted by the bottom blowing CO₂ during the whole blowing process. In all trials, the intensity of the bottom blowing gas was fixed at 0.04 Nm³/(t·min) in the early 12 minutes, and then increased to 0.06–0.11 Nm³/(t·min).

Table 2. Industrial trials scheme.

Trial number	Gas type		Blowing time (min)		Blowing intensity (Nm3/(t·min))		Amount of Data
	Earlier	Later	Earlier	Later	0 to12 mins	12 to 15 mins
No. 1	N2	Ar	9	6	0.04	0.060	16
					0.04	0.080	16
No. 2	CO2	Ar	3	12	0.04	0.060	28
					0.04	0.070	12
					0.04	0.080	18
No. 3	CO2	Ar	6	9	0.04	0.040	23
					0.04	0.080	12
No. 4	CO2	Ar	9	6	0.04	0.060	14
					0.04	0.070	18
					0.04	0.080	16
No. 5	CO2	Ar	12	3	0.04	0.060	41
					0.04	0.070	10
					0.04	0.080	16
					0.04	0.011	16
No. 6	CO2	Ar	15	0	0.04	0.060	23
					0.04	0.070	63
					0.04	0.080	14
					0.04	0.11	23

Fig. 1.

Trial parameters for the bottom blowing CO₂ in the converter. (Online version in color.)

3. Results and Discussion

3.1. The Reaction Mechanism between CO₂ and the Molten Steel

The thermodynamic software FactSage 7.1 was used to calculate the reaction between the CO₂ and the molten steel at different stages, and calculation details are shown in the research method. The content variation of [Si], [Mn] with [C] at the initial stage are shown in Fig. 2(a). The [Mn] content remained unchanged with the decreasing of [C] content. CO₂ only reacts with [C] in the molten steel when [C] content is above the turning point of 4.06 mass%. CO₂ reacts with [C] and [Si] simultaneously when [C] drops below that value. Therefore, 4.06 mass% [C] and 0.27 mass% [Si] were the turning point at which the Gibbs free energy change of CO₂ reacting with [C] and [Si] became the same, which was similar to the study of H. Ono-Nakazato et al.³¹⁾ The variation of gas generation with [C] content is shown in Fig. 2(b), the solid line represents the actual gas generated and the dotted line is the extension of the straight line before the turning point. The generated CO gas conforms to the rule in the Fig. 2(a). All the CO₂ reacted with [C] to produce double volume of CO before the turning point. After the turning point, CO₂ not only reacts with [C] to produce twice the volume of CO, but also reacts with [Si] to produce the same volume of CO, which results in the increase of the CO generation. Figure 2 also indicates that CO₂ has been fully reacted. In addition, [Mn] did not react with CO₂ since the Gibbs free energy change of the reaction between CO₂ with [Mn] was higher than that of CO₂ with [C] and [Si] at the initial stage.

Fig. 2.

Variation of elements content and gas generation with [C] in the molten steel at the initial stage: (a) [Si], [Mn] content and (b) gas generation. (Online version in color.)

The gas generation of CO₂ reacting with the molten steel at the middle stage of the converter is shown in Fig. 3. At the middle stage, [Si] and [Mn] in the molten steel have been reacted almost completely to be as low as 0.01 mass%, while the content of [C] is high, so the blowing CO₂ completely reacts with [C], and the CO generation increases linearly with the reduction of [C] content.

Fig. 3.

The variation of gas generation in the middle stage. (Online version in color.)

