Startseite Effect of TiO2 content in slag on Ti content in molten steel
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Effect of TiO2 content in slag on Ti content in molten steel

  • Gang Gao , Xiaofang Shi EMAIL logo , Zhenghai Zhu und Lizhong Chang
Veröffentlicht/Copyright: 20. April 2021
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High Temperature Materials and Processes
Aus der Zeitschrift High Temperature Materials and Processes Band 40 Heft 1

Abstract

A calculation model of activity for CaO–SiO2–MgO–Al2O3–TiO2 slag is established according to molecular-ion coexistence theory of slag structure to calculate the activities of Al2O3, SiO2, and TiO2 in the slag. The possibility of TiO2 reduction in the slag during refining is analyzed by thermodynamics and verified by laboratory and industrial experiments. Both theoretical analysis and laboratory experimental results show that the content of TiO2 in the ladle slag significantly influences the Ti content in molten steel. When the content of the dissolved aluminum in molten steel is 0.030–0.050%, the TiO2 content in the ladle slag should be controlled below 0.3% to prevent TiO2 reduction. The critical content of TiO2 decreases with an increasing amount of the dissolved aluminum in molten steel. In addition, silicon should be used as a deoxidizer during diffused deoxidization because aluminum as a deoxidizer would lead to the reduction of TiO2. The industrial experiments confirm the results of the laboratory experiments and thermodynamics analysis.

Keywords: clean steel; Ti; TiO2 ; slag; coexistence theory

1 Introduction

Ti is a harmful element in special steel, such as bearing steel, spring steel, and wheel steel. Ti exerts a strong combining force with dissolved nitrogen in steel and exists in the form of inclusions, such as Ti nitride and Ti carbonitride. These inclusions are hard and angular in shape and can severely affect the fatigue life of mechanical parts, such as bearings and wheels. Inclusions containing Ti are particularly harmful especially when the cleanliness is significantly improved, and there is very little oxide inclusion existing in steel. The effect of Ti nitride inclusions, with an average size of 6 µm, on the fatigue property of steel is equivalent to the oxide inclusions, with an average size of 25 µm [1,2,3,4,5]. Therefore, the amount of Ti nitride inclusions should be strictly controlled in the production of high-quality steel.

The contents of Ti and N in steel must be decreased to prevent the formation of TiN inclusions. Bearing and wheel steel with severe service conditions are generally treated by vacuuming to decrease the dissolved nitrogen content to as low as 0.004%. Further reducing the dissolved nitrogen content is extremely difficult; in this case, Ti content in the steel should be controlled.

The source of Ti should be first analyzed to control Ti content in molten steel. Based on the literature and production practices, Ti content in molten steel mainly originates from the following: (1) residual Ti in molten steel at the end of the converter or electric arc furnace smelting; (2) Ti as an impurity element from ferroalloy during alloying; (3) and reduction of TiO2 in the ladle slag into molten steel during refining [6,7,8]. According to the author’s surveys, oxidation smelting leads to be very low [Ti] content (approximately 0.0002–0.0005%) in molten steel at the end of converter or electric arc furnace smelting. Most Ti in ferroalloys are incorporated into molten steel during alloying. In general, low Ti content in the ferroalloy results in low [Ti] content in molten steel. However, ferroalloys with low [Ti] content are very expensive. Therefore, the [Ti] content in the ferroalloy should not be necessarily too low for the control of the cost of steel. Thus far, the appropriate amount of [Ti] in ferroalloys remains unknown. The [Ti] content in the product is determined by the customer before steelmaking and is generally determined at the end of the converter or electric arc furnace smelting. The [Ti] content in the ferroalloy can be determined if the [Ti] content from reducing TiO2 in the ladle slag is determined. To investigate TiO2 reduction and reduction in its amount, the reduction rule of TiO2 in ladle slag during the refining process is analyzed by thermodynamics calculation and is verified through laboratory experiment and industrial production.

2 Thermodynamic analysis

2.1 Reduction thermodynamics of TiO2 in the ladle slag

During the refining of bearing and wheel steels, a certain amount of dissolved aluminum exists in molten steel. Under suitable thermodynamic conditions, the dissolved aluminum can react with TiO2 in the ladle slag, thereby increasing the [Ti] content in molten steel. The reaction is given by equation (1) [8,9,10,11]:

(1) 2 3 [ Al ] + 1 2 ( TiO 2 ) = 1 3 ( Al 2 O 3 ) + 1 2 [ Ti ] Δ G 1 θ = − 71 , 653 + 17.72 T ( J/mol ) Δ G [ Al ] = Δ G 1 θ + RT ln a Al 2 O 3 1 / 3 ⋅ a [ Ti ] 1 / 2 a [ Al ] 2 / 3 ⋅ a TiO 2 1 / 2 ( J/mol ) ,

where a Al 2 O 3 is the activity of Al2O3 in the ladle slag; a TiO 2 is the activity of TiO2 in the ladle slag; and a [ Al ] , a [ Ti ] are the activities of dissolved aluminum and [Ti] in molten steel. Activity is calculated using 1% solution as the reference state.

