**1. Introduction**

Gears structure a fundamental part in the running of different machines and cars for transmitting power. There are a range of gears used in different areas according to the particular prerequisites, such as automotive gears, mining gears, marine gears, wind turbines, bicycle gears, instrumentation gears and conveyor systems. The automotive industry requires automotive gears for high force capacity, high fatigue strength and to convert mechanical energy efficiently and silently. However, machining of gear steels, generally, is linked with high energy consumption and decreased cutting tool life. The worsening of mechanical properties depends on the amount, size, morphology and dispersion of non-metallic inclusions. Since the machining costs (estimated ~40%) represent a large fraction of the total production expenses of most components, considerable attention has been paid to designing innovative techniques and technologies that make machinability viable with mechanical properties [1–3]. Adding a small amount of sulphur (typically from 100 to 500 ppm) is known to cause an outstanding improvement in the machinability of gear steels via manganese sulphide formation, which improves the machinability via chip embrittlement and increases tool life. Sulphides, especially MnS, with a round or shaft morphology are usually advantageous for this purpose. MnS inclusions have good ductility and can easily bend into long strip shapes after hot working, which causes anisotropy [4] in steel properties, consequently reducing the fatigue life [3] and altering

**Citation:** Ahmad, H.; Zhao, B.; Lyu, S.; Huang, Z.; Xu, Y.; Zhao, S.; Ma, X. Formation of Complex Inclusions in Gear Steels for Modification of Manganese Sulphide. *Metals* **2021**, *11*, 2051. https://doi.org/10.3390/ met11122051

Academic Editor: Eli Aghion

Received: 4 November 2021 Accepted: 16 December 2021 Published: 18 December 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

the transverse properties of steel, and is furthermore destructive to the machinability of steels [5]. Most of the oxide and sulphide inclusions are endogenous, formed as the result of deoxidation of steel. However, dual phase types with or without sulphides and pure sulphide inclusions are generated during all succeeding stages of steel processing, even during solidification [1,6–10]. In contrast, alumina (Al2O3) is another type of inclusion generated as the result of Al deoxidation in molten steel. Large and irregular shapes of alumina inclusions mostly float up towards the slag, while smaller alumina inclusions (less than 10 µm) unable to float up get stuck in the steel and solidify, which can act as a stress raiser or become a crack source or even help the crack propagation under cyclic loading, and suppress the anti-fatigue property of steel. In addition, too much Al2O<sup>3</sup> inclusions can cause nozzle clogging due to accumulation in the nozzle during the casting process, and because it is hard and brittle, it causes excessive tool wear in the machining process [7].

Normally, calcium addition is an effective way to alter the morphology and dispersion of non-metallic inclusions in the continuous casting process or in ingots. Calcium treatment is normally applied in Al-deoxidized steels to alter the solid alumina inclusions into liquid ones [11–13] which effortlessly pass through without spout impediment. In the current operations, the inclusions modification proficiency using Ca is usually not high; the surplus Ca may cause the development of high temperature stable inclusions such as CaS and Ca·Al12O19, leading to spout impediment [2,10,14]. In resulphurized steel, Ca is also used to transform the MnS inclusions by reducing the relative plasticity of MnS inclusions during hot working. Blais et al. proposed the optimum Ca/S ratio to be 0.7 for effective modification of sulphide inclusions [15]. The reaction formation of CaS was reported via the formation of a CaS transient phase right after the Ca injection treatment [16] or formed after modification of Al2O<sup>3</sup> to calcium aluminates [17]. Verma et al. proposed that transit CaS can react with the alumina to yield modified inclusions [18,19], while Wang et al. proposed that the reaction between Al2O3-CaO inclusions with the dissolved [S] and [Al] in the melt forms CaS around the inclusions [20]. Park et al. studied the mechanism of complex inclusions formation by using synthetic slag without adding Ca in the melt and predicted the critical sulphur [S] level required to precipitate CaS in the inclusions. By increasing the SiO<sup>2</sup> in the slag, the critical [S] level increases at a given [Al] content [21].

For uniform distribution of fine MnS precipitates, it is effective to use such oxides which have a high sulphide capacity and low melting temperature [11–13,22]. Miki [23] concluded that sulphur solubility in the inclusions increases with increasing MnO content in oxides, therefore increasing the probability of MnS precipitation on oxides. Thus, improvement of steel properties lies in the proper morphology control of oxides and sulphides. If the adverse effects of both sulphides (MnS) and pure oxides (Al2O3) can be minimized or even avoided via Ca treatment, the deformation of pure MnS precipitates will be easily confined, and also the oxide inclusions will not directly become the reason for steel failure. Although there is enough literature available on inclusions modification in Al-deoxidized and Ca-treated steels in lab scale models, there is very limited data available to improve the quality of industrial products because of the complex nature of reactions taking place in the ladle. To clearly define the role of Ca in modifying the sulphide and oxide inclusions in Al-killed Ca-treated gear steels, it is necessary to obtain the fundamental knowledge to understand the formation mechanism of inclusions including the composition of each phase from the viewpoint of thermodynamics and evolution during the steelmaking process on an industrial scale. Therefore, in this study we focused on the characterization and formation of complex oxide-sulphide inclusions in different sulphur-level industrial samples with an Al-deoxidation and Ca-treatment process. The formation mechanisms and thermodynamic prediction are discussed for the optimization of manganese sulphide modification.