The calculated results of CO₂ reacting with molten steel at the final stage are shown in Fig. 4. The content variation of [O] in the molten steel and (FeO) generation are shown in Fig. 4(a). The [C] content decreases with the CO₂ blowing, correspondingly [O] content continues to increase. When the [C] content decreases to approximately 0.035 mass%, the (FeO) generation gradually increases due to the reaction of CO₂ and Fe. The variation of CO and CO₂ generation with [C] content are shown in Fig. 4(b). CO and CO₂ increase with the decreasing of [C] content. The reaction percentage of CO₂ at the final stage is shown in Fig. 4(c). The reaction ratio of CO₂ with [Fe] is calculated from the (FeO) generation, the reaction ratio of CO₂ with [C] is obtained from the total CO generation minus the CO produced by the reaction of CO₂ and [Fe], and the ratio of CO₂ not participating in the reaction is obtained from the remaining CO₂. With the decreasing of [C] content, the reaction ratio of CO₂ with [C] gradually decreases. Then part of CO₂ reacts with [Fe] to generate the same volume of CO, and another part of CO₂ does not participate in the reaction. The FeO generation are attributed to two reasons: (a) FeO was formulated by the reaction between [Fe] and CO₂ as expressed in Eq. (1). (b) CO₂ is decomposed into CO(g) and [O] as expressed in Eq. (2), then [O] reacts with [Fe] to form FeO as expressed in Eq. (3).   

$$\mathrm{C}{\mathrm{O}}_2(\mathrm{g})+\mathrm{F}\mathrm{e}(\mathrm{l})=(\mathrm{F}\mathrm{e}\mathrm{O})+\mathrm{C}\mathrm{O}(\mathrm{g})$$(1)

  

$$\mathrm{C}{\mathrm{O}}_2(\mathrm{g})=\mathrm{C}\mathrm{O}(\mathrm{g})+[\mathrm{O}]$$(2)

  

$$[\mathrm{F}\mathrm{e}]+[\mathrm{O}]=(\mathrm{F}\mathrm{e}\mathrm{O})$$(3)

Fig. 4.

Calculation results of CO₂ reacting with the molten steel at the final stage: (a) [O] and (FeO) content, (b) gas generation, and (c) reaction percentage of CO₂. (Online version in color.)

Summarizing the above results of the thermodynamic calculation, the reaction mechanism between the CO₂ and the molten steel during the converter smelting process is shown in Fig. 5. The reaction behavior of the bottom blowing CO₂ is analyzed according to the typical three stage of the converter. (a) Initial stage: the temperature of molten steel is low as 1350°C, and the contents of [C], [Si], and [Mn] are relatively high. CO₂ reacts with [C] before the turning point and reacts with [C] and [Si] simultaneously after that. (b) Middle stage: the temperature increases to 1500°C, [Si] and [Mn] are almost completely oxidized, but the [C] content is still high. This stage is high-speed decarburization period. CO₂ reacts with [C] to generate twice volume of CO. (c) Final stage: the temperature increases to 1620°C, and the [C] content decreases to about 0.1 mass%. Firstly CO₂ reacts with [C] to generate CO. Then part of CO₂ begins to react with [Fe] and the other part of CO₂ does not react with the molten steel.

Fig. 5.

Schematic of the reaction mechanism of CO₂ bottom blowing with the molten steel at different stages of converter process. (Online version in color.)

3.2. Effect of Bottom Blowing CO₂ on the End-point State in the Converter

Compared with the inert gas, CO₂ reacts with elements in the molten steel. Some studies indicate that bottom blowing CO₂ increases the content of [O] in the molten steel and T.Fe content in the slag at the end of the converter.^8,9) However, other studies hold that the stirring energy of the molten bath is enhanced due to the reaction of CO₂ with [C], then the better kinetic conditions make the [O] and T.Fe content decrease.^10,12) The [C]·[O] value is the key index to evaluate the oxidation of molten steel at the end-point state. The analysis of [C]·[O] value avoid the influence of the variation of the [C] content. In this paper, [C]·[O] value and T.Fe content in the slag are used as indicators to measure the oxidizability of molten steel and slag. The decarburization reaction, the standard Gibbs free energy change, the equilibrium constant, and the [C]·[O] value are shown as follows:   

$$[C]+[O]=CO(g)$$(4)

  

$$\mathrm{\Delta }{G}^{\theta }=-22\mathrm{\hspace{0.17em} }000-38.34T$$(5)

