Effect of Molten Steel Composition on Inclusion Modification by Calcium Treatment in Al-Killed Tinplate Steel

Abstract

The effects of oxygen, magnesium, and sulfur content in Al-killed tinplate steel on the “liquid zone” of inclusion modification by calcium treatment were clarified through industrial experiments and thermodynamic calculation, and the characteristics of inclusions modification were studied. The results show that the inclusions in molten steel before calcium treatment are mainly Al₂O₃ inclusions. The inclusions in molten steel are CaO·Al₂O₃ after calcium treatment with a holding time of 10 min, while the inclusions are mainly 3CaO·Al₂O₃ with a holding time of 30 min. And 12CaO·7Al₂O₃ inclusions are observed in molten steel when T.O content increases to 40 ppm after calcium treatment with a holding time of 30 min. As the increase of T.O content from 10 ppm to 40 ppm, the difference between the upper and lower limits of the critical calcium content corresponding to the “liquid zone” increases from 5 ppm to 17 ppm. The increase of T.O content in molten steel will enlarge the “liquid zone” range of inclusion modification by calcium treatment, and increase the critical calcium content. With the increase of magnesium content in molten steel, the liquid phase ratio of inclusions modification by calcium treatment decreases. To obtain the liquid phase ratio of inclusions at least 50% in molten steel, not only the calcium content in steel should be strictly controlled, but also the magnesium content in steel should not be larger than 15 ppm. With the increase of sulfur content in molten steel, the “liquid zone” range of inclusion modification by calcium treatment becomes narrow.

1. Introduction

Non-metallic inclusions in tinplate steel have significant effects on the high surface quality of tinplate steel production, which have the characteristics of strong oxidation resistance, high strength, and good formability.^1,2,3,4) Aluminum is widely used as a strong deoxidizer during the RH refining process of tinplate steel production since it can effectively reduce the oxygen content in molten steel to a lower level.⁵⁾ However, the deoxidation products are Al₂O₃, which is possible to cause nozzle clogging and some defects on the surface quality of steel production as the formation of Al₂O₃ inclusions is inevitable.^6,7,8,9) To reduce the problems caused by residual Al₂O₃ particles, calcium treatment is usually employed to convert solid Al₂O₃ particles into liquid calcium aluminates.

Calcium treatment is a very common, direct, and effective measure to modify Al₂O₃ inclusions to liquid calcium aluminate inclusions during the steelmaking process, which is conducive not only to the mechanical properties of tinplate steel production but also to the steelmaking process, such as avoiding nozzle clogging in the continuous casting process, specifically in terms of Al deoxidated steel production.^{10,11,12,13,14)} There exists a “liquid zone” regarding Al₂O₃ inclusion modification by Ca-treatment, i.e., the appropriate range of calcium content (the lower and upper critical value of calcium content) for the liquid modification target. The lower critical value is the minimum content of calcium in molten steel required to achieve no-solid calcium aluminate, while the upper calcium content is the maximum amount of calcium in molten steel to avoid the formation of CaS inclusion.¹⁵⁾ Sulfur content in molten steel has a significant effect on inclusions during the calcium treatment process and calcium in molten steel may react with sulfur and form nondeformable CaS inclusion,¹⁶⁾ which prevents calcium from reacting with Al₂O₃ inclusion when the sulfur content in molten steel is high.^{13,16,17,18,19)} Therefore, insufficient or excessive calcium content can also have negative effects. Insufficient calcium addition leads to the formation of calcium aluminates with high melting points, deteriorating the castability of steel. On the other hand, excessive calcium addition may produce solid CaS, which is also considered to be harmful in terms of submerged entry nozzle clogging.^20,21)