#### **2. Materials and Methods**

To investigate the effect of sulphur and calcium on complex oxide and sulphide inclusions, two samples having different amounts of sulphur and calcium were taken from two industrial products. The chemical compositions of the samples are given in Table 1

below. Two steel grades (named GS1 and GS2) have almost the same Mn concentration: GS1 contains 290 ppm S and 14 ppm Ca, whereas GS2 contains 140 ppm S and 19 ppm Ca. The steelmaking process followed sequential steps through an electric arc furnace (EAF) → ladle (LF) refining → vacuum degassing (VD) → continuous casting (CC). Both samples underwent the same manufacturing route, i.e., the scrap metal along with hot metal from the converter was added into the electric arc furnace (Danieli & Co., Buttrio, Italy). During melting, oxygen lancing was used to reduce the carbon content to the desired level. After melting, the molten steel was tapped to the ladle furnace for further treatment, in which lumps of pure aluminum were added to reduce the oxygen content. Pure argon gas purging from the bottom was also employed to accelerate the mixing of aluminum and removal of oxide impurities in the slag. After ladle treatment, the furnace was transferred to the vacuum degassing process to ensure the complete removal of impurities. Ca in the form of CaSi was added into the vacuum degassing process to liquify the remaining impurities (in order to avoid nozzle blockage and surface defects), and finally, the furnace was transferred to the continuous casting plant to cast the steel into long bars of variable sizes. Finally, both samples were further rolled down into small diameter rods (after soaking in the furnace having a temperature ranging between 1000 and 1050 ◦C), i.e., GS1 was rolled to 40 mm diameter and GS2 to 25 mm.


**Table 1.** Chemical compositions of two gear steel samples (wt.%).

\*N, P, S, Al, Ca and O are in ppm.

#### *Analysis of Steel*

Steel samples were cautiously prepared for electron probe microanalyzer (EPMA) analysis. Longitudinal sections were cut for inclusion analysis. After setting into the epoxy resin, the samples were ground up to 4000 grit size paper and slightly polished. Extensive polishing and etching can easily wash away the inclusions. A 20 <sup>×</sup> 20 mm<sup>2</sup> area of each sample was manually scanned for inclusions to ensure no inclusion was left behind.

A JXA 8200 electron probe X-ray microanalyzer (JEOL, Tokyo, Japan) with wavelengthdispersive detectors was used for microstructure and compositional analysis. Parameters employed for analysis include an accelerating voltage of 15 kV, 15 nA current, zero probe diameter, peak measuring time of 30 s, background position of ±5 µm and background measuring time of 5 s. EDS was used to identify and locate non-metallic inclusions in the steel; WDS was performed to measure their composition. Standards used for analysis were spinel (MgO-Al2O3), calcium silicate (CaSiO3), iron sulphide (FeS2), spessartine (Mn3Al2Si3O12) and hematite (Fe2O3) for Al, Mg, Ca, Si, S, Mn and Fe, respectively. The built-in ZAF correction procedure supplied with the probe was applied. Different crystals to cover the entire X-ray spectrum were used, which were lithium fluoride (LIF), pentaerythritol (PET) and thallium acid phthalate (TAP). Assuming the valence state of each element, compositions of the phases were re-calculated. The overall accuracy of the EPMA measurement of each component is within 1 wt.%.

#### **3. Results and Discussion**

#### *3.1. Morphology and Composition of Inclusions*

Longitudinal sections of both GS1 and GS2 samples were scanned for a better understanding of the morphology and distribution of the inclusions. In GS1 and GS2 samples, there are approximately 300 and 370 inclusions respectively in the scanned area of 2 cm<sup>2</sup> , which was measured via EMPA. The inclusions are categorized into different groups based on their major chemical compositions and morphologies including single/dual phases and spherical/elongated sulphides. The classification of inclusions is given in Table 2, which

*Article* 

describes the overall composition range of oxides and sulphides in single phase and also in complex inclusions. The wt.% of alumina, silica, calcium and magnesium oxides in the inclusions along with sulphur and manganese can be easily seen. Figure 1 shows the typical inclusions in GS1 and GS2 samples based on their compositions and phases, i.e., single phase and dual phase.


**Table 2.** Classification of inclusions in both samples based on the chemical compositions and morphologies.

**Figure 1.** Typical morphology and composition of inclusions in both samples. (**a**) GS1 sample, (**b**) GS2 sample. **Figure 1.** Typical morphology and composition of inclusions in both samples. (**a**) GS1 sample, (**b**) GS2 sample.