In a very empirical manner, it is assumed that the activity coefficient is equal to unity when the concentration of element is very low, as occurs in the present steels studied. Therefore, a [ Al ] , a [ Ti ] are defined as the concentration (wt%) of the Al, Ti in the steel.

When Δ G [ Al ] is less than zero, TiO2 in the ladle slag will be reduced by the dissolved aluminum in molten steel.

A diffusion deoxidizer for slag deoxidation is added to the ladle slag for rapid deoxidation to create a reducing atmosphere during refining. The most commonly used diffusion deoxidizers are Al and SiFe. The diffusion deoxidizer can possibly reduce TiO2. The thermodynamics equation of the reaction between Al deoxidizer and TiO2 is given by equation (2) [8,9,10,11].

(2) 2 3 Al + 1 2 ( TiO 2 ) = 1 3 ( Al 2 O 3 ) + 1 2 Ti Δ G 2 θ = − 94 , 083 + 13.95 T ( J/mol ) Δ G Al = Δ G 2 θ + R T ln a Al 2 O 3 1 / 3 ⋅ a Ti 1 / 2 a Al 2 / 3 ⋅ a TiO 2 1 / 2 ( J/mol ) ,

where the activities of Al and Ti in molted steel are calculated using pure substance as the reference state, a Al = 1 , a Ti = 1 ; and the activities of the other components are as same as equation (1).

The thermodynamic equation of the reaction between Si deoxidizer and TiO2 is given by equation (3) [8,9,10,11].

(3) Si + ( TiO 2 ) = ( SiO 2 ) + Ti Δ G 3 θ = − 40 , 730 + 11.32 T ( J/mol ) Δ G Si = Δ G 3 θ + R T ln a SiO 2 ⋅ a Ti a Si ⋅ a TiO 2 ( J/mol ) ,

where activities of Si and Ti in molted steel are calculated using pure substance as the reference state, a Si = 1 , a Ti = 1 ; a SiO 2 is the activity of SiO2 in the ladle slag; and a TiO 2 is the activity of TiO2 in the ladle slag.

When Δ G Al and Δ G Si are less than zero, TiO2 in the ladle slag can be reduced by Al or Si.

Based on equations (1)–(3), the activities of different components in the ladle slag should be calculated first to determine whether TiO2 can be reduced.

2.2 Calculation model of activity for slag component

The structure of the high-temperature solution is very complicated. Directly determining the internal structure of liquid slag by experiments is difficult because of limitations of research and experimental methods. According to empirical and theoretical analyses, the following structures are proposed: molecular structure, ion structure theory, and molecular-ion coexistence theory model. Molecular and ion structure models exhibit limitations, but molecular-ion coexistence theory has achieved satisfactory results [12,13,14,15]. Molecular-ion coexistence theory is originally proposed by Chuiko [16] and amended by Zhang to become a complete set of model systems [17]. In this paper, molecular-ion coexistence theory is used to calculate the activity of slag component.

2.2.1 Hypothesis

  1. The structural units in CaO–SiO2–Al2O3–MgO–TiO2 molten slags are composed of simple ions, simple molecules, and complex molecules. Each structural unit is independent in slags.

  2. The forming of complex molecules is confirmed by reactions of chemical dynamic equilibrium.

  3. The chemical reactions of forming complex molecules obey the mass action law.

  4. The structural units in slags obey the law of conservation of mass.

2.2.2 Structural Units in CaO–SiO2–Al2O3–MgO–TiO2 Slag System

According to the molecular-ion coexistence theory, the structural units in the slag at 1,873 K are as follows:

  • Simple ions: Ca2+, O2−, Mg2+

  • Simple molecules: SiO2, Al2O3, TiO2

  • Complex molecules: (1) 3CaO·Al2O3, (2) 12CaO·7Al2O3, (3) CaO·Al2O3, (4) CaO·2Al2O3, (5) CaO·6Al2O3, (6) CaO·SiO2, (7) 2CaO·SiO2, (8) 3CaO·SiO2, (9) CaO·MgO·2SiO2, (10) 2CaO·MgO·2SiO2, (11) CaO·MgO·SiO2, (12) 3CaO·MgO·2SiO2, (13) CaO·TiO2, (14) 3CaO·2TiO2, (15) 4CaO·3TiO2, (16) Al2O3·TiO2, (17) CaO·SiO2·TiO2, (8) 3Al2O3·2SiO2, (19) 2CaO·Al2O3·SiO2, (20) CaO·Al2O3·2SiO2, (21) MgO·Al2O3, (22) 2MgO·SiO2, (23) MgO·SiO2, (24) MgO·2TiO2, (25) MgO·TiO2, (26) 2MgO·TiO2.

