Modification Mechanism of Spinel Inclusions in Medium Manganese

metals

Article Modiﬁcation Mechanism of Spinel Inclusions in Medium Manganese Steel with Rare Earth Treatment

Zhe Yu 1,2 and Chengjun Liu 1,2,*

1 Key Laboratory for Ecological Metallurgy of Multimetallic Ores (Ministry of Education), Shenyang 110819, China 2 School of Metallurgy, Northeastern University, Shenyang 110819, China * Correspondence: [email protected]; Tel.: +86-138-9885-0333

Received: 7 July 2019; Accepted: 19 July 2019; Published: 21 July 2019

Abstract: In aluminum deoxidized medium manganese steel, spinel inclusions are easily to form during refining, and such inclusions will deteriorate the toughness of the medium manganese steel. Rare earth inclusions have a smaller hardness, and their thermal expansion coefficients are similar to that of steel. They can avoid large stress concentrations around inclusions during the heat treatment of steel, which is beneficial for improving the toughness of steel. Therefore, rare earth Ce is usually used to modify spinel inclusions in steel. In order to clarify the modification mechanism of spinel inclusions in medium manganese steel with Ce treatment, high-temperature simulation experiments were carried out. Samples were taken step by step during the experimental steel smelting process, and the inclusions in the samples were analyzed by SEM-EDS. Finally, the experimental results were discussed and analyzed in combination with thermodynamic calculations. The results show that after Ce treatment, the amount of inclusions decrease, the inclusion size is basically less than 5 µm, and the spinel inclusions are transformed into rare earth inclusions. After Ce addition, Mn and Mg in the spinel inclusions are first replaced by Ce, and the spinel structure is destroyed to form CeAlO3. When the O content in the steel is low, S in the steel will replace the O in the inclusion, and CeAlO3 and spinel inclusions will be transformed into Ce2O2S. By measuring the total oxygen content of the steel, the total Ce content required for complete modification of spinel inclusions can be obtained. Finally, the critical conditions for the formation and transformation of inclusions in the Fe-Mn-Al-Mg-Ce-O-S system at 1873K were obtained according to thermodynamic calculations.

Keywords: medium manganese steel; spinel inclusions; Ce treatment; modiﬁcation mechanism

1. Introduction Medium manganese steel is a typical representative of third generation automotive steel with high strength and high plasticity. It can be applied to many body structure parts including automobile A-pillars, B-pillars, anticollision beams, and sill reinforcement. Industrial experimental studies have shown that (Mn,Mg)O Al O - and MgO Al O -type spinel inclusions will form during refining of · 2 3 · 2 3 aluminum deoxidized medium manganese steel [1]. Spinel inclusions are generally angular and have high hardness, which is the main cause of stamping and cracking defects in automotive steel [2,3]. Therefore, it is necessary to modify the spinel inclusions to improve the toughness of medium manganese steel. At present, low-melting CaO-MgO-Al2O3 inclusions obtained by calcium treatment are generally expected to avoid the damage of hard inclusions [4,5]. However, large-size inclusions cannot be completely modified due to kinetic conditions, which means that the spinel phase core of the complex inclusions will still exist and be exposed after rolling [6]. The rare earth element Ce has a good modification effect on the spinel [7–9], which can modify the hard oxide inclusions into rare earth inclusions with a soft hardness [10]. Huang et al. [7,9]

Metals 2019, 9, 804; doi:10.3390/met9070804 www.mdpi.com/journal/metals Metals 2019, 9, x FOR PEER REVIEW 2 of 13 Metals 2019, 9, 804 2 of 13 Wang et al. [8] respectively studied the modification behavior of Ce on inclusions in drill steel and high-speed railway steel, and the transformation mechanism of spinel to Ce2O3 and Ce2O2S was and Wang et al. [8] respectively studied the modification behavior of Ce on inclusions in drill steel described. However, there are relatively few reports on rare earth modified spinel inclusions. The and high-speed railway steel, and the transformation mechanism of spinel to Ce2O3 and Ce2O2S modification mechanism of Ce on the spinel inclusions is not comprehensive, and the effect of was described. However, there are relatively few reports on rare earth modified spinel inclusions. aluminum content on Ce treatment has not been reported. Therefore, in this paper, the modification The modification mechanism of Ce on the spinel inclusions is not comprehensive, and the effect of behaviors of spinel inclusions after Ce treatment in medium manganese steel with different aluminum content on Ce treatment has not been reported. Therefore, in this paper, the modification aluminum contents were studied. Meanwhile, the Ce content required for complete modification of behaviors of spinel inclusions after Ce treatment in medium manganese steel with different aluminum spinel inclusions was discussed in combination with thermodynamic calculations, which are contents were studied. Meanwhile, the Ce content required for complete modification of spinel intended to guide the appropriate addition of rare earth in the actual production process. inclusions was discussed in combination with thermodynamic calculations, which are intended to guide the appropriate addition of rare earth in the actual production process. 2. Experimental Procedure