Figure 2a,b represent the detailed size and percentage distribution of individual inclusions in GS1 and GS2 samples, respectively. According to Figure 1a,b, most of the inclusions are round, while some are in irregular shapes, especially the pure MnS inclusions, which are elongated due to rolling. ImageJ (LOCI, University of Wisconsin, Madison WI,

*3.2. Number Fraction and Size Range of Inclusions* 

*Metals* **2021**, *11*, 2051. https://doi.org/10.3390/met11122051 www.mdpi.com/journal/metals

#### *3.2. Number Fraction and Size Range of Inclusions*

Figure 2a,b represent the detailed size and percentage distribution of individual inclusions in GS1 and GS2 samples, respectively. According to Figure 1a,b, most of the inclusions are round, while some are in irregular shapes, especially the pure MnS inclusions, which are elongated due to rolling. ImageJ (LOCI, University of Wisconsin, Madison, WI, USA) was used to measure the size of the inclusions. Most of the inclusions in the samples are regular in shape and easy to measure. For irregular inclusions morphology, the inclusion size is simply defined by measuring the longest length. Inclusions percentage is calculated by dividing the number of inclusions of each type by the total number of inclusions. In Figure 2a, there are a greater number (around 15%) of pure MnS inclusions in the range of 10–20 µm, while around 17% of the pure MnS inclusions are larger than 20 µm, a smaller number (only 5%) of pure alumina inclusions and a higher number of calcium-aluminates, and also complex oxy-sulphides, whereas in Figure 2b, there are around 16% pure alumina having a size greater than 5 µm, and the spinel (MgO-Al2O3) inclusions are around 14% in that size range. It is shown that the pure alumina inclusions are more in number than the complex oxides and sulphides; these inclusions are less 10 µm in size, which is in an acceptable range for the mechanical properties of gear steels. Around 2% of the calcium aluminates are less or equal to 5 µm, and about 8% of the calcium-aluminate inclusions are in the 5–10 µm range. Around 9% of the total inclusions are duplex ones in which the solid solution of Ca and Mn sulphide encapsulated the alumina and spinel inclusions. Most of the inclusions are in the acceptable size range, i.e., 5–15 µm, except the pure MnS, which was plastically deformed during rolling of the steel. *Metals* **2021**, *11*, 2051 2 of 10 are regular in shape and easy to measure. For irregular inclusions morphology, the inclusion size is simply defined by measuring the longest length. Inclusions percentage is calculated by dividing the number of inclusions of each type by the total number of inclusions. In Figure 2a, there are a greater number (around 15%) of pure MnS inclusions in the range of 10–20 µm, while around 17% of the pure MnS inclusions are larger than 20 µm, a smaller number (only 5%) of pure alumina inclusions and a higher number of calciumaluminates, and also complex oxy-sulphides, whereas in Figure 2b, there are around 16% pure alumina having a size greater than 5 µm, and the spinel (MgO-Al2O3) inclusions are the 5–10 µm range. Around 9% of the total inclusions are duplex ones in which the solid solution of Ca and Mn sulphide encapsulated the alumina and spinel inclusions. Most of the inclusions are in the acceptable size range, i.e., 5–15 µm, except the pure MnS, which was plastically deformed during rolling of the steel.

**Figure 2.** The size of different types of inclusions in the gear steels. (**a**) GS1 sample, (**b**) GS2 sample. **Figure 2.** The size of different types of inclusions in the gear steels. (**a**) GS1 sample, (**b**) GS2 sample.

Figure 3 specifically represents the size comparison between pure MnS inclusions in both high and low sulphur samples, respectively. Steel with high sulphur content (GS1 sample) and only 14 ppm Ca shows a higher number of pure MnS inclusions which are elongated and also larger in size due to rolling of the steel because of the plastic (soft) nature of pure manganese sulphide, as has already been mentioned above. The inclusions of the GS2 sample with a relatively low sulphur content have a small number of pure MnS, also shown in Figure 3, but have a solid solution of (Ca,Mn)S. Overall, Figure 3 shows a better MnS modification result due to Ca treatment in the GS2 sample, because most of the portion of pure MnS inclusions was transformed into a solid solution of (Ca,Mn)S. Figure 3 specifically represents the size comparison between pure MnS inclusions in both high and low sulphur samples, respectively. Steel with high sulphur content (GS1 sample) and only 14 ppm Ca shows a higher number of pure MnS inclusions which are elongated and also larger in size due to rolling of the steel because of the plastic (soft) nature of pure manganese sulphide, as has already been mentioned above. The inclusions of the GS2 sample with a relatively low sulphur content have a small number of pure MnS, also shown in Figure 3, but have a solid solution of (Ca,Mn)S. Overall, Figure 3 shows a better MnS modification result due to Ca treatment in the GS2 sample, because most of the portion of pure MnS inclusions was transformed into a solid solution of (Ca,Mn)S.

**Figure 3.** Pure MnS size comparison between the two samples. **Figure 3.** Pure MnS size comparison between the two samples.