Let m 1 = n CaO , m 2 = n sio 2 , m 3 = n Al 2 O 3 , m 4 = n MgO , m 5 = n TiO 2 , N 1 = N CaO , N 2 = N MgO , N 3 = N Al 2 O 3 , N 4 = N SiO 2 , N 5 = N TiO 2 , N 6 = N 3 CaO ⋅ Al 2 O 3 , N 7 = N 12 CaO ⋅ 7 Al 2 O 3 , … , and N 31 = N 2 MgO ⋅ TiO 2 . The ∑X is the sum of the amounts of substance of each structural unit. m 1, m 2, m 3, m 4, and m 5 represent the mole percentages of CaO, SiO2, Al2O3, MgO, and TiO2 in the CaO–SiO2–Al2O3–MgO–TiO2 slag system, respectively. N i (i = 1, 2, 3,…, 31) represents the mole percentages of different structural units, that is, the activity of each structural unit.

According to the coexistence theory, the main reactions involved in the model are presented in Table 1 [10,16].

Table 1

Chemical reactions in CaO–SiO2–Al2O3–MgO–TiO2 slag system

Chemical reactions Standard Gibbs free energy (J/mol) Balanced relationships
3(Ca2+ + O2−)(s) + (Al2O3)(s) = (3CaO·Al2O3)(s) −12,600 − 24.69 T N 6 = K 1 N 1 3 N 3 (4)
12(Ca2+ + O2−)(s) + 7(Al2O3)(s) = (12CaO·7Al2O3)(s) −47,279 − 180.46 T N 7 = K 2 N 1 12 N 3 7 (5)
(Ca2+ + O2−)(s) + (Al2O3)(s) = (CaO·Al2O3)(s) −18,000 − 18.83 T N 8 = K 3 N 1 N 3 (6)
(Ca2+ + O2−)(s) + 2(Al2O3)(s) = (CaO·2Al2O3)(s) −16,700 − 25.52 T N 9 = K 4 N 1 N 3 2 (7)
(Ca2+ + O2−)(s) + 6(Al2O3)(s) = (CaO·6Al2O3)(s) −16,380 − 37.58 T N 10 = K 5 N 1 N 3 6 (8)
(Ca2+ + O2−)(s) + (SiO2)(s) = (CaO·SiO2)(s) −81,416 − 10.50 T N 11 = K 6 N 1 N 4 (9)
2(Ca2+ + O2−)(s) + (SiO2)(s) = (2CaO·SiO2)(s) −16,043 + 4.106 N 12 = K 7 N 1 2 N 4 (10)
3(Ca2+ + O2−)(s) + (SiO2)(s) = (3CaO·SiO2)(s) −93,366 − 23.03 T N 13 = K 8 N 1 2 N 4 (11)
(Ca2+ + O2−)(s) + (Mg2+ + O2−)(s) + 2(SiO2)(s) = (CaO·MgO·2SiO2)(s) −162,758 − 18.83 T N 14 = K 9 N 1 N 2 N 4 2 (12)
2(Ca2+ + O2−)(s) + (Mg2+ + O2−)(s) + 2(SiO2)(s) = (2CaO·MgO·2SiO2)(s) −73,688 − 63.64 T N 15 = K 10 N 1 2 N 2 N 4 2 (13)
(Ca2+ + O2−)(s) + (Mg2+ + O2−)(s) + (SiO2)(s) = (CaO·MgO·SiO2)(s) −124,683 + 3.77 T N 16 = K 11 N 1 N 2 N 4 (14)
(Ca2+ + O2−)(s) + (Mg2+ + O2−)(s) + (SiO2)(s) = (CaO·MgO·SiO2)(s) −315,469 + 24.786 T N 17 = K 12 N 1 3 N 2 N 4 2 (15)
(Ca2+ + O2−)(s) + (TiO2)(s) = (CaO·TiO2)(s) −79,900 − 3.35 T N 18 = K 13 N 1 N 5 (16)
3(Ca2+ + O2−)(s) + 2(TiO2)(s) = (3CaO·2TiO2)(s) −206,690 − 17.03 T N 19 = K 14 N 1 3 N 5 2 (17)
4(Ca2+ + O2−)(s) + 3(TiO2)(s) = (4CaO·3TiO2)(s) −292,900 − 17.57 T N 20 = K 15 N 1 4 N 5 3 (18)
(Al2O3)(s) + (TiO2)(s) = (Al2O3·TiO2)(s) −25,300 + 3.93 T N 21 = K 16 N 3 N 5 (19)
(Ca2+ + O2−)(s) + (SiO2)(s) + (TiO2)(s) = (CaO·SiO2·TiO2)(s) −122,591 + 10.88 T N 22 = K 17 N 1 N 4 N 5 (20)
3(Al2O3)(s) + 2(SiO2)(s) = (3Al2O3·2SiO2)(s) −8,600 − 17.41 T N 23 = K 18 N 3 2 N 4 2 (21)
2(Ca2+ + O2−)(s) + (Al2O3)(s) + (SiO2)(s) = (2CaO·Al2O3·SiO2)(s) −171125.6 + 8.79 T N 24 = K 19 N 1 2 N 3 N 4 (22)
(Ca2+ + O2−)(s) + (Al2O3)(s) + 2(SiO2)(s) = (CaO·Al2O3·2SiO2)(s) 138,969 + 17.15 T N 25 = K 20 N 1 N 3 N 4 2 (23)
(Al2O3)(s) + (Mg2+ + O2−)(s) = (MgO·Al2O3)(s) −35,530 − 2.09 T N 26 = K 21 N 2 N 3 (24)
2(Mg2+ + O2−)(s) + (SiO2)(s) = (2MgO·SiO2)(s) −67,200 + 4.13 T N 27 = K 22 N 2 2 N 4 (25)
(Mg2+ + O2−)(s) + (SiO2)(s) = (MgO·SiO2)(s) −41,100 + 6.1 T N 28 = K 23 N 2 N 4 (26)
(Mg2+ + O2−)(s) + 2(TiO2)(s) = (MgO·2TiO2)(s) −27,600 + 0.63 T N 29 = K 22 N 2 N 5 2 (27)
(Mg2+ + O2−)(s) + (TiO2)(s) = (MgO·TiO2)(s) −26,400 + 3.14 T N 30 = K 22 N 2 N 4 (28)
2(Mg2+ + O2−)(s) + (TiO2)(s) = (2MgO·TiO2)(s) −25,500 + 1.26 T N 31 = K 26 N 2 2 N 5 (29)

