Effect of Mn Content on the Reaction between Fe-xMn (x = 5, 10, 15, and 20 Mass Pct) Steel and CaO-SiO 2 -Al 2 O 3 -MgO Slag

: Medium- and high-Mn steels have excellent properties but are very difﬁcult to be commer-cially produced because of the high content of some alloy elements. To enhance the understanding of the reaction between medium/high-Mn steel and reﬁning slag which is signiﬁcantly different from the conventional steels, steel and slag composition and the inclusions were investigated by equilibrium reaction between Fe-xMn (x = 5, 10, 15, and 20 mass pct) and CaO-SiO 2 -Al 2 O 3 -MgO top slag at 1873 K in the laboratory. Furthermore, the effect of Mn content on inclusion transformation and steel cleanliness was also explored. After slag–steel reaction, both contents of MnO in slag and Si in steel increased. Most MnO inclusions in master steel transformed to MnO-SiO 2 and MnO-Al 2 O 3 -MgO. With the increase in Mn content, the amount share of MnO type inclusions decreased and that of MnO-Al 2 O 3 -MgO type increased. In addition, both the number density of observed inclusions and the calculated oxygen content in inclusions increased. Thermodynamic analysis indicates that the composition change of steel and slag and the transformation of inclusions are mainly the consequence of the reaction between Mn in molten steel and SiO 2 and MgO in top slag. The dissolved Mn in medium/high-Mn steel presents a strong reactivity.


Introduction
In the past decades, with the increasing desire for energy saving and environmental protection, medium-and high-Mn steels, with Mn content higher than 3%, have garnered great attention from the automotive industry and researchers due to their outstanding mechanical properties, broad application prospects, and very high production difficulties.
Some studies in metallurgy have been conducted to explore the mechanism, fundamental data, etc. for medium/high-Mn steels and to solve the problems met during the production of these steels. Pak [1] studied the thermodynamics of deoxidation equilibria and AlN formation in high-Mn-and high-Al-alloyed liquid steel. Yan [2] and Yu [3] investigated metallurgical characteristics of refining slag for high-manganese steel. Kim et al. [4,5] studied reactions between Al and Mn in liquid steel and molten mold flux, which is closely related to the difficulties met in continuous casting of medium/high-Mn steels. With regard to the studies in the field of steelmaking, Liu [6] investigated the effect of CO 2 and O 2 mixed injection on Mn retention in high-Mn steel. Gigacher [7] and Alba [8] investigated the inclusions in high-Mn steel with different Mn content or Al content in absence of top slag. As known, ladle slag is used in the production of medium/high-Mn steels. Because of the high content of alloys in these steels, the reactions between molten steel and top slag are significantly different from those of conventional steels. Yu [3] investigated the refining slag CaO-SiO 2 -Al 2 O 3 -MgO in equilibrium with Fe-(10, 20 mass pct)Mn. The MnO content in slag is much higher than that in equilibrium with conventional steel because of the reaction between [Mn] in steel and SiO 2 in slag. Peymandar [9] studied the reaction between CaO-SiO 2 -Al 2 O 3 -MgO slag (initial slag basicity = 1.3, Al 2 O 3 =15 mass pct) and high-Mn steel, with a focus on reaction kinetics. Deng [10] studied the reaction between Al-killed manganese steel and CaO-SiO 2 -Al 2 O 3 -MgO ladle slag. Strong reactions between [Al] and [Mn] in steel and SiO 2 and MgO in slag were observed in both above studies. Such reactions are also reported in Yu's studies [11,12]. So, Mn in medium/high-Mn steels has strong reactivity and plays an important role in the reaction between steel and top slag. It is well known that the reaction between molten steel and top slag has a great effect on the precise control of steel composition and improvement of cleanliness [13][14][15][16]. However, the effects of Mn content on the reaction between medium/high-Mn steels and ladle slag have received little attention and seldom been reported. In addition, inclusions were seldom investigated in the above studies.
In the present work, the reaction between medium/high-Mn Steel, with Mn content of 5, 10, 15, and 20 mass pct, and CaO-SiO 2 -Al 2 O 3 -MgO slag was investigated at 1873 K by laboratory equilibrium experiments. The composition evolution of steel and slag and the nonmetallic inclusions were analyzed. Based on the experimental results, the effects of Mn content on inclusion transformation and steel cleanliness are discussed with the aid of thermodynamic calculations.