#### **4. Thermodynamics of Inclusions Formation 4. Thermodynamics of Inclusions Formation**

Chemical compositions of Al2O3, Al2O3-MgO and Al2O3-CaO inclusions are projected onto the Al2O3-MgO-CaO ternary system as shown in Figure 4a,b of both samples respectively (where M is MgO, MA is MgO-Al2O3, CA is CaO-Al2O3, CA2 is Ca-Al4O7, CA6 is Ca-Al12O19, C3A is Ca3-Al2O6 and C12A7 is Ca12-Al14O34). The liquidus temperature of inclusions can be predicted by Figure 4. The liquidus temperature of Al2O3, Al2O3-rich CaO, CaO-rich Al2O3 and Al2O3-MgO spinel inclusions is much higher than 1600 °C, which is normally the molten steel refining temperature, while some Al2O3-CaO inclusions are below 1600 °C. When Al was added to molten steel as a deoxidizer, Al2O3 inclusions were initially formed. The dissolved [Al] and [O] often reacted with traces of dissolved [Mg] or (MgO) refractory to form (MgO-Al2O3) spinel inclusions. These inclusions were formed through the reduction of MgO in the slag or refractory by aluminum even though Mg was not intentionally added in the steel. Mg reacted with Al2O3 to form MgO-Al2O3 inclusions. The formation of spinel inclusions through a steel-slag-refractory reaction has been discussed elsewhere in detail [21,24–26]. Ca-treatment was used to transform solid spinel and solid alumina into liquid calcium aluminates [21]. However, some Al2O3 inclusions were not properly modified by Ca treatment and formed solid Al2O3-rich CaO and CaO-rich Al2O3 inclusions, which have a high liquidus temperature. Chemical compositions of Al2O3, Al2O3-MgO and Al2O3-CaO inclusions are projected onto the Al2O3-MgO-CaO ternary system as shown in Figure 4a,b of both samples respectively (where M is MgO, MA is MgO-Al2O3, CA is CaO-Al2O3, CA<sup>2</sup> is Ca-Al4O7, CA<sup>6</sup> is Ca-Al12O19, C3A is Ca3-Al2O<sup>6</sup> and C12A<sup>7</sup> is Ca12-Al14O34). The liquidus temperature of inclusions can be predicted by Figure 4. The liquidus temperature of Al2O3, Al2O3-rich CaO, CaO-rich Al2O<sup>3</sup> and Al2O3-MgO spinel inclusions is much higher than 1600 ◦C, which is normally the molten steel refining temperature, while some Al2O3-CaO inclusions are below 1600 ◦C. When Al was added to molten steel as a deoxidizer, Al2O<sup>3</sup> inclusions were initially formed. The dissolved [Al] and [O] often reacted with traces of dissolved [Mg] or (MgO) refractory to form (MgO-Al2O3) spinel inclusions. These inclusions were formed through the reduction of MgO in the slag or refractory by aluminum even though Mg was not intentionally added in the steel. Mg reacted with Al2O<sup>3</sup> to form MgO-Al2O<sup>3</sup> inclusions. The formation of spinel inclusions through a steel-slag-refractory reaction has been discussed elsewhere in detail [21,24–26]. Ca-treatment was used to transform solid spinel and solid alumina into liquid calcium aluminates [21]. However, some Al2O<sup>3</sup> inclusions were not properly modified by Ca treatment and formed solid Al2O3-rich CaO and CaO-rich Al2O<sup>3</sup> inclusions, which have a high liquidus temperature. *Metals* **2021**, *11*, 2051 4 of 10

**Figure 4.** The composition distribution of Al2O3-MgO-CaO inclusions on the Al2O3-MgO-CaO phase diagram. (**a**) GS1 **Figure 4.** The composition distribution of Al2O<sup>3</sup> -MgO-CaO inclusions on the Al2O<sup>3</sup> -MgO-CaO phase diagram. (**a**) GS1 sample, (**b**) GS2 sample.

sample, (**b**) GS2 sample.

calculated the Al2O3-CaO-CaS oxysulphide system using FactSage thermochemical software. To understand the solubility of CaS in the Al2O3-CaO at high temperature and to justify why pure CaS inclusions or CaS encapsulating oxide inclusions are not present in the present inclusions system, the Al2O3-CaO-CaS system was re-calculated using an updated version of the FactSage 7.3 (CRCT, Montreal, Canada and GTT Technologies, Aachen, Germany) phase diagram module using the FactOxide, FactPS. and FactMisc. databases with 1 atm pressure, and no other assumptions were considered [28], which is shown in Figure 5. The region of full-liquid phase at 1600 °C is delimited by the thinner lines on the diagram. Within the isothermal sections of 1600 °C, CaS solid precipitates in the molten steel during Ca-treatment can dissolve into liquid calcium aluminate inclusions in the primary phase fields of Ca3Al2O6 and CaAl2O4. In the CaS primary phase field, the solubility of CaS in the liquid calcium aluminate is up to 8.5 wt.%, and it decreases with increasing Al2O3 content in the inclusions at 1600 °C. The liquid inclusions formed in GS1 and GS2 samples were calculated, which are indicated in Figure 5 as solid points. The CaS composition in liquid calcium aluminate inclusions is around 3.2 wt.%. According to Higuchi et al. the dissolved CaS will re-precipitate from the liquid Al2O3-CaO-CaS oxysulphide inclusions to form Al2O3-CaO-CaS dual phases during solidification of steel [16], but in the present case it did not happen. Precipitation of MnS on CaS at low temperature

to form (Ca, Mn)S in solidified steels will be discussed in the next section.