There have been many studies investigating the inclusions modification by calcium treatment occurred by Ca transfer from slag phase. Liu et al.²²⁾ used the macroscopic processing characteristics of FactSage 7.2 to establish a model, and successfully predict the steel-slag reactions and evolution of the inclusion compositions in Fe–Al alloys. Jae Hong Shin et al.²³⁾ found that the spinel inclusion was modified to a CaO–Al₂O₃–MgO–SiO₂ liquid inclusion by transferring calcium from slag to molten steel without directly adding calcium to the molten steel. Yousef Tabatabaei et al.²⁴⁾ develop a kinetic model which allows determination of the change of composition of the steel, slag, and evolution of inclusions during Ca treatment. Jae Hong Shin et al.²⁵⁾ developed a refractory–slag–metal–inclusion multiphase reaction model by integrating the refractory–slag, slag–metal, and metal–inclusion elementary reactions in order to predict the evolution of inclusions during the secondary refining processes. Ren et al.²⁶⁾ proposed that the relationship of the equilibrated oxygen content and the calcium content could hardly be influenced by sulfur content. As the oxygen content increases, the desulfurization ability of calcium is weakened obviously.²⁷⁾ Xu et al.²⁸⁾ found that the type of inclusions after calcium treatment is determined by the sulfur and T.O contents of steel, and the mass ratio between Al₂O₃ and CaS is determined by the T.Ca and T.O contents of steel. The mechanism of alumina inclusion modification was investigated by studying inclusion evolution after calcium treatment, for heats with different sulfur contents. Verma et al.²⁹⁾ proposed that increased sulfur content had little effect on the extent of alumina modification, and the extent of calcium capture depended on the sulfur content in the steel. However, the insignificant modification effect of Al₂O₃ inclusions caused by unstable calcium content is still a key issue during the calcium treatment process of high-quality tinplate steel production. Meanwhile, the effect of calcium treatment with different oxygen, magnesium, and sulfur contents in molten steel is different.

In this work, the industrial experiment of Al₂O₃ inclusions modification by calcium treatment in molten steel was carried out. The influence of oxygen and sulfur content in molten steel on the “liquid zone” of inclusion modification by calcium treatment was investigated by industrial experiment and thermodynamic calculation. The effect of magnesium content in steel on the liquid phase ratio of inclusions after calcium treatment was studied by thermodynamic calculation.

2. Experiments

2.1. Experimental Procedures

The experiment of inclusion modification by calcium treatment was carried out in a steel plant in China. The industrial tests of tinplate steel were conducted through the production route of 180t BOF steelmaking → RH refining → Continuous casting → Hot rolling plate. No deoxidant agent was added into the ladle for pre-deoxidation during the tapping process. Subsequently, the ladle was transported to the RH refining station. During the RH refining process, the liquid steel was circulating degassing for 3 min firstly. After that, metallic aluminum was added into the liquid steel for deoxidation. At the middle stage of the RH refining process, MnFe alloy was added to adjust the composition of molten steel. RH refining time was 20 min. And after RH vacuum treatment, the bottom of ladle was soft blown with argon bubble for 5 min. In this industrial experiment, Ca–Fe alloy was added at the soft blowing stage after RH vacuum breaking. And adding Ca–Fe alloy (30pctCa-70pctFe) into molten steel by wire feeding equipment. The addition of Ca–Fe alloy wire is accompanied by soft blowing of argon at the bottom. Tinplate steel samples were taken before and after calcium treatment. The chemical composition of molten steel before calcium treatment is shown in Table 1. The schematic diagram of the experimental apparatus is shown in Fig. 1. After adding Ca–Fe alloy, different steel samples were taken and quenched rapidly in ice water at 0, 10, and 30 min respectively. Steel samples with different oxygen and sulfur content were taken to analyze inclusions and chemical components of molten steel after calcium treatment. The detailed sampling scheme is listed in Table 2.

Table 1. Chemical composition of 1-5# steel samples before calcium treatment, mass%.

Sample	C	Si	Mn	P	S	T.O	Al	Mg	Ca
1#~3#	0.0440	0.0112	0.2900	0.0076	0.0061	0.0032	0.0430	0.0007	0.0010
4-1#	0.0425	0.0108	0.2875	0.0077	0.0061	0.0041	0.0420	0.0007	0.0010
4-2#	0.0428	0.0112	0.2916	0.0072	0.0061	0.0050	0.0409	0.0007	0.0010
5#	0.0412	0.0110	0.2910	0.0082	0.0098	0.0032	0.0418	0.0007	0.0010

Fig. 1.

Schematic diagram of calcium wire feeding in the ladle. (Online version in color.)

Table 2. Sampling scheme of inclusions modification by Ca-treatment.

Sample	T.O content before Ca-treatment	Sulfur content before Ca-treatment	Holding time after Ca-treatment
	ppm	ppm	min
1#	32	61	Before Ca-treatment
2#	32	61	10
3#	32	61	30
4-1#	41	61	30
4-2#	50	61	30
5#	32	98	30

2.2. Analysis of Steel Sample

The T.O (total oxygen) content was determined by the fusion method, and the sulfur content was determined by the fusion and infrared absorption method. All the quenched steel samples after calcium treatment were detected and analyzed by using SEM-EDS to obtain the morphology and chemical composition of the inclusions. The contents of [Mg], [Ca], and [Al]s in the steel samples after calcium treatment were analyzed by an inductively coupled plasma optical emission spectrometer (ICP-OES).