Materials
Four master steels, with nominal Mn content of 5, 10, 15, and 20 mass pct were employed for slag-steel reaction experiments, which were prepared by using electrolytic iron and electrolytic manganese. Table 1 gives the composition of raw metal materials, in which the composition of electrolytic iron is from certificate and that of electrolytic manganese is by measurement with the following methods: Si by ICP-AES, Al by ICP-MS, and S by infrared absorption method. Initial slag was CaO-SiO 2 -Al 2 O 3 -MgO system with 20% Al 2 O 3 , 6% MgO, and slag basicity (B = CaO/SiO 2 ) of 4 by mixing chemical analytical reagents.

Experimental Method
Experiments were carried out in a vertical electric resistance furnace with Si-Mo heating, the schematic of which is shown in Figure 1. First, 180 g master steel and 36 g initial slag were charged together into a MgO crucible, which was then put in the constant temperature zone of an alumina reaction chamber. Heating began, and the moment when 1873 K was reached was set as the starting time of reaction. After slag and steel reacted for 90 min, which was determined by the preliminary experiment, the MgO crucible was taken out and water-cooled quickly. The whole experiment was carried out under argon protective atmosphere. The detailed experimental procedure is described in the authors' previous studies [3,12]. According to different Mn contents, the experiments were divided into four groups, named Fe-5Mn, Fe-10Mn, Fe-15Mn, and Fe-20Mn.
ASPEX observation of inclusions in the present work, the detected minimum inclusion size was set as 1 μm, and an area of no less than 20 mm 2 for each steel sample was observed to obtain statistical information. The program parameters of ASPEX for inclusion detection, beam energy, emission current, and working distance were set at 20 kV, 43.5 μA, and 17 mm, respectively. The detailed analysis methods are described in the authors' previous studies [12,17].

Chemical Compositions of Slag
Chemical compositions of slag samples after slag-steel reaction are shown in Table  2. A notable change in slag composition is that MnO content increased greatly from zero to 4.06-5.44% in the present study. The initial and final slag compositions with different Mn contents were plotted in the phase diagram of the multicomponent system (the CaO-SiO2-Al2O3-6%MgO-5%MnO system), which was calculated by FactSage 7.1, as shown in Figure 2. The compositions of the final slags of the four groups were located near the saturated line of (Mn, Mg)O, and the final slag of Exp. Fe-5Mn was a little different from those of Exp. Fe-10Mn, Fe-15Mn, and Fe-20Mn [12].  Chemical compositions of the obtained steel and slag samples were analyzed and nonmetallic inclusions were detected by a scanning electron microscope (SEM, JEOL JSM-6480LV) equipped with EDS (Thermo-NS6) and Aspex PSEM Explorer (ASPEX for short; Aspex Corporation). Total oxygen content (T.O) of steel was analyzed by hydrogen/nitrogen/oxygen determinator (TCH600, LECO Corporation, Saint Joseph, MI, USA). For the ASPEX observation of inclusions in the present work, the detected minimum inclusion size was set as 1 µm, and an area of no less than 20 mm 2 for each steel sample was observed to obtain statistical information. The program parameters of ASPEX for inclusion detection, beam energy, emission current, and working distance were set at 20 kV, 43.5 µA, and 17 mm, respectively. The detailed analysis methods are described in the authors' previous studies [12,17].

Chemical Compositions of Slag
Chemical compositions of slag samples after slag-steel reaction are shown in Table 2. A notable change in slag composition is that MnO content increased greatly from zero to 4.06-5.44% in the present study. The initial and final slag compositions with different Mn contents were plotted in the phase diagram of the multicomponent system (the CaO-SiO 2 -Al 2 O 3 -6%MgO-5%MnO system), which was calculated by FactSage 7.1, as shown in Figure 2. The compositions of the final slags of the four groups were located near the saturated line of (Mn, Mg)O, and the final slag of Exp. Fe-5Mn was a little different from those of Exp. Fe-10Mn, Fe-15Mn, and Fe-20Mn [12].