According to the law of conservation of mass, the equations (30–34) are listed as follows:

(30) ∑ i = 1 31 N i = 1 ,

(31) ∑ n CaO = ( N 1 + 3 N 6 + 12 N 7 + N 8 + N 9 + N 10 + N 11 + 2 N 12 + 3 N 13 + N 14 + 2 N 15 + N 16 + 3 N 17 + N 18 + 3 N 19 + 4 N 20 + 4 N 22 + 2 N 24 + N 25 ) ∑ X ,

(32) ∑ n MgO = ( N 2 + N 14 + N 15 + N 16 + N 17 + N 26 + 2 N 27 + N 28 + N 29 + N 30 + 2 N 31 ) ∑ X ,

(33) ∑ n Al 2 O 3 = ( N 3 + N 6 + 7 N 7 + N 8 + 2 N 9 + 6 N 10 + N 21 + 3 N 23 . + N 24 + N 25 + N 26 ) ∑ X ,

(34) ∑ n sio 2 = ( N 4 + N 11 + N 12 + N 13 + 2 N 14 + 2 N 15 + N 16 + 2 N 17 + N 22 + 2 N 29 + N 30 + N 31 ) ∑ X .

Equations (4)–(34) comprise the calculation models for CaO–SiO2–Al2O3–MgO–TiO2 slags. The calculating model is a system of nonlinear multivariable equations. Programming by MATLAB, the calculation model is solved.

2.3 Analysis

Under the actual production conditions, refining slags with different TiO2 contents are designed, as presented in Table 2.

Table 2

Mass percentage of different components in the refining slag, %

Scheme Component
CaO SiO2 MgO Al2O3 TiO2
1 55 11 5 28.9 0.1
2 55 11 5 28.8 0.2
3 55 11 5 28.7 0.3
4 55 11 5 28.6 0.4
5 55 11 5 28.5 0.5
6 55 11 5 28.4 0.6
7 55 11 5 28.3 0.7

According to molecular-ion coexistence theory, the activities of Al2O3, TiO2, and SiO2 are calculated, as presented in Table 3.

