Application of CO₂ and O₂ Mixed Blowing on the Decarburization and Manganese Retention

Abstract

Adding high carbon ferromanganese in the early decarburization stage of 200 series stainless steel can reduce the electrolytic manganese added during alloying; however, the oxidation loss of manganese is severe. This paper studies CO₂ and O₂ mixed blowing thermodynamics and kinetics on decarburization and manganese retention of 200 series stainless steel. The thermodynamic study indicates that manganese oxidation reduces when CO₂ is used instead of part of O₂ in molten steel decarburization. Meanwhile, the kinetic study determines that more bottom blowing gas enhances the stirring of molten steel by using CO₂ and O₂ mixed blowing technology; thus, the kinetic condition of decarburization can be improved. The industrial experimental study identifies that the tap-to-tap time increases by 4–5 minutes for every ton of high carbon ferromanganese increased during decarburization by using CO₂ and O₂ mixed blowing. Compared to pure oxygen blowing, applying CO₂ and O₂ mixed blowing significantly reduces manganese oxidation in decarburization, increases manganese yield by 3–5.6%, and exhibits a remarkable effect on decarburization and manganese retention.

1. Introduction

In 2020, the output of stainless crude steel in China reaches 30.139 million tons, whose global share rises from 56.3% in 2019 to 59.2%. With the increasingly serious environmental problems, the requirement for energy saving and emission reduction is gradually increasing, which has become a hot spot of current research in steel mill. Since the 1970s, scholars have studied the role of CO₂ gas in steel smelting,^1,2,3) confirming that CO₂ can be used for molten steel decarburization and discovered the possibility of bottom blowing CO₂ as a stirring gas. However, limited by the production conditions at that time, CO₂ was not used in actual industrial process. In the past ten years, the steelmaking process by using CO₂ has progressed significantly. In 2007, Jin Renjie et al.⁴⁾ noticed the decarburization effect of CO₂ in molten steel, followed by the further research on CO₂ in the field of steelmaking.⁵⁾ Researchers found that CO₂ could reduce the soot content in the converter, lower the iron loss of the slag,^{6,7,8,9,10,11)} and improve the dephosphorization effect.^6,12,13) However, Wan Xuefeng et al.¹⁴⁾ pointed out that although CO₂ could achieve the purpose of decarburization during the converter smelting process, it was necessary to pay attention to the temperature drop in molten steel. Wang Huan et al.¹⁵⁾ believed that it was feasible to use CO₂ to replace Ar as bottom blowing gas in EAF. The results of industrial experiments further indicated that the use of CO₂ to replace Ar could increase the desulfurization rate of EAF by 7%. Gu¹⁶⁾ and Dong¹⁷⁾ believed that the use of CO₂ to replace Ar in LF had a little effect on production process, and industrial experiments had verified the feasibility of this replacement. Han¹⁸⁾ and Wei¹⁹⁾ believed that CO₂ could also replace Ar in RH as the lift gas by grasping the timing of aluminum addition.

Based on known information, using CO₂ and O₂ mixed blowing to reduce the chromium loss has become an important means in traditional AOD smelting.^20,21) Previous studies have shown that the use of CO₂ instead of part of O₂ as an oxidant for decarburization is a feasible solution to increase the yield of alloying elements, because this substitution promotes the oxidation of [C] rather than alloy. However, there are still great difficulties in using CO₂ and O₂ mixed blowing on decarburization and manganese retention in actual production. And few studies have been taken in this field. Li Cheng et al.²²⁾ conducted preliminary explorations on the application of CO₂ and O₂ mixed blowing on the converter smelting of low-carbon stainless steel and found that the mixing of CO₂ and O₂ could achieve the purpose of decarburization and manganese retention. Liu Hongbo^23,24) discovered the reduction of the evaporation and oxidation loss of manganese with the addition of CO₂. Therefore, the introduction of CO₂ could increase the manganese yield as well as the decarburization efficiency.

Above research mainly focuses on high-carbon steels. The effect of CO₂ on the decarburization and manganese retention of low-carbon stainless steel is still unclear. Therefore, this article focuses on the 200 series stainless steel smelted by Guangxi Beibu Gulf New Materials Company. The thermodynamic and kinetic of CO₂ on decarbonization and manganese retention has been clarified through theoretical analysis, followed by industrial experiments to make further verification. This article aims to clarify and quantify the effect of CO₂ and O₂ mixed blowing on the decarburization and manganese retention of low-carbon stainless steel and provides theoretical and experimental guidance for improving the actual production process.

2. Theoretical Analysis of CO₂ on Decarbonization and Manganese Retention

At present, the research on the effect of CO₂ on decarburization and manganese retention is limited to the thermodynamic, which merely focuses on the competitive oxidation of carbon and manganese in molten steel, therefore there are certain limitations.^{20,21,22,23,24)} However, this research focuses on two aspects—thermodynamics and kinetics. And the theoretical principle of CO₂ on decarbonization and manganese retention is further analyzed by the thermodynamic calculation of element reaction and the kinetic calculation of gas work.