Chemical Compositions of Steel
Chemical compositions of steel samples after slag-steel reaction are shown in Table  3. The analyzed Mn contents were a little lower than the preset values. It also indicated that Mn yield improved with the increase in Mn content. Electrolytic manganese, which was used in this study, contains a few impurity elements, Al, Si, and S. Consequently, small amounts of these elements were taken into the steel bath with electrolytic manganese. Table 4 gives the calculated chemical composition of initial steel samples. After slagsteel reaction, Si content increased and S decreased. Furthermore, the increase in Si was higher than the amount that was taken in by electrolytic manganese and increased with the increase in Mn content in steel.

Chemical Compositions of Steel
Chemical compositions of steel samples after slag-steel reaction are shown in Table 3. The analyzed Mn contents were a little lower than the preset values. It also indicated that Mn yield improved with the increase in Mn content. Electrolytic manganese, which was used in this study, contains a few impurity elements, Al, Si, and S. Consequently, small amounts of these elements were taken into the steel bath with electrolytic manganese. Table 4 gives the calculated chemical composition of initial steel samples. After slag-steel reaction, Si content increased and S decreased. Furthermore, the increase in Si was higher than the amount that was taken in by electrolytic manganese and increased with the increase in Mn content in steel. The reason for the increase in Si content in steel and MnO in slag is discussed in the authors' previous studies [3,12]. During the reaction between top slag and molten steel, Equation (1) occurred [16,18]. (1) where a [i] or a (i) is the activity of i in molten steel or slag and K 1 is the equilibrium constant of Equation (1). Assuming the slag-steel reaction has reached equilibrium, the ratio of MnO to SiO 2 of slags, log(a 2 (MnO) /a (SiO 2 ) ), is expected to exhibit a linear relation to the log(a 2 [Mn] /a [Si] ) of steel melts with the slope of unity at a fixed temperature. The a (MnO) , a (SiO 2 ) , a [Mn] , and a [Si] of the four sets of experiments with different Mn contents were calculated. Here, a (MnO) and a (SiO 2 ) were calculated by FactSage 7.1, and a [Mn] and a [Si] were calculated by Equation (5) (Tables 3 and 5). Figure 3 shows the log(a 2 (MnO) /a (SiO 2 ) ) of final slags as a function of log(a 2 [Mn] /a [Si] ) of steel melts after slag-steel reaction. The log(a 2 (MnO) /a (SiO 2 ) ) of slags linearly increases by increasing the log(a 2 [Mn] /a [Si] ) of steel melts with the slope of 0.75. Thus, it can be inferred that the slag-steel system investigated in this work was in thermodynamic equilibrium [19].

Inclusion Type
The inclusions in steel samples were analyzed by ASPEX. The observed area of samples Fe-5Mn, Fe-10Mn, Fe-15Mn, and Fe-20Mn were 27.56, 25.59, 32.15, and 23.46 mm 2 , and the numbers of detected inclusions were 669, 752, 1406, and 1268, respectively. Inclusions in master medium-and high-Mn steels are mainly MnO type. Some contain a little Al2O3 and MnS [3]. After the reaction with the top slag of the CaO-SiO2-Al2O3-MgO system, most inclusions transformed into three types, which were MnO-SiO2, MnO-Al2O3-MgO, and MnO types, with the MnO-SiO2 type representing the majority; the typical morphologies of the three types of inclusions are shown in Figure 4. With the increase of Mn content, there was little change in inclusion types. That is, the above three types of inclusions existed in steel samples of the four groups of experiments after slag-steel reaction. sions in master medium-and high-Mn steels are mainly MnO type. Some contain a little Al2O3 and MnS [3]. After the reaction with the top slag of the CaO-SiO2-Al2O3-MgO system, most inclusions transformed into three types, which were MnO-SiO2, MnO-Al2O3-MgO, and MnO types, with the MnO-SiO2 type representing the majority; the typical morphologies of the three types of inclusions are shown in Figure 4. With the increase of Mn content, there was little change in inclusion types. That is, the above three types of inclusions existed in steel samples of the four groups of experiments after slag-steel reaction.