Table 3

The activities of Al2O3, TiO2, and SiO2 in the different refining slag

Component Scheme
1 2 3 4 5 6 7
Al2O3 0.0111536 0.0111101 0.0110845 0.0110518 0.0110197 0.0109885 0.0109578
TiO2 0.0000086 0.0000164 0.0000235 0.0000302 0.0000364 0.0000423 0.0000479
SiO2 0.0000324 0.0000324 0.0000325 0.0000325 0.0000325 0.0000326 0.0000326

According to Table 3 and equation (1), when the [Ti] content in the molten steel is 0.001% and the dissolved aluminum is 0.02–0.09%, Δ G [ Al ] varies with the change in TiO2 in the ladle slag (Figure 1). When the dissolved aluminum in molted steel is less than 0.05% and the TiO2 content in the ladle slag is less than 0.3%, Δ G [ Al ] is greater than zero, that is, the dissolved aluminum in molted steel could not reduce TiO2. However, Δ G [ Al ] would be less than zero with increasing TiO2 content in the ladle slag, indicating that the dissolved aluminum in steel could reduce TiO2 in the slag. When the dissolved aluminum in molted steel is larger than 0.07%, Δ G [ Al ] remains less than zero even if the TiO2 content in the ladle slag is 0.2%. With increasing [Al], the critical content of TiO2 reduces markedly.

Figure 1 Variation in Gibbs free energy of the reaction between [Al] and TiO2 in the slag at 1,873 K.
Figure 1

Variation in Gibbs free energy of the reaction between [Al] and TiO2 in the slag at 1,873 K.

According to equations (2)–(3) and Table 3, Δ G Al and Δ G Si vary with the change in TiO2 in the ladle slag when different deoxidizers are used in slag deoxidation (Figure 2). As shown in Figure 2(a), even if the TiO2 content in the slag is 0.1%, Δ G Al will remain less than zero. That is, Al could still possibly reduce the TiO2 in the slag. With increasing TiO2 content in the ladle slag, the tendency of Al reducing TiO2 is promoted. Figure 2(b) shows that even if the TiO2 content in the slag reaches 0.7%, Δ G Si is still greater than zero, that is, Si could not reduce TiO2 in the slag.

Figure 2 Variation in Gibbs free energy with different diffusion deoxidizer. (a) Al as diffusion deoxidizer; (b) Si as diffusion deoxidizer.
Figure 2

Variation in Gibbs free energy with different diffusion deoxidizer. (a) Al as diffusion deoxidizer; (b) Si as diffusion deoxidizer.

3 Laboratory experiment

3.1 Experimental materials and device

The diagram of experimental devices is shown in Figure 3. The main experimental equipment was a self-control style electronic high-temperature furnace with silicon molybdenum rods as a heating element, and an alumina crucible with the experimental sample was closed in a corundum tube within the furnace. To avoid the oxidation of molten steel, the Ar (600 mL/min) was injected into the furnace by corundum tube. The slag was the mixture of CaO, SiO2, MgO, Al2O3, and TiO2, all of which were analytically pure grade. The amount of metal and slag in each heat is 1,000 and 100 g, respectively.

Figure 3 Diagram of experimental devices.
Figure 3

Diagram of experimental devices.

3.2 Experimental scheme and process

The experiment was divided into two parts: the reaction between the dissolved aluminum and TiO2 in slag, and the reaction between the diffusion deoxidizer (Al or Si) and TiO2 in slag. The chemical composition of steel was as follows (wt%): C: 0.5–0.6, Si: 0.3–0.4, and Mn: 1.0–1.2.

  1. Reaction between the dissolved aluminum and TiO2 in slag

    After industrial pure iron in the alumina crucible was melted at 1,873 K, the carbon, metallic silicon, manganese, and pure aluminum were added to molten steel. A sample was obtained from molten steel to analyze initial [Ti] content. After sampling, refining slag with different TiO2 contents was added to molten steel. The sample was obtained after 10 and 15 min to analyze [Ti] and [Al] content through ARL-4460 photoelectric direct reading spectrometry analysis. According to the calculation results in Figure 1, the dissolved aluminum in molten steel was divided into two ranges: 0.03–0.050% and 0.07–0.09%. The composition of the refining slag and [Al] content in the steel is presented in Table 4.