2.1. Thermodynamic Analysis of CO₂ on Decarburization and Manganese Retention

For analysis connivence, the reaction distribution ratio (RDR) has been defined, that is, “the ratio of the amount of gas reacting with a certain element to the total amount of reacting gas when a certain gas reacts with multiple elements”. This study believes that the RDR of the blowing gas O₂ and CO₂ to the [C] and [Mn] in the molten steel is proportional to the Gibbs free energy when the reaction occurs:^25,26)   

$$x_{{\mathrm{O}}_2-\mathrm{C}}=\frac{\Delta G_{{\mathrm{O}}_2-\mathrm{C}}}{\Delta G_{{\mathrm{O}}_2-\mathrm{C}}+\Delta G_{{\mathrm{O}}_2-\mathrm{M}\mathrm{n}}}.$$(1)

  

$$x_{{\mathrm{O}}_2-\mathrm{M}\mathrm{n}}=\frac{\Delta G_{{\mathrm{O}}_2-\mathrm{M}\mathrm{n}}}{\Delta G_{{\mathrm{O}}_2-\mathrm{C}}+\Delta G_{{\mathrm{O}}_2-\mathrm{M}\mathrm{n}}}.$$(2)

  

$$x_{\mathrm{C}{\mathrm{O}}_2-\mathrm{C}}=\frac{\Delta G_{\mathrm{C}{\mathrm{O}}_2-\mathrm{C}}}{\Delta G_{\mathrm{C}{\mathrm{O}}_2-\mathrm{C}}+\Delta G_{\mathrm{C}{\mathrm{O}}_2-\mathrm{M}\mathrm{n}}}.$$(3)

  

$$x_{\mathrm{C}{\mathrm{O}}_2-\mathrm{M}\mathrm{n}}=\frac{\Delta G_{\mathrm{C}{\mathrm{O}}_2-\mathrm{M}\mathrm{n}}}{\Delta G_{\mathrm{C}{\mathrm{O}}_2-\mathrm{C}}+\Delta G_{\mathrm{C}{\mathrm{O}}_2-\mathrm{M}\mathrm{n}}}.$$(4)

where x_O2−C is the RDR of O₂ to [C]; x_O2−Mn is the RDR of O₂ to [Mn]; x_CO2−C is the RDR of CO₂ to [C]; x_CO2−Mn is the RDR of CO₂ to [Mn]; ΔG_O2−C is the Gibbs free energy when [C] reacts with O₂; ΔG_O2−Mn is the Gibbs free energy when [Mn] reacts with O₂; ΔG_CO2−C is the Gibbs free energy when [C] reacts with CO₂; ΔG_CO2−Mn is the Gibbs free energy when [Mn] reacts with CO₂.

The Gibbs free energy of the oxidation reaction of O₂ and CO₂ with [C] and [Mn] in molten steel^27,28) is also listed in Table 1. Among them, ΔG^Θ represents the standard Gibbs free energy of the reaction, J/mol; T is the temperature of molten steel, K; R is the gas constant, which is 8.314 J·mol⁻¹·K⁻¹; p_CO, p_CO2, p_O2 and p^Θ are respectively the partial pressures of CO, CO₂, O₂ and the standard atmospheric pressure; a_C, a_Mn and a_MnO are respectively the activities of C, Mn and MnO in molten steel. The first-order interaction parameters $e_i^j$ of C and Mn elements^29,30) is shown in Table 2.

Table 1. Reaction chemical formula of CO₂ and O₂ with elements in liquid steel.

CO2 reaction equations	ΔG/J·mol−1
[C]+CO2(g)=2CO(g)	${\displaystyle \mathrm{\Delta }{G}_{\mathrm{C}{\mathrm{O}}_2-\mathrm{C}}^{\mathrm{\Theta }}=\mathrm{\Delta }{G}_{\mathrm{C}{\mathrm{O}}_2-\mathrm{C}}-RTln\left(\frac{{\left(p_{\mathrm{C}\mathrm{O}}/p^{\mathrm{\Theta }}\right)}^2}{a_{\mathrm{C}}\cdot p_{\mathrm{C}{\mathrm{O}}_2}/p^{\mathrm{\Theta }}}\right)=137\mathrm{\hspace{0.17em} }890-126.52T}$
[Mn]+CO2(g)=MnO(s)+CO(g)	${\displaystyle \mathrm{\Delta }{G}_{\mathrm{C}{\mathrm{O}}_2-\mathrm{M}\mathrm{n}}^{\mathrm{\Theta }}=\mathrm{\Delta }{G}_{\mathrm{C}{\mathrm{O}}_2-\mathrm{M}\mathrm{n}}-RTln\left(\frac{a_{\mathrm{M}\mathrm{n}\mathrm{O}}\cdot p_{\mathrm{C}{\mathrm{O}}_{}}/p^{\mathrm{\Theta }}}{a_{\mathrm{M}\mathrm{n}}\cdot p_{\mathrm{C}{\mathrm{O}}_2}/p^{\mathrm{\Theta }}}\right)=-133\mathrm{\hspace{0.17em} }760+42.51T}$
[C]+1/2O2(g)=CO(g)	${\displaystyle \mathrm{\Delta }{G}_{{\mathrm{O}}_2-\mathrm{C}}^{\mathrm{\Theta }}=\mathrm{\Delta }{G}_{{\mathrm{O}}_2-\mathrm{C}}-RTln\left(\frac{p_{\mathrm{C}{\mathrm{O}}_{}}/p^{\mathrm{\Theta }}}{a_{\mathrm{C}}\cdot {\left(p_{{\mathrm{O}}_2}/p^{\mathrm{\Theta }}\right)}^{\frac{1}{2}}}\right)=-117\mathrm{\hspace{0.17em} }990-84.35T}$
[Mn]+1/2O2(g)=MnO(s)	${\displaystyle \mathrm{\Delta }{G}_{{\mathrm{O}}_2-\mathrm{M}\mathrm{n}}^{\mathrm{\Theta }}=\mathrm{\Delta }{G}_{{\mathrm{O}}_2-\mathrm{M}\mathrm{n}}-RTln\left(\frac{a_{\mathrm{M}\mathrm{n}\mathrm{O}}}{a_{\mathrm{M}\mathrm{n}}\cdot {\left(p_{{\mathrm{O}}_2}/p^{\mathrm{\Theta }}\right)}^{\frac{1}{2}}}\right)=-408\mathrm{\hspace{0.17em} }150+88.78T}$