3. Results and Discussion

3.1. Effect of T.O Content on Inclusion Modification after Calcium Treatment

To explore the transformation of inclusion after calcium treatment in the Fe-0.044C-0.0112Si-0.29Mn-0.0076P-0.043Al-0.0061S-0.0007Mg-Ca-O molten steel system, thermodynamic analysis is carried out at 1873 K by FactSage 7.0.³⁰⁾ Figure 2 shows the inclusion transformation during calcium treatment process for the molten steel with different T.O contents of 0.001%, 0.002%, 0.003% and 0.004% respectively. At the same time, for the different T.O contents, the critical calcium content and “liquid zone” range of inclusion modification are also shown in Fig. 2.

Fig. 2.

Effect of T.O content on the inclusion transformation and “liquid zone” at 1873 K. (Online version in color.)

For the molten steel with the T.O content of 10 ppm shown in Fig. 2(a), the critical calcium contents required for the formation of 12CaO·7Al₂O₃ should be controlled above 5 ppm, while CaS and CaO should be controlled below 10 ppm and 69 ppm, respectively, and the difference between the upper and lower calcium contents corresponding to the “liquid zone” range is 5 ppm. As the T.O content increases to 20 ppm and 30 ppm, the difference between the upper and lower calcium contents corresponding to the “liquid zone” range of inclusion transformation increases to 9 ppm and 13 ppm respectively, as shown in Figs. 2(b) and 2(c). With the T.O content increasing to 40 ppm, the “liquid zone” range becomes wide and the difference between the upper and lower calcium contents increase to 17 ppm, as shown in Fig. 2(d). It can be noted that the T.O content has a significant effect on the critical calcium content of inclusion transformation, and the decrease of T.O content makes the “liquid zone” narrow.

From Fig. 2, it can be seen that the critical calcium contents of 12CaO·7Al₂O₃, CaS and CaO increase with the increase of T.O content in molten steel, and the “liquid zone” range of inclusion modification by calcium treatment expands, which is conducive to the inclusion modification control. However, to improve the cleanliness of molten steel, it is necessary not only to reduce the T.O content in molten steel as much as possible but also to control the calcium content accurately in molten steel. In addition, industrial experiments were carried out to investigate the composition and morphology of inclusions before and after calcium treatment.

The chemical composition of molten steel in 1-5# samples is listed in Table 3. The typical irregular block morphology of Al₂O₃ inclusions larger than 5 μm observed in tinplate steel before calcium treatment (1# steel sample) is shown in Fig. 3. Figure 4 shows the morphology of typical CaO·Al₂O₃ inclusions in 2# steel sample with a holding time of 10 min. It can be seen that after calcium treatment, the Al₂O₃ inclusions were mainly modified to the spherical CaO·Al₂O₃ inclusions with Mg content of 1.45%. Similarly, after calcium treatment for 30 min, the Ca content in 3# steel sample is 18 ppm and the inclusions in 3# steel sample in were mainly 3CaO·Al₂O₃ inclusions with a melting point of 1812 K and Mg content of 2.04%, as shown in Fig. 5. It can be found from Fig. 2(c) that the inclusion in 2# steel sample is CaO·Al₂O₃, and the inclusion in 3# steel sample is liquid inclusion. The Ca content between 2# and 3# steel samples is different. Therefore, it can be concluded that the extending of holding time after calcium treatment increases the calcium content in the inclusions and is more conducive to the modification of alumina inclusion to calcium aluminate inclusions with a low melting point. After adding Ca–Fe alloy wire, increasing holding time is beneficial to increase mass transfer of calcium, so the calcium content in molten steel and inclusions increases with the increasing of holding time.

Table 3. Chemical composition of 2-5# steel samples after calcium treatment, mass%.

Sample	C	Si	Mn	P	S	T.O	Al	Mg	Ca
2#	0.0423	0.0115	0.2850	0.0080	0.0057	0.0033	0.0435	0.0008	0.0012
3#	0.0434	0.0117	0.3020	0.0074	0.0059	0.0030	0.0438	0.0010	0.0018
4-1#	0.0425	0.0108	0.2875	0.0077	0.0058	0.0038	0.0420	0.0011	0.0034
4-2#	0.0428	0.0112	0.2916	0.0072	0.0059	0.0047	0.0409	0.0010	0.0042
5#	0.0412	0.0110	0.2910	0.0082	0.0095	0.0029	0.0418	0.0012	0.0035

Fig. 3.