    Table 4

    The component of refining slag with different TiO2 and Al content in steel, %

    Experimental schemes Refining slag Al content in steel
    CaO SiO2 MgO Al2O3 TiO2 0.03–0.050 Al 0.07–0.090 Al
    1 55 11 5 28.90 0.10 0.042 0.077
    2 55 11 5 28.80 0.20 0.035 0.075
    3 55 11 5 28.75 0.25 0.041 0.082
    4 55 11 5 28.70 0.30 0.047 0.083
    5 55 11 5 28.60 0.40 0.039 0.074
    6 55 11 5 28.55 0.45 0.044 0.080
    7 55 11 5 28.50 0.50 0.047 0.085
    8 55 11 5 28.30 0.70 0.044 0.077

  2. Reaction between the diffusion deoxidizer (Al or Si) and TiO2 in slag

    After the alloying was finished and the initial [Ti] content in the sample was determined, refining slag with different TiO2 contents was added to molten steel. The refining slag was melted and added with 1.0 g metallic aluminum with a diameter of 2–4 mm or 2.0 g metallic silicon with a diameter of 0.5–1 mm. After 2 and 7 min, the [Ti] content in the sample was analyzed. The experimental schemes are presented in Table 5.

Table 5

Experiment schemes

Experimental schemes Refining slag, % Diffusion deoxidizer
CaO SiO2 MgO Al2O3 TiO2
1 55 11 5 28.9 0.1 Aluminum
2 55 11 5 28.7 0.3 Aluminum
3 55 11 5 28.5 0.5 Aluminum
4 55 11 5 28.3 0.7 Aluminum
5 55 11 5 28.7 0.3 Silicon
6 55 11 5 28.5 0.5 Silicon

3.3 Experimental results

When the dissolved aluminum in molten steel was 0.030–0.050%, [Ti] content in the molten steel varied with different TiO2 contents in the slag (Figure 4(a)). As shown in Figure 4(a), the initial [Ti] content in molten steel after alloying was between 0.0011 and 0.0015%. When the TiO2 content in the slag was less than 0.3%, the [Ti] content in the steel was almost unchanged. That is, the dissolved aluminum almost did not reduce TiO2 in the slag. When the TiO2 content in the slag was more than 0.3%, the [Ti] content increased obviously. For example, when the TiO2 content in the slag was 0.4%, the initial Ti content was 0.0012% and the [Ti] content increased to 0.0020% after 10 min. In addition, the [Ti] content was almost the same at 10 and 15 min, indicating that the reaction between [Al] and TiO2 reached the equilibrium at 10 min.

Figure 4 Content of [Ti] in steel varying with the content of TiO2 in slag. (a) 0.030–0.05% [Al]; (b) 0.070–0.09% [Al].
Figure 4

Content of [Ti] in steel varying with the content of TiO2 in slag. (a) 0.030–0.05% [Al]; (b) 0.070–0.09% [Al].

When the dissolved aluminum was 0.070–0.09%, the [Ti] content in the molten steel varied with different TiO2 contents in the slag (Figure 4(b)). As shown in Figure 4(b), the dissolved aluminum (less than 0.2%) almost could not reduce TiO2. When the TiO2 content was greater than 0.2%, TiO2 was rapidly reduced by dissolved aluminum, thereby significantly increasing the [Ti] content in steel. For example, when the TiO2 content in the slag was 0.3%, the initial [Ti] content was 0.0012% and the [Ti] content increased to 0.0022% at 10 min. When the TiO2 content was 0.7%, the [Ti] content increased from the initial 0.0013 to 0.0046%. Comparison of Figure 4(a) and (b) shows that the critical TiO2 content of the reaction between TiO2 and dissolved aluminum obviously decreased with increasing dissolved aluminum content. The increase in the [Ti] content was more obvious when the content of dissolved aluminum in steel was higher.

Figure 5 shows that the amount of [Ti] changed when the aluminum or silicon was added to the refining slag for diffused deoxidation. When using aluminum as a deoxidizer, the [Ti] content in the molten steel increased even if the TiO2 content in the slag was 0.1%. The [Ti] content in the molten steel increased rapidly with increasing TiO2 content in the slag. When silicon was used as the diffusion deoxidizer, the [Ti] content was almost unchanged even if the TiO2 in the slag increased to 0.5%. Therefore, TiO2 was not reduced when using silicon for diffusion deoxidation.

Figure 5 Variation in [Ti] content by adding different deoxidizer in refining slag. (a) Al as diffusion deoxidizer; (b) Si as diffusion deoxidizer.
Figure 5

Variation in [Ti] content by adding different deoxidizer in refining slag. (a) Al as diffusion deoxidizer; (b) Si as diffusion deoxidizer.

Based on Figures 1, 2, 4, and 5, the experimental results are consistent with the thermodynamics calculation. Therefore, the TiO2 content in the ladle slag significantly influences the [Ti] content in molten steel during refining. To prevent the reduction of TiO2, this work suggests the following: the amount of the dissolved aluminum in steel should not exceed 0.05%, the TiO2 content in the refining slag should be controlled below 0.3%, and ferrosilicon or silicon carbide should be used for diffusion deoxidation.