Table 2. Coefficient of primary interaction between elements.

i	j	$e_i^j$
C	C	0.1429)
	Mn	−0.008629)
Mn	C	−0.053830)
	Mn	030)

Calculating according to Wagner polynomial:   

$$lg{f}_{\mathrm{C}}=e_{\mathrm{C}}^{\mathrm{C}}[\%\mathrm{C}]+e_{\mathrm{C}}^{\mathrm{M}\mathrm{n}}[\%\mathrm{M}\mathrm{n}].$$(5)

  

$$lg{f}_{\mathrm{M}\mathrm{n}}=e_{\mathrm{M}\mathrm{n}}^{\mathrm{C}}[\%\mathrm{C}]+e_{\mathrm{M}\mathrm{n}}^{\mathrm{M}\mathrm{n}}[\%\mathrm{M}\mathrm{n}].$$(6)

Combining with the [C] and [Mn] mass fraction of the 200 series stainless steel smelted by Guangxi Beibu Gulf New Materials Company during the decarburization period, the activity coefficients of molten steel are calculated according to Eqs. (5) and (6) which is listed in Table 3.

Table 3. Coefficient of primary interaction between elements.

	component (%)	activity coefficients	activity
C	2	1.796	3.591
Mn	3	0.781	2.342

Based on the Gibbs free energy data in Table 1, it can be calculated that when the temperature is 1823 K, CO₂ can react with [C] and [Mn] in molten steel, indicating that it is feasible to use CO₂ replace part of O₂ for decarburization. According to the data in Table 1, Table 3 and Eqs. (1), (2), (3) and (4), x_CO2−C, x_O2−C, x_CO2−Mn and x_O2−Mn are further calculated, indicating that when the temperature is 1823 K, the partial pressure of CO increases from 0.1 to 0.4, and the partial pressure of O₂ (or CO₂) decreases from 0.9 to 0.6, which is shown in Fig. 1.

Fig. 1.

The distribution ratio between O₂, CO₂ and molten steel; (a) [C] distribution ratio; (b) [Mn] distribution ratio. (Online version in color.)

It can be seen from Fig. 1 that the partial pressure of CO has almost no effect on the RDR. The red line in Fig. 1(a) shows that x_CO2−C is about 64%, and the black line indicates that x_O2−C is about 55%. The red line in Fig. 1(b) shows that x_CO2−Mn is about 36%, and the black line indicates that x_O2−Mn is about 45%. Assuming sufficient reaction, using O₂ to oxidize 100 mol [C] in molten steel, 82 mol [Mn] is oxidized at the same time; while using CO₂ to oxidize the same amount of [C], only 56 mol [Mn] is oxidized. The oxidation of [Mn] is greatly reduced by using CO₂.

The thermodynamic calculation results show that, the introduction of CO₂ instead of O₂ can reduce the oxidation of [Mn] by 31.7% with the same amount of [C] being oxidized. That is, when the CO₂ and O₂ mixed blowing technology on decarburization and manganese retention is adopted, the more CO₂ brought in, the effect of manganese retention is better, which is based on the premise of actual production requirement.

2.2. Kinetic Analysis of CO₂ on Decarburization and Manganese Retention

The carbon concentration is relatively high in the early stage of decarburization, and the decarburization rate is proportional to the O₂ blowing rate at this stage. However, the carbon concentration becomes low as the decarburization entering final stage. And the mass transfer process of carbon in the molten steel becomes the limiting link of the decarburization reaction. Enhanced stirring of molten steel can improve the decarburization condition. In the process of AOD decarburization, the stirring energy of the bottom blowing gas to the molten steel includes: (a) the initial kinetic energy of the gas at the gas jet, E₁; (b) the expansion work done by gas expanded from room temperature to molten steel temperature, E₂; (3) the expansion work produced by gas reacting with molten steel elements to generate new gas, E₃; (4) the work done by buoyancy when a gas floats, E₄.

Supposing pure oxygen is used as bottom blowing gas, the O₂ flow rate is Q_O2. The decarburization amount of 1 mol O₂ is the same as that of 2 mol CO₂. Therefore, when O₂ and CO₂ are mixed as the bottom blowing gas, the O₂ flow rate $Q_{{\mathrm{O}}_2}^{\prime }$ is (1−x)Q_O2, and the CO₂ flow rate $Q_{{\text{CO}}_{\text{2}}}^{\prime }$ in the mixed gas is 2x·Q_CO2, where x is the ratio of O₂ reduction, 0≤x≤0.5.