Typical Al₂O₃ inclusions before Ca-treatment.

Fig. 4.

Typical CaO·Al₂O₃ composite inclusion after Ca-treatment.

Fig. 5.

Typical 3CaO·Al₂O₃ composite inclusion after Ca-treatment.

In addition, after calcium treatment for 30 min, spherical 12CaO·7Al₂O₃ inclusion was detected in 4-1# and 4-2# steel sample with T.O content of 41 ppm and 50 ppm, as shown in Figs. 6 and 7. When the T.O content in molten steel is 40 ppm, the “liquid zone” range of inclusion modification by calcium treatment is 23–40 ppm, as shown in Fig. 2(d), and when the T.O content in molten steel is 50 ppm, the “liquid zone” range of inclusion modification by calcium treatment is 25–50 ppm, as shown in Fig. 2(e). It can be seen that the detected calcium contents in 4-1# and 4-2# steel sample are 34 ppm and 42 ppm from Table 3. The calcium content in molten steel meets the content requirement of the “liquid zone” range.

Fig. 6.

Typical 12CaO·7Al₂O₃ composite inclusion after Ca-treatment with T.O content of 41 ppm.

Fig. 7.

Typical 12CaO·7Al₂O₃ composite inclusion after Ca-treatment with T.O content of 50 ppm.

In summary, the results of calcium modification for Al₂O₃ inclusions were verified by thermodynamic analysis and experimental study. When the calcium content in molten steel is controlled in the “liquid zone” range of inclusion modification by calcium treatment, the inclusions are mainly 12CaO·7Al₂O₃ with a low melting point, as shown in the test result of the 4# steel sample. While the calcium content is outside the “liquid zone” range, the inclusions are mainly incompletely modified CaO·Al₂O₃ and 3CaO·Al₂O₃, as shown in the test results of 2# and 3# steel samples. Compared with the experimental results of 3#, 4-1# and 4-2#, the inclusions of 4-1# and 4-2# samples are completely liquid inclusions after calcium treatment for the same time, which shows that the increase of T.O content in steel expands the range of “liquid zone” of inclusion modification for aluminum deoxidized killed steel.

The inclusions in 3#, 4-1#, and 4-2# steel samples were detected, and the inclusions were projected on the phase diagrams of CaO–Al₂O₃–MgO. The shaded region in the phase diagrams was the liquid phase region at 1873 K. Comparisons of inclusions in steel treated with calcium from 3#, 4-1# and 4-2# samples show that with the increase of T.O content in steel, the proportion of inclusions in the low melting point region after calcium treatment increases, and the average composition of inclusions tends to be close to the low melting point region, as shown in Fig. 8. Combined with the thermodynamic calculation results of the influence of T.O content on the “liquid zone”, the increase of T.O content enlarges the range of “liquid zone” is verified.

Fig. 8.

Composition of inclusions in steel samples with different T.O contents. (a) 3# steel sample, (b) 4-1# steel sample, (c) 4-2# steel sample. (Online version in color.)

T.O content has a significant effect on the critical calcium content of inclusion transformation. The thermodynamic calculation results in Fig. 2 are also verified, and it can be seen from thermodynamic calculation that the increase of T.O content makes the “liquid zone” wide. However, the critical calcium content of different types of inclusions modified into liquid inclusions such as 12CaO·7Al₂O₃, CaS and CaO is also increasing. For tinplate steel, the T.O content in steel is generally higher than 20 ppm, and at the same time, because the yield of calcium in molten steel is low and it is difficult to control calcium content in a narrow range, the calcium content in molten steel should be strictly controlled according to the T.O content in steel.

3.2. Effect of Mg Content on Inclusion Modification after Calcium Treatment

With the use of MgO in RH refining slag and Mg-based refractory materials in the ladle, MgO inevitably exists in Al-Killed tinplate steel, and there will be MgO·Al₂O₃ spinel inclusions in molten steel before calcium treatment. Therefore, Mg content in molten steel has an impact on the modification effect of inclusion by calcium treatment. To study the influence of Mg content on the effect of inclusions modification by calcium treatment, the change of Mg content in molten steel to the liquid phase ratio of inclusions at 1873 K is calculated by thermodynamic calculations. According to the composition of molten steel in Table 1, the composition of molten steel is Fe-0.044C-0.0112Si-0.29Mn-0.0076P-0.0061S-0.043Al-0.0032O-Ca-Mg, and the calculation result is shown in Fig. 9.