  

$$K=\frac{P_{CO}}{[C]\cdot [O]\cdot f_C\cdot f_O}$$(6)

  

$$[C]\cdot [O]=\frac{P_{CO}}{EXP\left(\frac{-\mathrm{\Delta }{G}^{\theta }}{RT}\right)\cdot f_C\cdot f_O}$$(7)

where ΔG^θ and K represent the standard Gibbs free energy change and equilibrium constant of reaction (4), respectively. T represents the temperature. f_C and f_O represent the activity coefficients for carbon and oxygen, respectively, R is a gas constant. [C] and [O] are the mass fraction of carbon and oxygen, respectively. P_CO is partial pressure of CO.

The effect of the gas type and the bottom blowing intensity on the [C]·[O] value are shown in Fig. 6. The box plots from the top to bottom are the maximum, upper quartile, median, lower quartile, and minimum values. The circle symbol is the average value. In the industrial trials of No. 1 to No. 5, Ar was blown at the final stage of 12–15 minutes. Trial No. 6 was conducted by the bottom blowing CO₂ during the whole process. The temperature was 1630–1670°C and the tapping [C] was from 0.03 mass% to 0.05 mass%. The results of No. 1 to No. 5 show the same trend that the [C]·[O] value decreased with the increasing of the bottom blowing intensity. The volume of Ar in the molten steel increases with the increasing of bottom-blown Ar intensity, and correspondingly the CO volume decreases, resulting in a decrease in CO partial pressure, which leads to a decrease in the [C]·[O] value. Interestingly, trial No. 6 shows the opposite trend that the [C]·[O] value increased with the decreasing of the blowing intensity at the final stage. The reasons were attributed to two aspects: (a) CO partial pressure is increased by the reactions of CO₂ with [C], resulting in the[C]·[O] value increasing; (b) FeO is generated by the reaction of CO₂ and [Fe] when the [C] content is low at the final stage, then FeO is decomposed into [Fe] and [O]. Therefore, the [C]·[O] value is mainly determined by the gas type and the bottom blowing intensity at the final stage. When the bottom blowing gas is inert gas at the final stage, [C]·[O] decreases with the increasing of the bottom blowing intensity for the inert gas reduces the partial pressure of CO. However, when the bottom blowing gas at the final stage is CO₂ gas, the [C]·[O] value increases with the increasing of the bottom blowing intensity for the reaction of CO₂ with the molten steel increases the CO partial pressure, in addition, the oxidation properties of CO₂ increase the [O] content of the molten steel.

Fig. 6.

The effect of gas type and bottom blowing intensity on the [C]·[O] value, CO₂ blowing time of (a) 0 min, (b) 3 min, (c) 6 min, (d) 9 min, (e) 12 min, and (f) 15 min. (Online version in color.)

The partial pressure of CO is calculated by Eq. (8), and the effect of the bottom blowing intensity on the partial pressure of CO is shown in Fig. 7. Under the condition of bottom blowing Ar, the CO partial pressure decreases from 0.70 atm to about 0.65 atm with the increasing of the bottom blowing intensity. With the bottom blowing CO₂ at the final stage, the CO partial pressure increases from 0.70 atm to 0.76 atm with the increasing of the bottom blowing intensity.   

$$P_{CO}=[C]\cdot [O]\cdot EXP\left(\frac{-\mathrm{\Delta }{G}^{\theta }}{RT}\right)\cdot f_C\cdot f_O$$(8)

Fig. 7.

Effect of bottom blowing intensity on the CO partial pressure. (Online version in color.)