Effect of Mn Content on Inclusion Type
The statistical information of inclusions in steel samples was obtained by ASPEX. Inclusion types change little with Mn content increasing, whereas, the amount proportion of each type changed greatly, as shown in Figure 5. The percentage of MnO type decreased and MnO-Al2O3-MgO type increased with the increase in Mn content. The contents of Mg, Al, Si, Ca, Mn, and S of the detected inclusions were recounted to the sum of 100%. Figure  6 shows the average mass percentages of main elements in inclusions. It is easily seen that Mn represents the majority in inclusions. With the increase in Mn content, Mn content in inclusions shows a decreasing trend while Al and Mg show an increasing trend.

Effect of Mn Content on Inclusion Type
The statistical information of inclusions in steel samples was obtained by ASPEX. Inclusion types change little with Mn content increasing, whereas, the amount proportion of each type changed greatly, as shown in Figure 5. The percentage of MnO type decreased and MnO-Al 2 O 3 -MgO type increased with the increase in Mn content. The contents of Mg, Al, Si, Ca, Mn, and S of the detected inclusions were recounted to the sum of 100%. Figure 6 shows the average mass percentages of main elements in inclusions. It is easily seen that Mn represents the majority in inclusions. With the increase in Mn content, Mn content in inclusions shows a decreasing trend while Al and Mg show an increasing trend.

Relationship between Mn Content and Steel Cleanliness
With the increase in Mn content from 5% to 10%, 15%, and 20%, total oxygen content (T.O) changed from 22.4 × 10 −6 to 17.6 × 10 −6 , 21.5 × 10 −6 , and 23.2 × 10 −6 , which shows no apparent tendency. The number density of inclusions in steel increased with the increase in Mn content, as shown in Figure 7.

Relationship between Mn Content and Steel Cleanliness
With the increase in Mn content from 5% to 10%, 15%, and 20%, total oxygen content (T.O) changed from 22.4 × 10 −6 to 17.6 × 10 −6 , 21.5 × 10 −6 , and 23.2 × 10 −6 , which shows no apparent tendency. The number density of inclusions in steel increased with the increase in Mn content, as shown in Figure 7.

Relationship between Mn Content and Steel Cleanliness
With the increase in Mn content from 5% to 10%, 15%, and 20%, total oxygen content (T.O) changed from 22.4 × 10 −6 to 17.6 × 10 −6 , 21.5 × 10 −6 , and 23.2 × 10 −6 , which shows no apparent tendency. The number density of inclusions in steel increased with the increase in Mn content, as shown in Figure 7.

Evolution Mechanism of Inclusions
After reaction with CaO-SiO 2 -Al 2 O 3 -MgO top slag, the main type of inclusions transformed from MnO in master steel to MnO-SiO 2 . This is because the reaction [Mn] + (SiO 2 ) = [Si] + (MnO) occurred between medium/high-Mn steel and top slag, which is explored in detail in the authors' previous studies [3]. In the present study, the change in Gibbs free energy of Reaction (1)  Another phenomenon is that, with the increase in Mn content in steel, the proportion of MnO type decreased greatly and that of MnO-Al 2 O 3 -MgO type increased, as shown in Figure 4. As reported in Deng's research [10], when the Mn content is high enough, for example, w(Mn) = 20%, the dissolved Mn can not only reduce SiO 2 in slag but also react with MgO in slag. Park [22] investigated the reaction between CaO-SiO 2 -MgO-Al 2 O 3 flux and Fe-xMn-yAl (x = 10 and 20 mass pct, y = 1, 3, and 6 mass pct) steel at 1873 K. Several small solid compounds, i.e., [Mg, Mn]Al 2 O 4 spinel, were found in the slag reacted with Fe-20Mn-1Al but not found in the slag with Fe-10Mn-1Al. Therefore, the dissolved Mn in molten steel presents strong reactivity with the increase in Mn content. In the present study, Reaction (6) would occur with the increase in Mn content [10,23]. MgO in slag would be reduced by Mn in steel, and then the generated Mg would combine with oxygen in the steel bath or existing inclusions. As a result, inclusions of MnO-Al 2 O 3 -MgO type formed.