Fig. 9.

Effect of Mg content in molten steel on modification of inclusions at 1873 K. (Online version in color.)

As can be seen from Fig. 9, when the calcium content in molten steel is less than 20 ppm and constant, the liquid phase ratio of inclusions increases first and then decreases with the increase of Mg content. When the calcium content in the steel is 10 ppm and the required Mg content is 6 ppm, the liquid phase ratio of inclusions is up to 75%. When the calcium content in the steel is 20 ppm, the liquid phase ratio of inclusions starts to decrease from 100% as the Mg content in the steel exceeds 15 ppm, but it is still more than 80% as the Mg content increases to 20 ppm. When the calcium content in the steel is 30 ppm, the liquid phase ratio of inclusions starts to decrease from 100% as the Mg content in the steel exceeds 2 ppm. When the Ca content increases to 40 ppm and Mg content increases to 20 ppm, the liquid phase ratio of inclusions in molten steel is still more than 50%. When the calcium content in the steel is more than 80 ppm, the liquid phase ratio of inclusions after calcium treatment in molten steel is less than 50%.

Furthermore, Fig. 10 shows the inclusion of CaO–MgO–Al₂O₃ system with multiple MgO·Al₂O₃ cores in 3# steel sample. The calcium content in 3# steel sample is 18 ppm, as shown in Table 3. The formation of CaO–MgO–Al₂O₃ inclusion was due to the high Mg content in molten steel, which is 10 ppm, as shown in Table 3. Ca could easily react with MgO·Al₂O₃ inclusions and form CaO–MgO–Al₂O₃ inclusion with a low melting point. The formation reaction of CaO–MgO–Al₂O₃ inclusion is shown in Eqs. (1), (2), (3).³¹⁾   

$$\begin{array}{l}4{(\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3)}_{\mathrm{i}\mathrm{n}\mathrm{c}\mathrm{l}\mathrm{u}\mathrm{s}\mathrm{i}\mathrm{o}\mathrm{n}}+3\left[\mathrm{M}\mathrm{g}\right]=3{(\mathrm{M}\mathrm{g}\mathrm{O}\cdot \mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3)}_{\mathrm{i}\mathrm{n}\mathrm{c}\mathrm{l}\mathrm{u}\mathrm{s}\mathrm{i}\mathrm{o}\mathrm{n}}+2\left[\mathrm{A}\mathrm{l}\right]\\ log{K}_1=34.37-46\mathrm{\hspace{0.17em} }950/T\end{array}$$(1)

  

$$K_1=\frac{a_{\mathrm{M}\mathrm{g}\mathrm{O}\cdot \mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3}^3\cdot a_{[\mathrm{A}\mathrm{l}]}^2}{a_{\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3}^4\cdot a_{[\mathrm{M}\mathrm{g}]}^3}$$(2)

  

$$\begin{array}{l}x\left[\mathrm{C}\mathrm{a}\right]+{(\mathrm{M}\mathrm{g}\mathrm{O}\cdot \mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3)}_{\mathrm{i}\mathrm{n}\mathrm{c}\mathrm{l}\mathrm{u}\mathrm{s}\mathrm{i}\mathrm{o}\mathrm{n}}=\\ {[x\mathrm{C}\mathrm{a}\mathrm{O}\cdot \left(1-x\right)\mathrm{M}\mathrm{g}\mathrm{O}\cdot \mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3]}_{\mathrm{i}\mathrm{n}\mathrm{c}\mathrm{l}\mathrm{u}\mathrm{s}\mathrm{i}\mathrm{o}\mathrm{n}}+x\left[\mathrm{M}\mathrm{g}\right]\end{array}$$(3)

Since the inclusions in the steel sample after calcium treatment are mainly CaO·Al₂O₃, here, the value of x can be taken as 1, and thus Eq. (3) can be simplified to Eq. (4).   

$$\begin{array}{l}\left[\mathrm{C}\mathrm{a}\right]+{(\mathrm{M}\mathrm{g}\mathrm{O}\cdot \mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3)}_{\mathrm{i}\mathrm{n}\mathrm{c}\mathrm{l}\mathrm{u}\mathrm{s}\mathrm{i}\mathrm{o}\mathrm{n}}={\left(\mathrm{C}\mathrm{a}\mathrm{O}\cdot \mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3\right)}_{\mathrm{i}\mathrm{n}\mathrm{c}\mathrm{l}\mathrm{u}\mathrm{s}\mathrm{i}\mathrm{o}\mathrm{n}}+\left[\mathrm{M}\mathrm{g}\right]\\ \mathrm{l}\mathrm{o}\mathrm{g}{K}_2=-0.40+2\mathrm{\hspace{0.17em} }476/T\end{array}$$(4)