The effect of the bottom blowing intensity on the T.Fe content in the slag is shown in Fig. 8. Due to the interaction between [O] in the molten steel and the T.Fe in the slag, the effect rule of the bottom blowing gas and the intensity on the T.Fe content and [C]·[O] value was similar. The results from No. 1 to No. 5 show the same trend that the T.Fe content decreased with the increasing of the bottom blowing intensity. That was mainly due to the decreased [O] content at the final stage with increasing of the bottom blowing intensity. Similarly, trial No. 6 shows the opposite trend that the T.Fe content in the slag gradually increased with the increasing of the bottom blowing intensity. The [O] content in the molten steel increased when CO₂ was blown at the final stage, and FeO was generated by the reaction of CO₂ with [Fe]. Therefore, T.Fe content in the slag increased with the increasing of the bottom blowing intensity of CO₂ at the final stage.

Fig. 8.

The effect of bottom blowing intensity on the T.Fe tontent in the slag, CO₂ blowing time of (a) 0 min, (b) 3 min, (c) 6 min, (d) 9 min, (e) 12 min, and (f) 15 mins. (Online version in color.)

Under the blowing intensity at the final stage of 0.06 Nm³/(t·min), the effect of CO₂ blowing time on [C]·[O] value is shown in Fig. 9(a). Compared to the CO₂ blowing time from 0 min to 12 min, [C]·[O] value slightly increased when the CO₂ blowing time increased to 15 min. When the blowing intensity at the final stage was 0.08 Nm³/(t·min), the effect of CO₂ blowing time on [C]·[O] value is shown in Fig. 9(b). Compared to the CO₂ blowing time from 0 min to 12 min, [C]·[O] value significantly increased when the CO₂ blowing time increased to 15 min.

Fig. 9.

The effect of CO₂ blowing time on the [C]·[O] value, CO₂ blowing intensity at final stage of (a) 0.06 Nm³/(t·min) and (b) 0.08 Nm³/(t·min). (Online version in color.)

Similarly to variation pattern of [C]·[O] value, the effect of CO₂ blowing time on the T.Fe content in the slag are shown in Fig. 10. With the bottom blowing intensity at the final stage of 0.06 Nm³/(t·min), compared to the CO₂ blowing time from 3 min to 12 min, there was a slight increase as for T.Fe content (maximum value and average value) when the CO₂ blowing time increased to 15 min. With the bottom blowing intensity at the final stage of 0.08 Nm³/(t·min), compared to the CO₂ blowing time from 0 min to 12 min, T.Fe content significantly increased from 17.5 mass% to 20.5 mass% when the CO₂ blowing time increased to 15 min.

Fig. 10.

The effect of CO₂ blowing time on the T.Fe content in the slag, CO₂ blowing intensity at final stage of (a) 0.06 Nm³/(t·min) and (b) 0.08 Nm³/(t·min). (Online version in color.)

3.3. Effect of Bottom Blowing CO₂ on the Decarburization Rate

The role of CO₂ in the converter process mainly depends on the reaction between CO₂ and [C] in the molten steel. Therefore, it is necessary to analyze the effects of the bottom blowing inert gas and the bottom blowing CO₂ on the decarburization rate. Figure 11 shows the variation of exhaust gas under the inert gas mode (No. 1) and CO₂ mode (No. 6). CO was generated by the primary combustion of [C]. CO₂ was derived from secondary combustion of CO gas with the oxygen in the converter and the air inhaled from the gap between the furnace and the exhaust gas hood. The sum of CO and CO₂ represent the consumption of carbon in the molten steel. (CO+CO₂) continues to increase to maximum value at 4 to 5 min which was estimated as the first critical point, shown as I_CP in the Fig. 11. After the first critical point I_CP, (CO+CO₂) was stable about 80 volume% for a period of time. (CO+CO₂) decreased sharply at about 12 to 13 min, which was estimated as the second critical point, shown as II_CP in the Fig. 11.

Fig. 11.

Variation of exhaust gas under (a) inert gas mode and (b) CO₂ mode. (Online version in color.)