4 Industrial experiments

Laboratory experiments were small crucible experiments, whereas actual industrial refining was in a large steel ladle, which can provide a better kinetic condition for steel/slag reaction. The bottom blow system in industrial production was also beneficial for steel/slag reaction. Therefore, the slag/steel equilibrium in industrial production can realize within 80–90 min ladle refining [18], and the production of the steel with refining slags of low TiO2 content was carried out based on the thermodynamic calculation and laboratory experimental results.

The steel composition was the same as that shown in Section 3.2, and the dissolved aluminum was determined as 0.01–0.04%. The 80-t EAF–LF–RH–LF–CCM process was used in the production in a domestic steel company, and 10 heats melted. The roughing slag amount during EAF tapping was less than 0.35 kg per ton of steel and LF refining time was 1 h. The weight of refining slag accounted for 2% of the weight of liquid steel. Liquid steel samples were obtained during refining and used to determine [Ti] content through ARL-4460 photoelectric direct reading spectrometry analysis. The specific sampling schemes were as follows:

  1. At the end of EAF smelting, sample 1 was obtained before tapping when the composition and temperature met the requirement.

  2. During tapping, alloys, such as Al, Si, Mn, and C, and two-third of the refined slag were added to the ladle. The ladle was transferred to LF. Sample 2 was obtained at this time.

  3. Heating and soft stirring were conducted to homogenize molten steel and the slag. According to component requirements, ferroalloys and the rest of the refined slag were added to molten steel. Sample 3 was then obtained.

  4. During subsequent LF refining, samples 4 and 5 were obtained at 15–20 min intervals.

  5. After LF refining was completed, the ladle was transferred to RH vacuum degassing for 25 min. Samples 6 and 7 were obtained before and after vacuuming, respectively.

  6. After RH refining was completed, the soft stirring lasted for 10 min and the steel was transferred to the tundish where the sample 8 was taken.

During the refining process, 1.5 kg SiC per ton of steel was added for diffusion deoxidization. At the end of the refining, slag samples were obtained to determine TiO2 content through X-ray fluorescence spectrometry analysis (ARLAdvant’X Intellipower™ 3600). The TiO2 content in the slag was determined to be 0.01–0.03%. The variation in the [Ti] content in molten steel in all heats is shown in Figure 6.

Figure 6 Variation in [Ti] content in molten steel during the smelting process.
Figure 6

Variation in [Ti] content in molten steel during the smelting process.

As shown in Figure 6, at the end of EAF, the [Ti] content was ∼0.0003% (average Ti content in 10 heats) and then rapidly increased to ∼0.0007% (sample 2) because of deoxidation and alloying process during tapping process. The ferroalloys were continuously added to the liquid steel to adjust the composition from sample 2 to sample 3 during LF refining process, which leads to the increase in Ti content. Therefore, the [Ti] content in sample 3 further increased to ∼0.0011% after alloying. After sample 3, the alloying of molten steel was complete and no additional ferroalloys were added to the liquid steel. During the alloying process, the Ti content in the steel increased from ∼0.0003 to ∼0.0011%. The [Ti] content varied slightly from samples 3 to 8, indicating that TiO2 could not be reduced. The industrial experiments confirm the accuracy of the results of the laboratory experiments and thermodynamics analysis.

5 Conclusion

  1. Molecular-ion coexistence theory is adopted to calculate the activities of the slag components. According to calculation results, the possibility of the reaction between aluminum and TiO2 is analyzed from a thermodynamics point of view. The results show that TiO2 can be reduced by the dissolved aluminum when the TiO2 content in the refining slag and the dissolved aluminum in the molten steel are kept at a suitable level at 1,873 K.

  2. Laboratory experiments show that the Ti content in the molten steel almost does not increase when the aluminum content is 0.03–0.50%, and the TiO2 content in the ladle slag is less than 0.3% at 1,873 K. However, even if the TiO2 content in the ladle slag is 0.2%, it can be reduced by 0.07–0.09% dissolved aluminum. When the aluminum is used as a diffusion deoxidizer, TiO2 is easily reduced. Opposite results are obtained when using silicon as a deoxidizer. The industrial experimental results are consistent with that of the laboratory experimental results.

  3. The reduction of TiO2 can be prevented through the following: the dissolved aluminum in steel should not exceed 0.05%, the TiO2 content in the refining slag should be controlled below 0.3%, and ferrosilicon or silicon carbide should be used for diffusion deoxidation.

Acknowledgments

The authors gratefully acknowledge financial support from the National Nature Science Foundations of China (Grant No. 51974002).