Effect of Mn Content on Steel Cleanliness
Alba [8] investigated the inclusions in Fe-xMn-3%Al (x = 2%, 5%, 20%) steel in absence of top slag. In his research, the observed inclusions were mainly in Figure 6. To clarify the effect of Mn content on steel cleanliness, dissolved oxygen in steel and oxygen in inclusions were calculated, and then the relationship between T.O, oxygen in inclusions, and number density of inclusions was explored with the increase in Mn content.
T.O in steel is the sum of the free oxygen content ([O] free ) and the oxygen content in the inclusions ((O) inc ), as shown in Equation (8). In this study, the Mn content of steel was high, and the main component of the inclusions was MnO, so it can be assumed that the free oxygen content in the steel bath was controlled by manganese-oxygen equilibrium, as shown in Reaction (9) [24]. T where K 3 is the equilibrium constant of Reaction (9); a (MnO) , a [Mn] , and a [O] are the activities of MnO in slag, Mn in molten steel, and O in molten steel, respectively. According to Equations (5), (10), and (11); the compositions in Tables 2 and 3; and the respective interaction coefficients in Table 5, the free oxygen content in the steel bath with different Mn contents can be obtained, which is shown as the sign " " in Figure 8. In the calculation, the a (MnO) was calculated by FactSage 7.1; its value is 0.0788, 0.0799, 0.0888, and 0.0999 for Exp. Fe-5Mn, Fe-10Mn, Fe-15Mn, and Fe-20Mn, respectively.  Tables 2 and 3; and the respective interaction coefficients in Table 5, the free oxygen content in the steel bath with different Mn contents can be obtained, which is shown as the sign "□" in Figure 8. In the calculation, the

[O] in Equilibrium in Fe-xMn Melts
In the present study, the steel bath had a high content of Mn and a small amount of Si and Al solute elements, as given in Table 3. The detected inclusions in steel contained such components as MnO, SiO2, and Al2O3. This indicates that the solute elements, Mn, Si, and Al, participate in the reactions in molten steel or between slag and steel. On the other hand, for the above calculation of oxygen content, the calculated dissolved oxygen content was less than the measured T.O content. Furthermore, the calculated oxygen content in inclusions agrees well with the number density of the observed inclusions under the assumption that Equation (9) is the main controlled reaction. So, all these results give strong evidence that the content of dissolved oxygen in this study is in equilibrium with Mn content in molten steel, although other metal elements, such as Si and Al, also participate in the reactions. That is, the dissolved oxygen content is controlled by Reaction (9).
After the reaction between top slag and molten steel, the main inclusions transformed from MnO to MnO-SiO2 and MnO-Al2O3-MgO. Thermodynamic analysis shows that the reactions between Mn in molten steel and SiO2 and MgO in top slag occur. The dissolved Mn in medium/high-Mn steel presents a strong reactivity.

Conclusions
Laboratory experiments were carried out to investigate the reaction between Fe-xMn (x = 5, 10, 15, and 20 mass pct) and CaO-SiO2-Al2O3-MgO refining slag. The following conclusions are drawn:

[O] in Equilibrium in Fe-xMn Melts
In the present study, the steel bath had a high content of Mn and a small amount of Si and Al solute elements, as given in Table 3. The detected inclusions in steel contained such components as MnO, SiO 2 , and Al 2 O 3 . This indicates that the solute elements, Mn, Si, and Al, participate in the reactions in molten steel or between slag and steel. On the other hand, for the above calculation of oxygen content, the calculated dissolved oxygen content was less than the measured T.O content. Furthermore, the calculated oxygen content in inclusions agrees well with the number density of the observed inclusions under the assumption that Equation (9) is the main controlled reaction. So, all these results give strong evidence that the content of dissolved oxygen in this study is in equilibrium with Mn content in molten steel, although other metal elements, such as Si and Al, also participate in the reactions. That is, the dissolved oxygen content is controlled by Reaction (9).
After the reaction between top slag and molten steel, the main inclusions transformed from MnO to MnO-SiO 2 and MnO-Al 2 O 3 -MgO. Thermodynamic analysis shows that the reactions between Mn in molten steel and SiO 2 and MgO in top slag occur. The dissolved Mn in medium/high-Mn steel presents a strong reactivity.
The dissolved Mn in medium/high-Mn steel presents a strong reactivity in the slagsteel reaction. The composition change of steel and slag and the transformation of inclusions are mainly the consequence of the reaction between Mn in molten steel and SiO 2 and MgO in top slag.

2.
After slag-steel reaction, MnO content in slag increased greatly from zero to 4.06-5.44%, and Si content in steel increased. With the increase in Mn content in steel, the contents of MnO in slag and Si in steel show an increasing trend.