When pure oxygen is used as bottom blowing gas, the work done of the bottom blowing gas E_A1, E_A2, E_A3, E_A4 is as follows:   

$$E_{\mathrm{A}1}=\frac{1}{2}{\rho }_{{\mathrm{O}}_2}{Q}_{{\mathrm{O}}_2}{\left(\frac{Q_{{\mathrm{O}}_2}}{S}\right)}^2.$$(7)

  

$$E_{\mathrm{A}2}=\underset{V_{\mathrm{A}0}}{\overset{V_{\mathrm{A}1}}{\int }}{P}_{1}dV=P_1\left(V_{\mathrm{A}1}-V_{\mathrm{A}0}\right)=n_{\mathrm{A}}R\left(T-T_0\right).$$(8)

  

$$\begin{array}{l}{E}_{\mathrm{A}3}=\underset{V_{\mathrm{A}1}}{\overset{V_{\mathrm{A}2}}{\int }}{P}_{1}dV=P_1\left(V_{\mathrm{A}2}-V_{\mathrm{A}1}\right)\\ \hspace{1em}\hspace{1em}=\left(\left(1-{\eta }_{{\mathrm{O}}_2}\right)n_{\mathrm{A}}+2n_{\mathrm{A}}{\eta }_{{\mathrm{O}}_2}{x}_{{\mathrm{O}}_2-\mathrm{C}}-n_{\mathrm{A}}\right)RT.\end{array}$$(9)

  

$$\begin{array}{l}{E}_{\mathrm{A}4}=\underset{0}{\overset{H}{\int }}{V}^{\prime }\rho gdz=-\underset{P_1}{\overset{P_2}{\int }}{V}^{\prime }dp\\ \hspace{1em}\hspace{1em}=-\underset{P_1}{\overset{P_2}{\int }}\frac{\left(1-{\eta }_{{\mathrm{O}}_2}\right)n_{\mathrm{A}}+2n_{\mathrm{A}}{\eta }_{{\mathrm{O}}_2}{x}_{{\mathrm{O}}_2-\mathrm{C}}}{P}RTdp\\ \hspace{1em}\hspace{1em}=\left(\left(1-{\eta }_{{\mathrm{O}}_2}\right)n_{\mathrm{A}}+2n_{\mathrm{A}}{\eta }_{{\mathrm{O}}_2}{x}_{{\mathrm{O}}_2-\mathrm{C}}\right)RTln\left(\frac{P_1}{P_2}\right).\end{array}$$(10)

where ρ_O2 is the gas density of O₂ in the standard state, 1.429 Kg·m⁻³; Q_O2 is the flow rate of bottom blowing O₂, Nm³·s⁻¹; S is the cross-sectional area of the gas jet, m²; V_A0 is the gas volume flow rate blown into the furnace at room temperature, Nm³·s⁻¹; V_A1 is the gas volume flow rate at molten steel temperature, Nm³·s⁻¹; V_A2 is the gas volume flow rate escaped from the molten steel at molten steel temperature, Nm³·s⁻¹; P₁ is the pressure around the gas jet, Pa; P₂ is the pressure at the surface of the molten steel, Pa; T is the temperature of the molten steel, K; T₀ is the initial temperature of the gas, K; η_O2 is the utilization rate of O₂, which is 0.8 based on experience; n_A is the amount of substance of blowing gas into molten steel when pure oxygen is used as bottom blowing gas, mol·s⁻¹; x_O2−C is 55% which is calculated by Eq. (1).

Under the same conditions, when O₂ and CO₂ mixed gas is used as bottom blowing gas, the work done of the bottom blowing gas E_B1, E_B2, E_B3, E_B4 is as follows:   

$$\begin{array}{l}{E}_{\mathrm{B}1}=\frac{1}{2}{\rho }_{{\mathrm{O}}_2}{Q^{\prime }}_{{\mathrm{O}}_2}{\left(\frac{{Q^{\prime }}_{{\mathrm{O}}_2}+{Q^{\prime }}_{\mathrm{C}{\mathrm{O}}_2}}{S}\right)}^2+\frac{1}{2}{\rho }_{\mathrm{C}{\mathrm{O}}_2}{Q^{\prime }}_{\mathrm{C}{\mathrm{O}}_2}{\left(\frac{{Q^{\prime }}_{{\mathrm{O}}_2}+{Q^{\prime }}_{\mathrm{C}{\mathrm{O}}_2}}{S}\right)}^2\\ \hspace{1em}\hspace{1em}=\frac{1}{2}{\rho }_{{\mathrm{O}}_2}\left(1-x\right)Q_{{\mathrm{O}}_2}{\left(\frac{\left(1+x\right)Q_{{\mathrm{O}}_2}}{S}\right)}^2\\ \hspace{1em}\hspace{1em}+\frac{1}{2}{\rho }_{\mathrm{C}{\mathrm{O}}_2}2x{Q}_{{\mathrm{O}}_2}{\left(\frac{\left(1+x\right)Q_{{\mathrm{O}}_2}}{S}\right)}^2.\end{array}$$(11)

  

$$E_{\mathrm{B}2}=\underset{V_{\mathrm{B}0}}{\overset{V_{\mathrm{B}1}}{\int }}{P}_{1}dV=P_1\left(V_{\mathrm{B}1}-V_{\mathrm{B}0}\right)=n_{\mathrm{B}}R\left(T-T_0\right).$$(12)