  

$$K_{\text{2}}=\frac{a_{\text{CaO}\cdot {\text{Al}}_{\text{2}}{\text{O}}_{\text{3}}}\cdot a_{[\text{Mg}]}^{}}{a_{\text{MgO}\cdot {\text{Al}}_{\text{2}}{\text{O}}_{\text{3}}}\cdot a_{[\text{Ca}]}^{}}$$(5)

Fig. 10.

Typical CaO–Al₂O₃–MgO composite inclusion after Ca-treatment.

The standard states of the oxides are taken as pure solids in Eqs. (1), (3) and (4). And the activities of elements in molten steel are taken relative to a dilute solution taking unit as the mass percent. The activity of element i can be calculated based on Eqs. (6) and (7).   

$$log{f}_i=\sum e_i^j[\text{mass}\%j]$$(6)

  

$$a_i=f_i\cdot [\text{mass}\%i]$$(7)

In Eq. (6), $e_i^j$ is the interaction coefficient of element j to i and f_i is the activity coefficient of element i, while a_[i] is the activity of constituent [i] in molten steel in Eq. (7). The chemical composition of molten steel is given in Table 3. In the calculations, the interaction coefficients used in thermodynamic calculations are listed in Table 4, and the activity coefficients of element i are listed in Table 5. The phase stability diagram of calcium aluminate system is calculated.³⁸⁾ For the above calculations, when [Ca] content reaches 18 ppm and [Al] content in 3# steel sample reaches 0.0438%, it is also verified that the CaO·Al₂O₃ inclusions would be generated, as shown in Fig. 11. And thermodynamic analysis agrees well with the industrial experimental results.

Table 4. Interaction coefficients used in thermodynamic calculations at 1873 K.

j i	C	Si	Mn	S	P	Al	O	Ca
Ca	−0.34	−0.097	−0.0156	−336	−0.097	−0.072	−2500	−0.002
Mg	−0.15	−0.09	/	−1.38	/	−0.12	−430	0
Al	0.091	0.0056	0.035	0.030	0.033	0.045	−1.98	−0.047
O	−0.436	−0.131	−0.021	−0.133	0.07	−1.17	−0.20	−515
S	0.11	0.063	−0.026	−0.028	0.029	0.035	−0.27	−259

Table 5. Activity coefficients of element i.

Sample	fC	fSi	fMn	fS	fAl	fO	fCa	fMg
3#	0.943	1.029	0.932	1.102	1.127	0.422	0.0012	0.026
4-1#	0.946	1.028	0.992	0.997	1.018	0.644	0.0006	0.012
4-2#	0.943	1.028	0.991	0.995	1.015	0.649	0.0006	0.005
5#	0.943	1.029	0.992	0.996	1.023	0.645	0.0015	0.032

Fig. 11.

Phase stability diagram of CaO–Al₂O₃ system. (Online version in color.)

Therefore, with the increase of magnesium content in molten steel, the liquid phase ratio of inclusions in steel decreases. To completely modify inclusions by calcium treatment under the current conditions of molten steel composition, that is, the liquid phase ratio of inclusions should be at least 50%, not only the calcium content in molten steel should be strictly controlled to be no more than 60 ppm, but also the Mg content in the steel should not be higher than 15 ppm.

3.3. Effect of Sulfur Content on Inclusion Modification after Calcium Treatment

The sulfur content in molten steel also has a significant effect on the composition of inclusions after calcium treatment. To explore the inclusion transformation after calcium addition in the Fe-0.044C-0.0112Si-0.29Mn-0.0076P-0.0032O-0.043Al-0.0007Mg-Ca-S molten steel system, a thermodynamic calculation is carried out at 1873 K by FactSage 7.0. Figure 12 shows the inclusion transformation during the calcium treatment process for the molten steel with different sulfur contents of 0.006%, 0.008%, and 0.01% respectively. Meanwhile, for the different sulfur contents, the critical calcium content and “liquid zone” range of inclusion modification is also shown in Fig. 12.

Fig. 12.

Effect of sulfur content on the inclusion transformation and “liquid zone” at 1873 K. (Online version in color.)