(Ca,Mn)S was observed to be associated with Al2O3-CaO inclusions to form Al2O3- CaO-(Ca, Mn)S oxysulphide inclusions. Kang et al. [27] measured the phase equilibria and calculated the Al2O3-CaO-CaS oxysulphide system using FactSage thermochemical software. To understand the solubility of CaS in the Al2O3-CaO at high temperature and to justify why pure CaS inclusions or CaS encapsulating oxide inclusions are not present in the present inclusions system, the Al2O3-CaO-CaS system was re-calculated using an updated version of the FactSage 7.3 (CRCT, Montreal, Canada and GTT Technologies, Aachen, Germany) phase diagram module using the FactOxide, FactPS. and FactMisc. databases with 1 atm pressure, and no other assumptions were considered [28], which is shown in Figure 5. The region of full-liquid phase at 1600 ◦C is delimited by the thinner lines on the diagram. Within the isothermal sections of 1600 ◦C, CaS solid precipitates in the molten steel during Ca-treatment can dissolve into liquid calcium aluminate inclusions in the primary phase fields of Ca3Al2O<sup>6</sup> and CaAl2O4. In the CaS primary phase field, the solubility of CaS in the liquid calcium aluminate is up to 8.5 wt.%, and it decreases with increasing Al2O<sup>3</sup> content in the inclusions at 1600 ◦C. The liquid inclusions formed in GS1 and GS2 samples were calculated, which are indicated in Figure 5 as solid points. The CaS composition in liquid calcium aluminate inclusions is around 3.2 wt.%. According to Higuchi et al. the dissolved CaS will re-precipitate from the liquid Al2O3-CaO-CaS oxysulphide inclusions to form Al2O3-CaO-CaS dual phases during solidification of steel [16], but in the present case it did not happen. Precipitation of MnS on CaS at low temperature to form (Ca, Mn)S in solidified steels will be discussed in the next section. *Metals* **2021**, *11*, 2051 5 of 10

**Figure 5.** Calculated liquidus projection of CaO-Al2O3-CaS system in low CaS region. **Figure 5.** Calculated liquidus projection of CaO-Al2O<sup>3</sup> -CaS system in low CaS region.

Thermodynamics of inclusions precipitation and evolution in the equilibrium cooling of steels from 1600 °C to 1000 °C are shown in Figure 6a,b for GS1 and GS2 samples based on the chemical compositions in Table 1, respectively. Figure 6a,b show liquid calcium aluminates formed in molten GS1 and GS2 steels at 1600 °C. The ratio of CaO/Al2O3 in the liquid oxide inclusions mainly depends on the modification extent of Al2O3 by Ca and concentrations of Ca, Al and O in Al-killed steels. The liquid calcium aluminate inclusions transformed to the CaAl2O4 phase at around 1600 °C, then altered from CaAl4O7 to CaAl12O19 and finally transformed to Al2O3 inclusions below 1400 °C. A large number of Al2O3 or Al2O3-rich inclusions were observed in the GS1 and GS2 solidified steels, as shown in their compositions in Figure 4 above, which is consistent with the thermodynamic calculations. The Al2O3 or Al2O3-rich inclusions might be formed from the cooling of liquid inclusions or a deficient modification of Al2O3 due to Ca approaching equilibrium. One thing worth noting is the amount of AlN formed in both steels; GS2 has more mass % of AlN than GS1. This is due to the fact that a greater amount of Al is used for deoxidation of steel. Nitrogen is usually added in the form of a ferroalloy for grain refinement, to prevent the austenite grain coarsening during heat treatment and to enhance the mechanical properties of the steels via grain boundary pinning through aluminum-nitride Thermodynamics of inclusions precipitation and evolution in the equilibrium cooling of steels from 1600 ◦C to 1000 ◦C are shown in Figure 6a,b for GS1 and GS2 samples based on the chemical compositions in Table 1, respectively. Figure 6a,b show liquid calcium aluminates formed in molten GS1 and GS2 steels at 1600 ◦C. The ratio of CaO/Al2O<sup>3</sup> in the liquid oxide inclusions mainly depends on the modification extent of Al2O<sup>3</sup> by Ca and concentrations of Ca, Al and O in Al-killed steels. The liquid calcium aluminate inclusions transformed to the CaAl2O<sup>4</sup> phase at around 1600 ◦C, then altered from CaAl4O<sup>7</sup> to CaAl12O<sup>19</sup> and finally transformed to Al2O<sup>3</sup> inclusions below 1400 ◦C. A large number of Al2O<sup>3</sup> or Al2O3-rich inclusions were observed in the GS1 and GS2 solidified steels, as shown in their compositions in Figure 4 above, which is consistent with the thermodynamic calculations. The Al2O<sup>3</sup> or Al2O3-rich inclusions might be formed from the cooling of liquid inclusions or a deficient modification of Al2O<sup>3</sup> due to Ca approaching equilibrium. One thing worth noting is the amount of AlN formed in both steels; GS2 has more mass % of AlN than GS1. This is due to the fact that a greater amount of Al is used for deoxidation of steel. Nitrogen is usually added in the form of a ferroalloy for grain refinement, to prevent the austenite grain coarsening during heat treatment and to enhance the mechanical properties of the steels via grain boundary pinning through aluminum-nitride precipitates

precipitates (AlN) [29]. AlN was not detected during EPMA analysis, maybe because of

during continuous casting is not at equilibrium, so maybe that is why AlN was unable to be detected by the probe despite the prediction during thermodynamic calculations.

(AlN) [29]. AlN was not detected during EPMA analysis, maybe because of the detection limit of the probe, which mostly measures inclusions larger than or equal to 1 µm. AlN may have a sub-micron or nano size, and also the cooling rate in the ladle and during continuous casting is not at equilibrium, so maybe that is why AlN was unable to be detected by the probe despite the prediction during thermodynamic calculations. *Metals* **2021**, *11*, 2051 6 of 10

**Figure 6.** Equilibrium precipitation of inclusions and evolution during steel solidification. (**a**) GS1 sample, (**b**) GS2 sample. **Figure 6.** Equilibrium precipitation of inclusions and evolution during steel solidification. (**a**) GS1 sample, (**b**) GS2 sample.