  1. Funding information: This study was funded by the National Nature Science Foundations of China (Grant No. 51974002).

  2. Author contributions: Gang Gao: experiment and data. Xiaofang Shi: writing and funding. Zhenghai Zhu: experiment. Lizhong Chang: revision.

  3. Conflict of interest: Authors state no conflict of interest.

  4. Data availability statement: All authors can confirm that all data used in this article can be published the Journal “High Temperature Materials and Processes”.

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Received: 2021-01-25
Revised: 2021-02-20
Accepted: 2021-02-22
Published Online: 2021-04-20

© 2021 Gang Gao et al., published by De Gruyter

This work is licensed under the Creative Commons Attribution 4.0 International License.

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https://doi.org/10.1515/htmp-2021-0015
Schlagwörter für diesen Artikel
clean steel; Ti; TiO2; slag; coexistence theory
Creative Commons
BY 4.0

Artikel in diesem Heft

  1. Research Articles
  2. Fused deposition modeling of poly(ether ether ketone) scaffolds
  3. Investigation of the microstructure evolution in TP347HFG austenitic steel at 700°C and its characterization method
  4. Hot deformation behavior and processing maps of 9Cr3W3Co oxide dispersion-strengthened steel
  5. Evolution of physicochemical properties of quick lime at converter-smelting temperature
  6. Influence of phase distribution of converter slag microzones on the occurrence of P
  7. Investigation on ultrasonic assisted friction stir welding of aluminum/steel dissimilar alloys
  8. Analysis of oxide scale thickness and pores position of HCM12A steel in supercritical water
  9. Behavior of MnS inclusions during homogenization process in low-alloyed steel FAS3420H
  10. Preparation and cutting performance of nano-scaled Al2O3-coated micro-textured cutting tool prepared by atomic layer deposition
  11. Prediction of hot metal temperature based on data mining
  12. Effect of TiO2 content in slag on Ti content in molten steel
  13. Performance evaluation of titanium-based metal nitride coatings and die lifetime prediction in a cold extrusion process
  14. Effect of different drilling techniques on high-cycle fatigue behavior of nickel-based single-crystal superalloy with film cooling hole
  15. Effect of CO2 injection into blast furnace tuyeres on the pulverized coal combustion
  16. Microstructure and properties of Co–Al porous intermetallics fabricated by thermal explosion reaction
  17. Evolution regularity of temperature field of active heat insulation roadway considering thermal insulation spraying and grouting: A case study of Zhujidong Coal Mine, China
  18. Evolution of reduction process from tungsten oxide to ultrafine tungsten powder via hydrogen
  19. A thermodynamic assessment of precipitation, growth, and control of MnS inclusion in U75V heavy rail steel
  20. Effect of basicity on the reduction swelling properties of iron ore briquettes
  21. Effect of Cr and Al alloying on the oxidation resistance of a Ti5Si3-incorporated MoSiBTiC alloy
  22. Microstructure and mechanical properties of 2060 Al–Li alloy welded by alternating current cold metal transfer with high-frequency pulse current
  23. Effects of composition and strain rate on hot ductility of Cr–Mo-alloy steel in the two-phase region
  24. Effect of K and Na on reduction swelling performance of oxidized roasted briquettes
  25. Dephosphorization mechanism and phase change in the reduction of converter slag
  26. Parametric investigation and optimization for CO2 laser cladding of AlFeCoCrNiCu powder on AISI 316
  27. Optimization of heat transfer and pressure drop of the channel flow with baffle
  28. Quantitative analysis of microstructure and mechanical properties of Nb–V microalloyed high-strength seismic reinforcement with different Nb additions
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  30. Research on high-temperature mechanical properties of wellhead and downhole tool steel in offshore multi-round thermal recovery
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  33. Effects of ball milling on powder particle boundaries and properties of ODS copper
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  35. Solidification microstructure characteristics of Cu–Pb alloy by ECP treatment
  36. Luminescence properties of Eu2+ and Sm3+ co-doped in KBaPO4
  37. Research on high-temperature oxidation resistance, hot forming ability, and microstructure of Al–Si–Cu coating for 22MnB5 steel
  38. The differential analysis for temperature distribution diagnostics of arc current-carrying region in sheet slanting tungsten electrode inert gas welding with the electrostatic probe
  39. Reactions at the molten flux-weld pool interface in submerged arc welding
  40. The effect of liquid crystalline graphene oxide compared with non-liquid crystalline graphene oxide on the rheological properties of polyacrylonitrile solution
  41. Study on manganese volatilization behavior of Fe–Mn–C–Al twinning-induced plasticity steel
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  47. Copper-based kesterite thin films for photoelectrochemical water splitting
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