From Fig. 12(a), under the current composition condition with 0.0061% sulfur content, the critical calcium contents required for the formation of 12CaO·7Al₂O₃ should be controlled above 18 ppm, CaS and CaO should be controlled below 32 ppm and 107 ppm, respectively. And the difference between the upper and lower calcium contents corresponding to the “liquid zone” range is 14 ppm. As the sulfur content increases to 80 ppm and 100 ppm, the difference between the upper and lower calcium contents corresponding to the “liquid zone” range of inclusion transformation decreases to 10 ppm and 9 ppm respectively, as shown in Figs. 12(b) and 12(c). When the sulfur content in molten steel is too high during calcium treatment, it is necessary to decrease the amount of Ca–Fe wire to promote the transformation of inclusions into low melting point 12CaO·7Al₂O₃.

From Fig. 12 and previous research results,^9,39) it is reasonable to conclude that the sulfur content has a significant influence on the critical calcium content of inclusion transformation and the “liquid zone” of the inclusions in the calcium treatment. The required calcium content for the inclusion modification is relatively stable. When the sulfur content in molten steel is 60–100 ppm, the “liquid zone” range shall be controlled in 9–14 ppm. It also can be noted that the increase of sulfur content makes the “liquid zone” range of inclusions modification by calcium treatment narrower. At the same time, the increase of Ca–Fe wire makes the content of calcium aluminate sulfide compound inclusions in molten steel increase continuously and even precipitates CaS inclusions, which also causes nozzle clogging in the casting process. To enlarge the “liquid zone” range of the calcium treatment, therefore, the desulfurization of molten steel should be carried out as far as possible under the present conditions of molten steel composition.

Sulfide is not found in 2#, 3#, 4-1# and 4-2# steel samples after calcium treatment with the sulfur content of 61 ppm, and the inclusions are mainly incompletely modified CaO–Al₂O₃ inclusions. When the sulfur content of the steel sample increases to 98 ppm (5# steel sample), the main component of inclusion is CaO–MgO–Al₂O₃–(CaS) inclusion. The elemental mapping of CaO–MgO–Al₂O₃–(CaS) inclusions is shown in Fig. 13. The diameter of CaO–MgO–Al₂O₃–(CaS) inclusion is about 10 μm, and the elements of Al, Ca, and Mg overlap and distribute evenly, which indicates that Ca has modified the whole inclusion and CaO–MgO–Al₂O₃ with CaS rings was caused by the dissolution of [S] into CaO–MgO–Al₂O₃ composite inclusion.

Fig. 13.

Elemental mapping of typical CaO–MgO–Al₂O₃–(CaS) inclusion after Ca-treatment. (Online version in color.)

The [S] in the molten steel dissolves into CaO in the calcium aluminate inclusions, as shown in Eq. (8), and the ΔG^θ of Eq. (8) is −341.18 kJ·mol⁻¹. The equilibrium relationship between [Al], [S] and CaO in the calcium aluminate inclusions at different temperatures is shown in Eq. (10), where the activity of Al₂O₃ and the activity of CaO in the different calcium aluminate inclusions is shown in Table 6.⁴⁰⁾ The activity of CaS used in the calculation for Eq. (10) is 0.76 at 1873 K.¹³⁾   

$$3{\text{CaO}}_{(\mathrm{i}\mathrm{n}\mathrm{c}\mathrm{l}\mathrm{u}\mathrm{s}\mathrm{i}\mathrm{o}\mathrm{n})}+2\left[\mathrm{A}\mathrm{l}\right]+3\left[\mathrm{S}\right]=3\mathrm{C}\mathrm{a}\mathrm{S}+\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_{3(\mathrm{i}\mathrm{n}\mathrm{c}\mathrm{l}\mathrm{u}\mathrm{s}\mathrm{i}\mathrm{o}\mathrm{n})}$$(8)

  

$$\mathrm{\Delta }{G}^{\theta }=-\text{963}\mathrm{\hspace{0.17em} }\text{016}+\text{332}T\text{(J}\cdot {\text{mol}}^{-\text{1}}\text{)}$$(9)

  

$$\begin{array}{l}\text{2log[Al]}+\text{3log[S]}=\frac{\mathrm{\Delta }{G}^{\theta }}{\text{2}\text{.303}RT}+3log{a}_{\text{CaS}}+log{a}_{{\text{Al}}_{\text{2}}{\text{O}}_{\text{3}}}\\ \hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}-3log{a}_{\text{CaO}}-3log{f}_{\text{S}}-2log{f}_{\text{Al}}\end{array}$$(10)

Table 6. Activity of CaO and Al₂O₃ in the calcium aluminate inclusions at 1873 K.