The variation of CaS and MnS components in the (Ca, Mn)S solid solution in equilibrium solidification in both samples is specifically shown in Figure 7. According to FactSage calculations, nearly pure CaS precipitated in molten Ca-treated Al-killed GS1 and GS2 steels at 1600 °C. The formation of solid CaS is usually caused by excessive sulphur and calcium in liquid steel. At steel melting temperature, both Ca and S have high activities and the amount of CaS generated in both samples is more than 90%. In GS1, with the decrease in temperature, this amount decreases until it reaches the balance with MnS at around 1330 °C, and after that, MnS begins to grow at the expense of CaS and reaches more than 90% at around 1000 °C, whereas CaS is only less than 10% at that temperature. On the other hand, in GS2, with the decrease in temperature the amount of CaS decreases until 1270 °C. The analysis of samples showed different results in the case of CaS inclusions, as proved by Figure 5. Individual or complex oxy-CaS inclusions were not observed during EPMA analysis because CaS was dissolved in the CaO-Al2O3 and did not solidify as CaO-Al2O3-CaS. Detailed mechanism of inclusions evolution during solidification can be seen in Figure 8. Decrease in CaS in the GS2 sample at a lower temperature can provide The variation of CaS and MnS components in the (Ca, Mn)S solid solution in equilibrium solidification in both samples is specifically shown in Figure 7. According to FactSage calculations, nearly pure CaS precipitated in molten Ca-treated Al-killed GS1 and GS2 steels at 1600 ◦C. The formation of solid CaS is usually caused by excessive sulphur and calcium in liquid steel. At steel melting temperature, both Ca and S have high activities and the amount of CaS generated in both samples is more than 90%. In GS1, with the decrease in temperature, this amount decreases until it reaches the balance with MnS at around 1330 ◦C, and after that, MnS begins to grow at the expense of CaS and reaches more than 90% at around 1000 ◦C, whereas CaS is only less than 10% at that temperature. On the other hand, in GS2, with the decrease in temperature the amount of CaS decreases until 1270 ◦C. The analysis of samples showed different results in the case of CaS inclusions, as proved by Figure 5. Individual or complex oxy-CaS inclusions were not observed during EPMA analysis because CaS was dissolved in the CaO-Al2O<sup>3</sup> and did not solidify as CaO-Al2O3-CaS.Detailed mechanism of inclusions evolution during solidification can be seen in Figure 8. Decrease in CaS in the GS2 sample at a lower temperature can provide the complete casting of several heats with no nozzle blockage and a lower amount of MnS generated.

the complete casting of several heats with no nozzle blockage and a lower amount of MnS

generated.

**Figure 7. Figure 7.** Mass percentage of CaS and MnS in the solid solution of (Ca,Mn)S. ( Mass percentage of CaS and MnS in the solid solution of (Ca,Mn)S. (**aa** ) GS1 sample, ( ) GS1 sample, (**b b** ) GS2 Sample. ) GS2 Sample

**Figure 8.** The schematic diagram of inclusions formation at the different stages of Al deoxidation, Ca treatment and solidification. **Figure 8.** The schematic diagram of inclusions formation at the different stages of Al deoxidation, Ca treatment and solidification.

#### **5. Formation Mechanism of Inclusions 5. Formation Mechanism of Inclusions**

The reactions during modification of alumina inclusions by calcium can be divided into the following kinetic steps: The reactions during modification of alumina inclusions by calcium can be divided into the following kinetic steps:

1. Ca injection into the ladle, which immediately melts the Ca, because its melting point is much lower than the melting temperature of steel, followed by the dissolution of calcium from the gas bubbles-steel interface to the bulk of the steel and finally transfer of the dissolved calcium from the bulk steel to the steel-inclusion interface: 1. Ca injection into the ladle, which immediately melts the Ca, because its melting point is much lower than the melting temperature of steel, followed by the dissolution of calcium from the gas bubbles-steel interface to the bulk of the steel and finally transfer of the dissolved calcium from the bulk steel to the steel-inclusion interface:

$$\text{Ca(s)} = \text{Ca(l)} + \text{Ca(g)} = \text{[Ca]} \tag{1}$$

[Ca] + [S] = (CaS) (3)

Ca(s) = Ca(l) + Ca(g) = [Ca] (1) 2. Diffusion of calcium from the steel-inclusion interface into the alumina core fol-2. Diffusion of calcium from the steel-inclusion interface into the alumina core followed by chemical reaction of the calcium with alumina:

3. The sulphur content varies in both samples, consequently the extent of CaS for-

In the above equations, square brackets denote an element dissolved in molten steel, and round brackets indicate the compounds in the inclusions. Before the calcium-alloy wire injection, mainly Al2O3 was produced due to the Al deoxidation of liquid steel. During the calcium treatment, the injection of Ca will lead to a severe splashing of liquid steel due to the high vapor pressure of Ca, and the added [Ca] reacts with [O] and Al2O3 inclusions to transform solid Al2O3 to liquid calcium aluminate inclusions. Some semi-solid Al2O3-rich core-shell inclusions might be partially modified due to kinetic diffusion of [Ca] controlling limitation [30–32]. When the [S] content is higher than the critical value for the precipitation of CaS, the CaO in the liquid inclusions reacts with sulphur [S] and aluminum [Al] in the molten steel. Therefore, the composition of the inclusions in the core is transformed to a MgOꞏAl2O3-rich system (close to spinel), and a CaS shell is formed. On the other hand, when the [S] content is lower than the critical value for the precipitation of CaS, liquid inclusions are formed [21]. The solubility of [Mn] and [S] decreases during

$$\text{[Ca]} + \text{[O]} + (\text{x} + 1/\text{3})\\\text{(Al}\_2\text{O}\_3) = \text{(CaO} \cdot \text{xAl}\_2\text{O}\_3) + 2/3[\text{Al}] \tag{2}$$

[Ca] + [O] + (x + 1/3)(Al2O3) = (CaO·xAl2O3) + 2/3[Al] (2)

(CaO) + 2/3[Al] +[S] = (CaS) + 1/3 (Al2O3) (4)

3. The sulphur content varies in both samples, consequently the extent of CaS formation changes. Ca can react with sulphur in two ways:

$$\text{[Ca]} + \text{[S]} = \text{(CaS)}\tag{3}$$

$$\text{(CaO)} + \text{2/3[Al]} + \text{[S]} = \text{(CaS)} + 1/\text{3 (Al}\_2\text{O}\_3) \tag{4}$$

In the above equations, square brackets denote an element dissolved in molten steel, and round brackets indicate the compounds in the inclusions. Before the calcium-alloy wire injection, mainly Al2O<sup>3</sup> was produced due to the Al deoxidation of liquid steel. During the calcium treatment, the injection of Ca will lead to a severe splashing of liquid steel due to the high vapor pressure of Ca, and the added [Ca] reacts with [O] and Al2O<sup>3</sup> inclusions to transform solid Al2O<sup>3</sup> to liquid calcium aluminate inclusions. Some semisolid Al2O3-rich core-shell inclusions might be partially modified due to kinetic diffusion of [Ca] controlling limitation [30–32]. When the [S] content is higher than the critical value for the precipitation of CaS, the CaO in the liquid inclusions reacts with sulphur [S] and aluminum [Al] in the molten steel. Therefore, the composition of the inclusions in the core is transformed to a MgO·Al2O3-rich system (close to spinel), and a CaS shell is formed. On the other hand, when the [S] content is lower than the critical value for the precipitation of CaS, liquid inclusions are formed [21]. The solubility of [Mn] and [S] decreases during the solidification of steels. Segregation of [Mn] and [S] takes the primary inclusions as the heterogeneous nucleation sites for MnS precipitation, then reacts with primary CaS wrapping on the oxides to form (Ca,Mn)S solid solution [32–35]. A schematic diagram of inclusions formation is demonstrated in Figure 8 and can be visualized via SEM-EDS analysis as shown in Figure 9. Figure 9 with EDS scanning explains the formation of a crescent shaped (Ca,Mn)S layer surrounding the spinels and in the other image it shows the oval shape (Ca,Mn)S associated with the spinels. *Metals* **2021**, *11*, 2051 9 of 10 the solidification of steels. Segregation of [Mn] and [S] takes the primary inclusions as the heterogeneous nucleation sites for MnS precipitation, then reacts with primary CaS wrapping on the oxides to form (Ca,Mn)S solid solution [32–35]. A schematic diagram of inclusions formation is demonstrated in Figure 8 and can be visualized via SEM-EDS analysis as shown in Figure 9. Figure 9 with EDS scanning explains the formation of a crescent shaped (Ca,Mn)S layer surrounding the spinels and in the other image it shows the oval shape (Ca,Mn)S associated with the spinels.

**Figure 9.** Duplex inclusions formed by (Ca,Mn)S precipitated on and collided with Al2O3-MgO inclusions. **Figure 9.** Duplex inclusions formed by (Ca,Mn)S precipitated on and collided with Al2O<sup>3</sup> - MgO inclusions.

The stable inclusions which can be formed based on the compositions of GS1 and GS2 in Table 1 by varying [Al] and [Ca] were calculated using FactSage 7.3 software with a phase diagram module with 1 atm pressure (FactOxide, FactPS and FactMisc. databases) to understand the controlling of inclusions. Figure 10 shows the stability diagram of the inclusions formed in the Fe-C-Si-Mn-Cr-S-O-Al-Ca systems at 1600 °C. The compositions of GS1 and GS2 are located close to the boundary of CaS + liquid and totally liquid inclusions region, which indicates the amount of CaS is relatively small. The small amount of CaS might not cause the problem of nozzle clogging in one heat casting but could block The stable inclusions which can be formed based on the compositions of GS1 and GS2 in Table 1 by varying [Al] and [Ca] were calculated using FactSage 7.3 software with a phase diagram module with 1 atm pressure (FactOxide, FactPS and FactMisc. databases) to understand the controlling of inclusions. Figure 10 shows the stability diagram of the inclusions formed in the Fe-C-Si-Mn-Cr-S-O-Al-Ca systems at 1600 ◦C. The compositions of GS1 and GS2 are located close to the boundary of CaS + liquid and totally liquid inclusions region, which indicates the amount of CaS is relatively small. The small amount of CaS might not cause the problem of nozzle clogging in one heat casting but could block the

the nozzle by accumulating in the nozzle after several castings in practice. The liquid inclusions region is the target for successful modification of inclusions but not the whole

other oxides, specially FeO and MnO, or decrease in [Ca] content less than 10 ppm will not completely modify the alumina inclusions but form (high temperature stable) CaAl4O7 inclusions which may create problems during casting. The modification window of Ca-

treatment is quite narrow and should be controlled precisely.