  

$$\begin{array}{l}{E}_{\mathrm{B}3}=\underset{V_{\mathrm{B}1}}{\overset{V_{\mathrm{B}2}}{\int }}{P}_{1}dV=P_1\left(V_{\mathrm{B}2}-V_{\mathrm{B}1}\right)\\ =\left(\begin{array}{l}\left(\left(1-{\eta }_{{\mathrm{O}}_2}\right)n_{\mathrm{B}-{\mathrm{O}}_2}+2n_{\mathrm{B}-{\mathrm{O}}_2}{\eta }_{O_2}{x}_{{\mathrm{O}}_2-\mathrm{C}}\right)\\ +\left(\left(1-{\eta }_{\mathrm{C}{\mathrm{O}}_2}\right)n_{\mathrm{B}-\mathrm{C}{\mathrm{O}}_2}+2n_{\mathrm{B}-\mathrm{C}{\mathrm{O}}_2}{\eta }_{\mathrm{C}{\mathrm{O}}_2}{x}_{\mathrm{C}{\mathrm{O}}_2-\mathrm{C}}+n_{\mathrm{B}-\mathrm{C}{\mathrm{O}}_2}{\eta }_{\mathrm{C}{\mathrm{O}}_2}{x}_{\mathrm{C}{\mathrm{O}}_2-\mathrm{M}\mathrm{n}}\right)\\ -n_{\mathrm{B}}\end{array}\right)RT.\end{array}$$(13)

  

$$\begin{array}{l}{E}_{\mathrm{B}4}=\underset{0}{\overset{H}{\int }}{V}^{\prime }\rho gdz=-\underset{P_1}{\overset{P_2}{\int }}{V}^{\prime }dp\\ =-\underset{P_1}{\overset{P_2}{\int }}\left(\begin{array}{l}{\displaystyle \frac{\left(\left(1-{\eta }_{{\mathrm{O}}_2}\right)n_{\mathrm{B}-{\mathrm{O}}_2}+2n_{\mathrm{B}-{\mathrm{O}}_2}{\eta }_{{\mathrm{O}}_2}{x}_{{\mathrm{O}}_2-\mathrm{C}}\right)}{P}}\\ +{\displaystyle \frac{\left(\left(1-{\eta }_{\mathrm{C}{\mathrm{O}}_2}\right)n_{\mathrm{B}-\mathrm{C}{\mathrm{O}}_2}+2n_{\mathrm{B}-\mathrm{C}{\mathrm{O}}_2}{\eta }_{\mathrm{C}{\mathrm{O}}_2}{x}_{\mathrm{C}{\mathrm{O}}_2-\mathrm{C}}+n_{\mathrm{B}-\mathrm{C}{\mathrm{O}}_2}{\eta }_{\mathrm{C}{\mathrm{O}}_2}{x}_{\mathrm{C}{\mathrm{O}}_2-\mathrm{M}\mathrm{n}}\right)}{P}}\end{array}\right)RTdp\\ =\left(\begin{array}{l}\left(\left(1-{\eta }_{{\mathrm{O}}_2}\right)n_{\mathrm{B}-{\mathrm{O}}_2}+2n_{\mathrm{B}-{\mathrm{O}}_2}{\eta }_{{\mathrm{O}}_2}{x}_{{\mathrm{O}}_2-\mathrm{C}}\right)\\ +\left(\left(1-{\eta }_{\mathrm{C}{\mathrm{O}}_2}\right)n_{\mathrm{B}-\mathrm{C}{\mathrm{O}}_2}+2n_{\mathrm{B}-\mathrm{C}{\mathrm{O}}_2}{\eta }_{\mathrm{C}{\mathrm{O}}_2}{x}_{\mathrm{C}{\mathrm{O}}_2-\mathrm{C}}+n_{\mathrm{B}-\mathrm{C}{\mathrm{O}}_2}{\eta }_{\mathrm{C}{\mathrm{O}}_2}{x}_{\mathrm{C}{\mathrm{O}}_2-\mathrm{M}\mathrm{n}}\right)\end{array}\right)RTln\left(\frac{P_1}{P_2}\right).\end{array}$$(14)

where ρ_CO2 is the gas density of CO₂ in the standard state, 1.964 Kg·m⁻³; x is the ratio of O₂ reduction, 0≤x≤0.5; $Q_{{\mathrm{O}}_2}^{\prime }$ is the flow rate of O₂ by using the mixed gas as bottom blowing gas, Nm³·s⁻¹; $Q_{{\text{CO}}_{\text{2}}}^{\prime }$ is the flow rate of CO₂ by using the mixed gas as bottom blowing gas, Nm³·s⁻¹; V_B0 is the gas volume flow rate blown into the furnace at room temperature, Nm³·s⁻¹; V_B1 is the gas volume flow rate at molten steel temperature, Nm³·s⁻¹; V_B2 is the gas volume flow rate escaped from the molten steel at the temperature of molten steel after the reaction of the mixed gas, Nm³·s⁻¹; η_CO2 is the utilization rate of CO₂, which is 0.8 based on experience; n_B is the amount of the substance blown into the molten steel of the mixed gas, n_B=(1+x)n_A, mol·s⁻¹; n_B−O2 is the amount of substance blown into the molten steel of the O₂, n_B−O2=(1−x)n_A, mol·s⁻¹; n_B−CO2 is the amount of substance blown into the molten steel of the CO₂, n_B−CO2=2xn_A, mol·s⁻¹; x_CO2−C is 64% which is calculated by Eq. (3); x_CO2−Mn is 36% which is calculated by Eq. (4).

The ratio of the work done by pure oxygen bottom blowing to the work done by mixed gas bottom blowing is as follows:   

$$\frac{E_{\mathrm{B}1}}{E_{\mathrm{A}1}}={\left(1+x\right)}^2\left(1+\frac{2\mathrm{\hspace{0.17em} }499}{1\mathrm{\hspace{0.17em} }429}x\right),$$(15)

  

$$\frac{E_{\mathrm{B}2}}{E_{\mathrm{A}2}}=1+x,$$(16)

  

$$\frac{E_{\mathrm{B}3}}{E_{\mathrm{A}3}}=1+29.3x,$$(17)

  

$$\frac{E_{\mathrm{B}4}}{E_{\mathrm{A}4}}=1+1.8x,$$(18)

When 0≤x≤0.5, the results of Eqs. (15), (16), (17), and (18) increase as x increasing, and the ratio of each item is greater than 1. It can be concluded that by using CO₂ and O₂ mixed blowing technology on decarbonization and manganese retention, the amount of gas blown increases compared to the original pure oxygen blowing process. And more bottom blowing gas enhances the stirring of molten steel, as a result the kinetic condition of decarburization is improved.