Exhaust gas analysis was used to calculate the dynamic [C] content and decarburization rate. According to the mass balance of carbon elements in the molten steel, the decarburization rate is obtained from the variation of the exhaust gas in the converter. No. 1 and No. 6 are compared as pure inert gas and pure CO₂ bottom mode. The Eqs. (9), (10), and (11) are used to calculate the [C] content in the converter process by the mass balance of carbon.   

$$C_0=W_{hot\mathrm{\hspace{0.17em} }metal}\cdot {[C]}_{hot\mathrm{\hspace{0.17em} }metal}^0+W_{scrap}\cdot C_{scrap}^0$$(9)

  

$$\begin{array}{l}\mathrm{\Delta }{C}_t=\\ \underset{0\to t}{\int }\left[k\cdot \frac{12\mathrm{\hspace{0.17em} }000}{22.4}\cdot V_{waste\mathrm{\hspace{0.17em} }gas}\cdot \left(CO{\%}_{waste\mathrm{\hspace{0.17em} }gas}+C{O}_2{\%}_{waste\mathrm{\hspace{0.17em} }gas}\right)\right]\times dt\end{array}$$(10)

  

$${[C]}_t=\left(C_0-\mathrm{\Delta }{C}_t\right)/\left(W_{hot\mathrm{\hspace{0.17em} }metal}+W_{scrap}\right)$$(11)

where C₀ is the total carbon mass brought by the hot metal and the scrap. W_{hot metal} and W_scrap represent the weight of hot metal and scrap, respectively. [C]0 hot metal and C0 scrap are the carbon content of hot metal and scrap, respectively. t is the blowing time. ΔC_t is the consumption of carbon at the blowing time t. k is the correction coefficient. [C]_t is the carbon content of the molten steel at the blowing time t.

Figure 12 shows the relationship between the decarburization rate and the carbon content under two modes. The solid line and dashed line represent the decarburization rate of N₂–Ar blowing mode and CO₂ blowing mode, respectively. The first stage was mainly for the oxidation reaction of [Si] and [Mn]. The decarburization rate increased to 0.37 mass%[C]/min with the [C] content decreasing from 4.3 mass% to I_CP of 3.3 mass%. The second stage was the high-speed decarburization period with the [C] content decreasing from I_CP to II_CP of 0.7 mass%, and the decarburization rate was maintained constantly 0.35 to 0.4 mass%[C]/min. The third stage was the final stage of decarburization, carbon mass transfer became the limiting link at the final stage, and the decarburization rate decreased from 0.37 mass%[C]/min to 0 mass%[C]/min. Therefore, it was mutually confirmed with the thermodynamic calculation results in the previous section. Comparing the decarburization rate of the N₂–Ar blowing mode and CO₂ blowing mode, the variation of decarburization rate of these two modes was the same. The values of decarbonization rate at the first critical point (I_CP), the second critical point (II_CP), and the peak decarbonization period are quite close. CO₂ has little effect on the decarburization rate of molten steel, for the flow of CO₂ (800 Nm³/h) accounts for less than 1 volume% of the total flue gas flow (240000 Nm³/h). In order to improve the reaction rate of CO₂ and [C] in the molten steel, CO₂ gas is blown into the molten steel during the high-speed decarburization. It is necessary for the CO₂ to be blown during the high-speed decarburization period when the [C] content is between 0.7 mass% and 3.3 mass% and the time is between 5 and 12 minutes.

Fig. 12.

Relationship between the decarburization rate and the carbon content under two different modes. (Online version in color.)

4. Conclusions

In the current study, the reaction mechanism of CO₂ with molten steel was studied by the thermodynamic software, the effect of CO₂ bottom blowing time and the blowing intensity on the end-point state were studied by industrial trials in a 300 t converter. The main conclusions are as follows:

(1) The reaction behavior of CO₂ after the bottom blowing into the molten steel is mainly divided into three stages: (a) Initial stage: CO₂ reacts with [C] before the turning point, and reacts the same with [C] and [Si] after that. (b) Middle stage: CO₂ completely reacts with [C] to produce twice the volume of CO. (c) Final stage: with the decreasing of [C] content, CO₂ begins to react with [Fe], and the ratio of unreacted CO₂ gradually increases.