2. ExperimentalHigh-temperature Procedure simulation experiments were carried out by using a MoSi2 furnace (TianJin Muffle Technology Co. Ltd, Tianjin, China). According to the general composition of medium High-temperature simulation experiments were carried out by using a MoSi2 furnace (TianJin Mumanganeseffle Technology steel, the Co. target Ltd, Al Tianjin,content China).was set to According three levels: to low the aluminum general composition (0.02%), numbered of medium R1; manganesemedium aluminum steel, the target(0.2%), Al contentnumbered was R2; set toand three high levels: aluminum low aluminum (2.0%), (0.02%), numbered numbered R3. The R1; mediumexperimental aluminum steps (0.2%),are as follows. numbered Approximately R2; and high aluminum400 g of pure (2.0%), iron numbered and an MnFe R3. Thealloy experimental was placed stepsin a corundum are as follows. crucible Approximately and then placed 400 gin of the pure cons irontant and temperature an MnFe alloy zone wasof the placed furnace. in a The corundum master crucibleirons (pure and iron then and placed MnFe in alloy) the constant were heated temperature to 1873 K zone under of argon the furnace. atmosphere The master with a ironsflow rate (pure of 0.4 L min−1. Subsequently, the oxygen activity in the molten steel was measured by an electrolyte1 iron and MnFe alloy) were heated to 1873 K under argon atmosphere with a flow rate of 0.4 L min− . Subsequently,oxygen probe, the and oxygen it was activitydefined in as the the molten initial steeloxygen was activity measured (No. by 1 ana[O] electrolyte). Based on oxygen the target probe, Al content, a certain amount of Al wire was added, and NiMg alloy was added 10 min later. No. 2 a[O] and it was defined as the initial oxygen activity (No. 1 a[O]). Based on the target Al content, a certain was measured by electrolyte oxygen probe after another 10 min from the addition of NiMg alloy, and amount of Al wire was added, and NiMg alloy was added 10 min later. No. 2 a[O] was measured by electrolytethen sample oxygen 0 was probetaken afterby a quartz another tube 10 min and from quench theed addition in an ice of bath. NiMg Finally, alloy, and CeFe then alloy sample was added 0 was takeninto the by molten a quartz steel tube to and modify quenched inclusions. in an iceThereafter bath. Finally,, the molten CeFe alloy steel was was added respectively into the held molten for 1, steel 10, toand modify 30 min inclusions. at 1873 K Thereafter,(1600 °C) and the moltenthen successively steel was respectively sampled as held above for (sample 1, 10, and 1–sample 30 min at 3). 1873 After K taking sample 2, the oxygen activity was measured by an electrolyte oxygen probe, which was (1600 ◦C) and then successively sampled as above (sample 1–sample 3). After taking sample 2, the oxygendefined activityas the final was measuredoxygen activity by an electrolyte(No. 3 a[O]). oxygen The experimental probe, which process was defined is provided as the finalin Figure oxygen 1. Table 1 shows the compositions of raw materials, including pure iron, MnFe alloy, Al wire, NiMg activity (No. 3 a[O]). The experimental process is provided in Figure1. Table1 shows the compositions ofalloy, raw and materials, CeFe alloy. including pure iron, MnFe alloy, Al wire, NiMg alloy, and CeFe alloy.