Calcium aluminate	3CaO·Al2O3	12CaO·7Al2O3	CaO·Al2O3	CaO·2Al2O3	6CaO·Al2O3
a(CaO)	1	0.34	0.15	0.1	0.043
a(Al2O3)	0.017	0.064	0.275	0.414	0.637

Figure 14 shows the thermodynamic calculation results of sulfur precipitation in different types of inclusions (CaO·Al₂O₃, 3CaO·Al₂O_3, and 12CaO·7Al₂O₃). According to Table 3, when the calcium content in molten steel is 35 ppm, 418 ppm [Al], 98 ppm [S], the content of [Al], [S] is in the stable zone of CaO·Al₂O₃ and 12CaO·7Al₂O₃, the sulfur element can dissolve into the surface of calcium aluminate and precipitate CaS. When the sulfur content in steel is 100 ppm, the inclusions after calcium treatment are mainly CaO–MgO–Al₂O₃–(CaS) composite inclusions. Thermodynamic calculation shows that the “liquid zone” range of inclusion calcium treatment is 23–32 ppm, as shown in Fig. 12(c), while the detected calcium content in molten steel is 35 ppm, as shown in Table 3, which is larger than the calcium content required for the “liquid zone” range of inclusion modification by Ca-treatment, and CaS inclusions will precipitate under this experimental condition.

Fig. 14.

Precipitation of sulfur and aluminum in different types of calcium aluminate inclusions at different temperatures. (Online version in color.)

The inclusions in 3# and 5# steel samples were detected, and the inclusions were projected on the phase diagrams of CaO–Al₂O₃–CaS systems, respectively. The shaded region in the phase diagrams was the liquid phase region at 1873 K. Comparing the composition of inclusions in steel treated with Ca from 3# and 5# samples, it can be found that the proportion of inclusions treated with Ca decreases in the low melting point area when the sulphur content in steel increases, and the average composition of inclusions tends to be far away from the low melting point region, as shown in Fig. 15. Combined with the thermodynamic calculation results of the influence of sulfur content on the “liquid zone”, the result that the increase of sulphur content narrows the “liquid zone” range is verified.

Fig. 15.

Composition of inclusions in steel samples with different sulfur contents. (a) 3# steel sample, (b) 5# steel sample. (Online version in color.)

Therefore, from the above thermodynamic analysis and experimental results, it can be concluded that the increase of sulfur content in molten steel narrows the “liquid zone” range of inclusions modification by calcium treatment. To enlarge the “liquid zone” range of inclusions modification by calcium treatment, therefore, the desulfurization of molten steel should be carried out as much as possible.

4. Conclusions

The calcium content and characteristics of inclusions in Al-killed tinplate after calcium treatment were studied by industrial experiments and thermodynamic calculation with different holding time and different initial oxygen, magnesium, and sulfur contents. The following conclusions are obtained.

(1) The results of calcium treatment for alumina inclusions in Al-killed tinplate steel were clarified by thermodynamic analysis and industrial experiment. When the T.O content in molten steel increases from 10 ppm to 40 ppm, the difference between the upper and lower critical calcium contents corresponding to the “liquid zone” increases from 5 ppm to 17 ppm. The increase of T.O content in molten steel enlarges the “liquid zone” range of inclusion modification by calcium treatment, and the critical calcium content of different types of inclusions modified into liquid inclusions such as 12CaO·7Al₂O₃, CaS and CaO is also increasing.

(2) With the increase of magnesium content in molten steel, the liquid phase ratio of modified inclusions treated by calcium decreases. To obtain the liquid phase ratio of inclusions in molten steel reach more than 50%, not only the calcium content in steel should be controlled strictly, but also the magnesium content in steel should be lower than 15 ppm.

(3) Sulfur content in molten steel plays an important influence on the inclusion modification. With the increase of sulfur content in steel, the upper and lower calcium contents corresponding to the “liquid zone” range of inclusion transformation decrease, and the “liquid zone” range of inclusion modified by calcium treatment narrows. And the results of thermodynamic calculation are verified by industrial test.

Acknowledgments

The authors gratefully acknowledge the National Natural Science Foundation of China. [Grant numbers: 52074077, 52174301 and 52274325], the Fundamental Research Funds for the Central Universities was supported by Chinese Education Ministry [Grant numbers. N2125018].

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