(2) The [C]·[O] value is mainly affected by the CO partial pressure at the final stage. CO partial pressure decreases from 0.70 atm to 0.65 atm with the increasing of bottom blowing intensity when Ar is bottom blown at the final stage, and the [C]·[O] value and the T.Fe content in the slag both decrease correspondingly. CO partial pressure increases from 0.70 atm to 0.76 atm with the increasing of bottom blowing intensity when CO₂ is bottom blown at the final stage, the [C]·[O] value and the T.Fe content in the slag both increase correspondingly.

(3) CO₂ bottom blowing has little effect on the decarburization rate. In order to improve the reaction rate of CO₂ and the [C] in molten steel, it is necessary for the CO₂ to be blown during the high-speed decarburization period when the [C] content is between 0.7 mass% and 3.3 mass% and the time is between 5 and 12 minutes.

Acknowledgments

The authors deeply appreciate Shougang Group Co., Ltd. for permission to publish this paper.

References

• 1)   K.  Han,  C. K.  Ahn and  M. S.  Lee: Int. J. Greenhouse Gas Contr., 27 (2014), 239. https://doi.org/10.1016/j.ijggc.2014.05.014
• 2)   P.  Cavaliere: Clean Ironmaking and Steelmaking Processes: Efficient Technologies for Greenhouse Emissions Abatement, Springer International Publishing, Cham, (2019), 1. https://doi.org/10.1007/978-3-030-21209-4
• 3)   D.  Gielen: Energy Convers. Manage., 44 (2003), 1027. https://doi.org/10.1016/S0196-8904(02)00111-5
• 4)   C. C.  Xu and  D.-q.  Cang: J. Iron Steel Res. Int., 17 (2010), 1. https://doi.org/10.1016/S1006-706X(10)60064-7
• 5)   K.  Dong and  X.  Wang: Metals, 9 (2019), 273. https://doi.org/10.3390/met9030273
• 6)   Y.  Zhou,  G.  Wei and  R.  Zhu: J. CO2 Util., 60 (2022), 102016. https://doi.org/10.1016/j.jcou.2022.102016
• 7)   W.  Wu,  R.  Zhu,  Z.  Li,  C.  Wang and  G.  Wei: Ironmaking Steelmaking, 48 (2021), 852. https://doi.org/10.1080/03019233.2021.1874623
• 8)   A. S.  Normanton,  D. M.  Bryce,  E.  Craggs and  K.  McGregor: Steel Times, 216 (1988), 503.
• 9)   R. J.  Selines: Metal. Int., 2 (1989), 133.
• 10)   G.  Jin: Steelmaking, 3 (1985), 96 (in Chinese).
• 11)   B.  Hao,  G.  Jin,  J.  Gao and  E.  Ma: Angang Technol., 4 (1983), 12 (in Chinese).
• 12)   G.  Wei,  Q.  Zhu,  S.  Hu,  R.  Zhu and  C.  Feng: Steelmaking, 37 (2021), 8 (in Chinese).
• 13)   Z.  Ibaraki,  M.  Okashima,  Y.  Ueda,  M.  Kanemoto and  K.  Arima: Tetsu-to-Hagané, 71 (1985), S175 (in Japanese). https://doi.org/10.2355/tetsutohagane1955.71.4_S151
• 14)   H.  Iso,  H.  Jiyono,  M.  Honda,  K.  Arima,  M.  Kanemoto and  Y.  Ueda: Tetsu-to-Hagané, 69 (1983), S1012 (in Japanese). https://doi.org/10.2355/tetsutohagane1955.69.12_S963