nozzle by accumulating in the nozzle after several castings in practice. The liquid inclusions region is the target for successful modification of inclusions but not the whole region is the target because either decrease in [Al] content less than 150 ppm will yield other oxides, specially FeO and MnO, or decrease in [Ca] content less than 10 ppm will not completely modify the alumina inclusions but form (high temperature stable) CaAl4O<sup>7</sup> inclusions which may create problems during casting. The modification window of Ca-treatment is quite narrow and should be controlled precisely. *Metals* **2021**, *11*, 2051 10 of 10

**Figure 10.** Stability diagram of inclusions formed in the Fe-C-Si-Mn-Cr-S-O-Al-Ca system at 1600 °C. (**a**) GS1 sample, (**b**) GS2 sample. (**b**) GS2 sample.

#### **6. Conclusions 6. Conclusions**

In the present study two gear steel samples with different sulphur and calcium contents were analyzed using an electron probe micro-analysis (EPMA) technique coupled with formal analysis, S.L.; investigation, H.A.; resources, H.A.; data curation, S.L.; writing—original **Figure 10.** Stability diagram of inclusions formed in the Fe-C-Si-Mn-Cr-S-O-Al-Ca system at 1600 ◦C. (**a**) GS1 sample, In the present study two gear steel samples with different sulphur and calcium contents were analyzed using an electron probe micro-analysis (EPMA) technique coupled with thermodynamic studies. The main findings are:


**Author Contributions:** Conceptualization, S.L. and H.A.; methodology, S.Z., Y.X. and Z.H.; software, H.A.; validation, S.L., X.M. and B.Z.; formal analysis, S.L.; investigation, H.A.; resources, H.A.; data curation, S.L.; writing—original draft preparation, H.A.; writing—review and editing, X.M.; supervision, X.M.; project administration, B.Z.; funding acquisition, B.Z. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research and APC were funded by the Australian Government through the Australian Research Council Research Hub for Computational Particle Technology, grant number IH140100035.

**Data Availability Statement:** The manuscript does not contain any data that has already been reported or archived elsewhere.

**Acknowledgments:** This research was supported by the Australian Government through the Australian Research Council Research Hub for Computational Particle Technology (IH140100035) and Baosteel through Baosteel-Australia Joint Research and Development Centre. The Australian Microscopy & Microanalysis Research Facility is acknowledged for providing characterization facilities. Technical support for the EPMA facility from Ron Rasch and Ying Yu of the Centre for Microscopy and Microanalysis at the University of Queensland is gratefully acknowledged.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


**Yang Li 1,\*, Chenhui Wu <sup>1</sup> , Xin Xie <sup>1</sup> , Lian Chen <sup>1</sup> , Jun Chen <sup>1</sup> , Xiaodong Yang <sup>2</sup> and Xiaodong Ma 3,\***


**Abstract:** During continuous casting, argon blowing from tundish cover (ABTC) can greatly prevent the flow of the remaining air and decrease the reoxidation of molten steel in tundish. In the current study, a numerical model based on a tundish of a two-strand slab continuous casting machine was established to investigate the feasibility and evaluate the protective casting effect of the ABTC process. The influence of operation parameters, including sealing schemes of tundish cover holes and the argon flow rate of the remaining oxygen content, were studied in tundish. Then, industrial trials based on the operation parameters from the numerical model were carried out to evaluate the protective effect of ABTC. The results indicate that the ABTC process has a great protective effect in avoiding increasing levels of nitrogen and losing titanium and aluminum. With the ABTC process applied, the average increment of nitrogen (4*w*[N]) in steel from the end of RH to tundish decreases by 90% from 10 <sup>×</sup> <sup>10</sup>−<sup>6</sup> to 1 <sup>×</sup> <sup>10</sup>−<sup>6</sup> , the average loss of titanium (4*w*[Ti]) by 12.7% from 63 <sup>×</sup> <sup>10</sup>−<sup>6</sup> to <sup>55</sup> <sup>×</sup> <sup>10</sup>−<sup>6</sup> , and the amount of aluminum (4*w*[Al]) decreases by 7.1% from 70 <sup>×</sup> <sup>10</sup>−<sup>6</sup> to 55 <sup>×</sup> <sup>10</sup>−<sup>6</sup> . The injecting hole and baking holes should be sealed during the period of empty tundish to efficiently discharge the air. In order to ensure that the oxygen volume fraction in tundish is less than 1%, the argon flow rate should be <sup>≥</sup>220 Nm3/h during the period of empty tundish and <sup>≥</sup>80 Nm3/h during the period of normal casting.

**Keywords:** continuous casting; tundish; argon blowing; protective casting; numerical simulation