3. Experimental Scheme

Guangxi Beibu Gulf New Materials Company adopts the “BF→AOD→LF” process to produce the 200 series stainless steel. Firstly, the molten iron is added into the AOD furnace, followed by the decarburization period as O₂ blown in and the temperature increased. And 1 ton or 2 tons of high-carbon ferromanganese is added during this period. After the decarburization period, reducing agent such as silico-manganese alloy is added to reduce the metal oxide in the slag and electrolytic manganese is added to improve the composition of manganese, then the steel can be tapped. Since manganese is an easily oxidizable element, which is easily oxidized to manganese oxide during the smelting process. The amount of high-carbon ferromanganese added in the decarburization period is limited, and only 1 ton or 2 tons can be added. Therefore, a large amount of silico-manganese and electrolytic manganese need to be added during the reduction period, which results in the high cost of the alloying production. The use of the mixed CO₂ and O₂ as blowing gas on decarbonization and manganese retention can increase the amount of high-carbon ferromanganese added in the decarburization process under the existing production conditions, and significantly increase the manganese content in the steel by improving the decarburization and manganese retention effect. And the cost of alloying production can also be reduced.

Liu Hongbo et al.²³⁾ showed that the amount of O₂ replaced by CO₂ should not exceed 50%. Considering the influence of molten steel temperature, the amount of O₂ replaced should be from 10% to 30%. Yuewen Fan³¹⁾ used the isotopes ¹⁸O₂ and ¹³CO₂ to determine the source of gas generated by the decarburization reaction. The study finds that when CO₂ accounts for 20% of the total gas, that is, when the volume ratio of O₂ to CO₂ is 4:1, the volume ratio of O₂ reacted with C to CO₂ reacted with C is still 4:1, indicating that the decarburization capacity of O₂ and CO₂ is basically the same. But the volume of O₂ reacting with iron is 7 times the volume of CO₂, indicating the oxidation reaction capacity of CO₂ with iron is weaker than that of O₂, as a result, using CO₂ to replace part of O₂ can also reduce iron oxidation loss.

In this study, the original smelting process on decarburization is set to pure oxygen group, while the use of the mixed CO₂ and O₂ as blowing gas on decarbonization is set as the mixed blowing group. The gas flow rate of the pure oxygen group is set to 140 parts O₂ and 300 parts N₂, and the gas flow rate of the mixed blowing group is set to 120 parts O₂, 40 parts CO₂ and 300 parts N₂. During the actual production process, the gas flow rate is affected by factors such as the composition, temperature, alloying amount, and tap-to-tap time. Therefore, there are certain fluctuations of results.

When the 200 series stainless steel is produced by the original process, 1 ton of high-carbon ferromanganese is added during the decarburization period, and the production data of 120 melts is continuously recorded. Excluding 55 melts with add billet, and briquettes or no high-carbon ferromanganese, the production data of remaining 65 melts is analyzed and marked as group A. And 2 tons of high-carbon ferromanganese are added during the decarburization period, and the production data of 136 melts are continuously recorded. Excluding 95 melts with add billet, and briquettes, and no or one ton of high-carbon ferromanganese added, the production data of remaining 41 melts is analyzed and marked as group C.

By using CO₂ and O₂ mixed blowing process, 1 ton of high-carbon ferromanganese is added during the decarbonization period to produce a total of 3 melts, which is marked as group B, while 2 tons of high-carbon ferromanganese are added during the decarburization period to produce a total of 1 melt, which is marked as group D. During the decarburization period, 3 tons of high-carbon ferromanganese are added, and the experimental production data of 16 melts are continuously recorded. Excluding 1 melt with the mixture of lime and manganese alloy, and the data of remaining 15 melts is analyzed. Among these 15 melts, the first 12 melts are produced in accordance with the designed CO₂ and O₂ mixed blowing process, which is marked as Group E. The last 3 melts are marked as Group F due to the insufficiency of CO₂. The specific scheme of the above six groups is shown in Table 4.

Table 4. Experimental scheme.

			A	B	C	D	E	F
experimental variable (final stage of decarburization)	Gas flow rate	O2 (Nm3/h)	1400	1200	1400	1200	1200	1300
		N2 (Nm3/h)	3000	3000	3000	3000	3000	3000
		CO2 (Nm3/h)	0	400	0	400	400	200
	Gas flow time	minute	40	40	45	45	50	50
	High-carbon ferromanganese	composition	C: 6.7%; Si: 1%; Mn: 75%; P: 0.2%; S: 0.05%
		amount (t)	1	1	2	2	3	3
manufacturing parameter	HM	composition	C: 4.78%; Si: 0.7%; Mn: 0.97%; P: 0.04%; Cr: 3.42%; Ni: 1.51%; Cu: 0.04%;
		amount (t)	45–55
	slag formers	composition	CaO
		amount (t)	6–7
	Electrolytic manganese	composition	Mn: 99.8%
		amount (t)	2–3.5
	High silicon manganese silicon	composition	C: 0.25%; Si: 27%; Mn: 65%;
		amount (t)	2–3
	Medium silicon manganese silicon	composition	C: 1%; Si: 17%; Mn: 65%; P: 0.17%; S: 0.05%;
		amount (t)	2–3
	high carbon ferrochrome	composition	C: 7%; Si: 3.1%; P: 0.03%; Cr: 55%; S: 0.05%
		amount (t)	12–17
	early stage of decarburization	O2 (Nm3/h)	8000–9000
		minute	40–45
	reduction period	Ar (Nm3/h)	4000
		minute	10–15