• 15)   M.  Sato,  K.  Ichihara and  T.  Okada: Tetsu-to-Hagané, 70 (1984), S945 (in Japanese). https://doi.org/10.2355/tetsutohagane1955.70.12_S923
• 16)   T.  Takahashi,  F.  Kitani,  Y.  Nimura,  H.  Hasegawa,  Y.  Shiratani and  N.  Hiraga: Tetsu-to-Hagané, 68 (1982), S995 (in Japanese). https://doi.org/10.2355/tetsutohagane1955.68.11_S965
• 17)   H.  Kobayashi,  Y.  Nimura,  S.  Kuriyama,  Y.  Shiratani,  M.  Hanmyo and  Y.  Miyawaki: Tetsu-to-Hagané, 70 (1984), S253 (in Japanese). https://doi.org/10.2355/tetsutohagane1955.70.4_S251
• 18)   G.  Wei,  R.  Zhu,  Y.  Wang,  X.  Wu and  K.  Dong: J. Iron Steel Res. Int., 26 (2019), 909. https://doi.org/10.1007/s42243-018-0163-7
• 19)   B.  Tian,  Y.  Zhou,  G.  Wei,  R.  Zhu,  W.  Wu and  Z.  Yang: Ceram. Int., 48 (2022), 36936. https://doi.org/10.1016/j.ceramint.2022.08.260
• 20)   W.  Dong,  A.  Xu,  H.  Li,  S.  Guan,  C.  Ji,  N.  Hao and  X.  Deng: Metall. Mater. Trans. B, 53 (2022), 3575. https://doi.org/10.1007/s11663-022-02621-3
• 21)   Y.  Zhou,  R.  Zhu and  G.  Wei: Energy Rep., 8 (2022), 7274. https://doi.org/10.1016/j.egyr.2022.05.234
• 22)   M.  Lv,  R.  Zhu,  X.  Wei,  H.  Wang and  X.  Bi: Steel Res. Int., 83 (2012), 11. https://doi.org/10.1002/srin.201100166
• 23)   C.  Yi,  R.  Zhu,  B.  Chen,  C.  Wang and  J.  Ke: ISIJ Int., 49 (2009), 1694. https://doi.org/10.2355/isijinternational.49.1694
• 24)   R.  Zhu,  X.  Bi,  M.  Lv,  R.  Liu and  X.  Bao: Adv. Mater. Res., 284–286 (2011), 1216. https://doi.org/10.4028/www.scientific.net/AMR.284-286.1216
• 25)   W. H.  Wu,  R.  Zhu,  H.  Wang,  C.  Wang and  G.  Wei: Ironmaking Steelmaking, 49 (2022), 147. https://doi.org/10.1080/03019233.2021.1972738
• 26)   Y.  Gu,  H.  Wang,  R.  Zhu,  J.  Wang,  M.  Lv and  H.  Wang: Steel Res. Int., 85 (2014), 589. https://doi.org/10.1002/srin.201300106
• 27)   K.  Dong,  R.  Zhu,  R.  Liu,  H.  Wang and  C.  Zhou: J. Univ. Sci. Technol. Beijing, 36 (2014), 226 (in Chinese). https://doi.org/10.13374/j.issn1001-053x.2014.s1.042
• 28)   G.  Chen,  J.  Yang,  L.  Li,  M.  Zhang and  S.  He: J. CO2 Util., 50 (2021), 101586. https://doi.org/10.1016/j.jcou.2021.101586
• 29)   B.  Han,  G.  Wei,  R.  Zhu,  W.  Wu,  J.  Jiang,  C.  Feng,  J.  Dong,  S.  Hu and  R.  Liu: J. CO2 Util., 34 (2019), 53. https://doi.org/10.1016/j.jcou.2019.05.038
• 30)   Q.  Li,  H.  Wang,  R.  Zhu,  D.  Shou,  R.  Liu and  Y.  Gu: Contin. Cast., 40 (2015), 5 (in Chinese). https://doi.org/10.13228/j.boyuan.issn1005-4006.20140053
• 31)   H.  Ono-Nakazato,  Y.  Morita,  K.  Tamura,  T.  Usui and  K.  Marukawa: ISIJ Int., 41 (2001), S61. https://doi.org/10.2355/isijinternational.41.Suppl_S61