There are also some input materials in Group A–F, including hot material, CaO, electrolytic manganese, high silicon-manganese, middle silicon-manganese and high carbon ferrochrome, all of whose amount and composition are listed in Table 4. O₂ has been blown in all six groups for 40–45 minutes continuously in the beginning. After the decarbonization period, Ar has been blown in all six groups for 10–15 minutes continuously. And the gas flow rates are also listed in Table 4.

More experimental variable and manufacturing parameter have been shown in Table 4 to make a clear display of the experimental scheme.

4. Results and Discussion

4.1. Tap-to-tap Time

The tap-to-tap time of groups A, C, and E + F is shown in Fig. 2. The number of smelting melts of groups B and D are too few to be concluded in the Fig. 2.

Fig. 2.

Comparison of tap-to-tap time of different smelting processes. (Online version in color.)

Experimental data shows that the average tap-to-tap time of group A, C and E + F is respectively 103, 107 and 112 minutes. The tap-to-tap time of 3 melts of group B is respectively 110, 97 and 110 minutes, and the tap-to-tap time of 1 melt of group D is 109 minutes.

The number of smelting melts in the two groups B and D is too few to calculate an accurate average time, but the tap-to-tap time of group B is within 25%–75% of that of group A, so as the group D and group C. It indicates that the tap-to-tap time of group B and D is at a normal level. Comparing the two groups A and B with 1 ton of high carbon ferromanganese and the C and D groups with 2 tons of high carbon ferromanganese, the CO₂ and O₂ mixed blowing process on decarbonization and manganese retention has no significant effect on the tap-to-tap time.

In addition, experiments have also observed that the average tap-to-tap time increases with the addition of high-carbon ferromanganese during the smelting process. This is because the addition of high-carbon ferromanganese increases the decarburization time. In theory, for every additional ton of high-carbon ferromanganese, extra 4 minutes of decarburization time is required. The average tap-to-tap time of Group E and Group F with 3 tons of high-carbon ferromanganese increases by 5 minutes compared with Group C with 2 tons of high-carbon ferromanganese, because one more steel sample needs to be taken during the experimental production for further calculation, which takes about 1 minute and leads to an increase of 1 minute of the tap-to-tap time. When the CO₂ and O₂ mixed blowing process is applied in actual production, there is no need of sampling, which can save the tap-to-tap time. Since the number of smelting melts in the two groups B and D is relatively few, the comparison between B, D and E+F is not made.

Comparing the pure oxygen group and the mixing blowing group, the tap-to-tap time does not change significantly when 1 to 2 tons of high-carbon ferromanganese are added by using the CO₂ and O₂ mixed blowing process on decarbonization and manganese retention. When 3 tons of high-carbon manganese are added, although the tap-to-tap time has increased slightly, it has no effect on the entire production and the tap-to-tap time is within the acceptable range.

4.2. Manganese Retention Effect

The manganese content in the molten steel at the end of decarburization of decarburization of the 6 groups is shown in Fig. 3.

Fig. 3.

Content of [Mn] at the end of decarburization of different smelting processes. (Online version in color.)

It can be concluded in Fig. 3 that the average manganese content of group A, B, C, D, E and F is respectively 1.2%, 1.52%, 2.0%, 2.45%, 2.8% and 2.2%. Figure 4 shows the relation between the average manganese content and the addition of high-carbon ferromanganese of the 6 groups. For the convenience of analysis, these 6 groups are divided into 3 teams, namely A–C, B–D–E and F.

Fig. 4.

Figure of addition amount of high carbon ferromanganese and manganese content. (Online version in color.)

The CO₂ and O₂ mixed blowing process on decarbonization and manganese retention can effectively reduce the oxidation of manganese in the molten steel and improve manganese content at the end of decarburization with the same amount addition of high-carbon ferromanganese by comparing teams A–B and C–D in Fig. 4. The average manganese content of team B–D–E shows that the manganese content at the end of decarburization increases as the addition of high-carbon ferromanganese increasing from 1 ton to 3 tons.

In addition, the increase rate of manganese content of group E with 3 tons of high-carbon ferromanganese added slightly drops which is compared with groups B and D. This is mainly for two reasons. First, CO₂ decarburization is an endothermic reaction. To ensure the stability of the temperature of molten steel,¹⁴⁾ the amount of CO₂ instead of O₂ set in this paper is not high, and in the decarburization process, the more high-carbon ferromanganese is added, the more obvious of the temperature of the molten steel decreases. The endothermic reaction is inhibited to a certain extent, resulting in a more stringent condition on decarburization and manganese retention; Second, the addition of high-carbon ferromanganese contributes to higher activity of manganese in the molten steel, which promotes the manganese oxidation, therefore it becomes more difficult to decarburize and preserve manganese.

In the smelting process of Group F, the average manganese content is low due to the insufficiency of CO₂ blowing, which indicates the decisive effect of CO₂ on decarbonization and manganese retention effect. If the CO₂ blowing is insufficient, the decarbonization and manganese retention cannot achieve the expected effect.

4.3. Manganese Yield

Manganese yield can be calculated by measured manganese content to actual added manganese content, where the measured manganese content is the quality of manganese in molten steel. The actual added manganese content is the total of the manganese quality added in molten iron, including high-carbon ferromanganese, high-silicon ferromanganese, middle-silicon manganese and electrolytic manganese. Among the 65 melts in group A, 21 melts are mixed with other molten iron ladle, so only 44 melts are calculated, and the average manganese yield is 93.8%; among the 41 melts in group C, 18 melts are mixed with other molten iron ladle, therefore only 23 melts are calculated, and the average manganese yield is 94.8%; among the 12 melts in group E, 4 melts are mixed with other molten iron ladle, and only the remaining 8 melts are calculated, and the average manganese yield is 96.7%.

The average manganese yields of groups A and C and 3 melts of group B, 1 melt of group D, and 8 melts of group E are shown in Fig. 5. Compared to the original process, more high-carbon ferromanganese can be added during the production by using CO₂ and O₂ mixed blowing process, reducing the subsequent addition of other manganese sources such as electrolytic manganese, which consequently lowers the cost of alloying and improves the manganese yield. With 1 ton of high carbon ferromanganese addition, the average manganese yield in group B is 99.4%, increased by 5.6% compared to group A, and the manganese yield in group D is 97.8% with 2 tons of high carbon ferromanganese addition, increased by 3% compared with group C.

Fig. 5.

Mn recovery rates date and the average Mn recovery rates. (Online version in color.)

By comparing the groups B, D, and E, the manganese yield gradually decreases with the increase of high-carbon ferromanganese addition, which contributes to more stringent conditions of decarburization and manganese retention. But the average manganese yield is still higher than that in the original process.

4.4. Analysis of CO₂ and O₂ Mixed Blowing on Decarbonization and Manganese Retention

Combined with the results of industrial production, the effects of CO₂ on decarbonization and manganese retention are mainly analyzed from the following three aspects:

1. Using CO₂ and O₂ mixed blowing process can reduce manganese volatilization in molten steel. Compared with pure oxygen blowing process, the temperature of the bubbles blown into the molten steel is reduced by using CO₂ and O₂ mixed blowing because the reaction of CO₂ with the [C] is an endothermic reaction, which contributes to the lowering of manganese volatilization. Liu Hongbo et al.²⁴⁾ used Factsage software to simulate the smelting process of TWIP steel with a manganese content of 18%. The smelting used 130 tons converter and adopted the CO₂ and O₂ mixed blowing process. The results showed that the increase of manganese volatilization in molten steel was in proportion to the reduction of CO₂.

2. CO₂ is a weak oxidizing gas. The manganese oxidation can be reduced when CO₂ reaches the same decarburization capacity as O₂ in molten steel. Comparing the results of Group E with F, it can be obtained that due to the insufficiency of CO₂, more manganese is oxidized during the decarburization process, and the manganese content in the sample is reduced. This is consistent with the calculation results of thermodynamic analysis in this paper, which confirms that CO₂ can further reduce manganese oxidation than O₂ as the same amount of carbon being oxidized.

3. During the final stage of decarburization, the carbon concentration in the molten steel is lower, as a result, the mass transfer process of carbon is limited, which influences the kinetic condition of decarburization reaction. However, the kinetic condition during the final stage of decarburization can be improved by adopting CO₂ and O₂ mixed blowing process mainly for two reasons. First, the total amount of gas blown increases by using CO₂ and O₂ mixed blowing, because 120 parts of O₂ and 40 parts of CO₂ are blown in instead of the original 140 parts of O₂. Therefore, the stirring of the molten steel increases. Second, CO₂ can also generate CO when it reacts with manganese in molten steel, further enhancing the stirring of the molten steel.

5. Conclusions

This paper takes the 200 series stainless steel produced by Guangxi Beibu Gulf New Material Company as the research object. Through theoretical analysis of CO₂ on decarbonization and manganese retention and combined with industrial experimental production, the effect of the CO₂ and O₂ mixed blowing on decarbonization and manganese retention is verified. And the conclusions are:

(1) According to the calculation results of the thermodynamic theory, the use of CO₂ instead of part of O₂ can reduce the manganese oxidation with the same amount of carbon being oxidized. The calculation results of the kinetic theory show that by using twice the amount of CO₂ instead of O₂, the amount of gas blown into molten steel increases with more CO being generated during decarburization. The mass transfer rate of carbon in the final stage of decarburization increases because of the enhanced stirring of the molten steel. As a result, the kinetic condition of decarburization improves greatly.

(2) The results of the industrial experiment show that by using CO₂ and O₂ mixed blowing process, the tap-to-tap time increases by 4–5 minutes for every additional ton of high-carbon ferromanganese during the decarbonization period. But it has no effect on the entire production and the tap-to-tap time is within the acceptable range.

(3) The composition of the 200 series stainless steel produced in the industrial experiment shows that the CO₂ and O₂ mixed blowing process can reduce the oxidation of manganese in the molten steel and increase the manganese content. And the manganese yield is improved by 3–5.6%. In addition, CO₂ has a decisive effect on the decarburization and manganese retention. If the CO₂ is insufficient, the decarburization and manganese retention effect cannot be guaranteed.

Acknowledgements

The authors gratefully acknowledge the financial support of this research by “the Fundamental Research Funds for the Central Universities” (Grant No. FRF-MP-20-55).

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