Figure 1. Schematic diagram of the experimental process. Figure 1. Schematic diagram of the experimental process. Table 1. Chemical composition of raw materials (mass fraction/%). Table 1. Chemical composition of raw materials (mass fraction/%). Type Fe Ni Mg Al C Si Mn Ce S P Others Type Fe Ni Mg Al C Si Mn Ce S P Others Pure iron 99.9 0.01 - 0.001 0.0021 0.01 0.03 - 0.003 0.005 0.03 PureAl wire iron - 99.9 - 0.01 - - 99.99 0.001 0.0021 - -0.01 0.03 - - - 0.003 - 0.005 - 0.03 0.01 MnFeAl wire alloy 12.57- -- -- 99.99 - 1.5- 1.5- 84.2- - - - 0.03 - 0.20.01 - NiMgMnFe alloy alloy 0.9312.57 80.12- 17.98- -- 0.781.5 0.191.5 84.2 - - - 0.03 - 0.2 -- - CeFe alloy 79 ------19 - - 2 NiMg alloy 0.93 80.12 17.98 - 0.78 0.19 - - - - - CeFe alloy 79 ------19 - - 2 To observe inclusions of steel, the steel samples were polished by SiC paper and diamond suspensions.To observe The inclusions chemical compositionsof steel, the steel and samp morphologyles were of polished inclusions by wereSiC paper analyzed and through diamond a scanningsuspensions. electron The chemical microscope compositions (SEM, Phenom and morp Prox,hology Eindhoven, of inclusions The Netherlands) were analyzed and anthrough energy a dispersivescanning electron spectroscope microscope (EDS, Phenom (SEM, Prox,Phenom Eindhoven, Prox, Eindhoven, The Netherlands). The Netherlands) The contents and of aluminum,an energy manganese,dispersive spectroscope magnesium, and(EDS, cerium Phenom in the Prox, steel wereEindhoven, determined The byNetherlands). using an inductively The contents coupled of

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plasmaMetals optical 2019, 9, emissionx FOR PEER REVIEW spectrometer (ICP-OES, PerkinElmer, Waltham, MA, USA). The content3 of 13 of sulfur was measured by a carbon and sulfur analyzer. The total oxygen content was measured by an oxygenaluminum, and nitrogen manganese, analyzer. magnesium, The contents and cerium of each in th elemente steel were in the determined three steels by using (numbered an inductively R1, R2, and R3) arecoupled listed plasma in Table optical2. emission spectrometer (ICP-OES, PerkinElmer, Waltham, MA, USA). The content of sulfur was measured by a carbon and sulfur analyzer. The total oxygen content was measured by anTable oxygen 2. Chemical and nitrogen composition analyzer. of experimental The contents steels of each (mass element fraction in/%). the three steels (numbered R1, R2, and R3) are listed in Table 2. a No. Mn Mg [O] C Table 2. ChemicalAl compositionCe of experimental S steels (mass fraction/%). TO No. 1 No. 2 No. 3 a[O] No.R1 C0.23 Mn 8.95 0.019 Al Mg 0.001 0.014 Ce 0.016 S 0.0149 0.0014 0.0005 0.0045TO R2 0.21 9.18 0.21 0.001 0.015 0.016No. 0.0124 1 No. 0.0002 2 No. 0.0002 3 0.0021 R1R3 0.230.25 8.95 9.35 0.0192.03 0.001 0.002 0.014 0.013 0.016 0.014 0.0149 0.0149 0.0014 0.0001 0.0005 0.0001 0.0045 0.0006 R2 0.21 9.18 0.21 0.001 0.015 0.016 0.0124 0.0002 0.0002 0.0021 R3 0.25 9.35 2.03 0.002 0.013 0.014 0.0149 0.0001 0.0001 0.0006 3. Results

3.1. Inclusion3. Results Transformation The3.1. Inclusion types of Transformation typical inclusions in the experimental steel are shown in Figure2; there are ﬁve types in common.The types of The typical type inclusions 1 inclusions in the are experiment Al2O3, asal shown steel are in shown Figure in2a. Figure Almost 2; there no obviousare five Mn elementtypes was in detectedcommon. inThe this type type 1 inclusions of inclusion. are Al Both2O3, the as typeshown 2 andin Figure type 32a. are Almost spinel no inclusions, obvious Mn namely (Mn,Mg)OelementAl wasO detectedand MgO in thisAl typeO spinel,of inclusion. respectively. Both the type The 2 type and type 4 and 3 are type spinel 5 inclusions inclusions, are namely rare earth · 2 3 · 2 3 inclusions,(Mn,Mg)O·Al and they2O3 appearand MgO·Al bright2O3 white spinel, under respectively. SEM backscatter The type 4 and conditions. type 5 inclusions The type are 4 rare inclusions earth are inclusions, and they appear bright white under SEM backscatter conditions. The type 4 inclusions are CeAlO3, which are mostly block-shaped and have a small size of about 2~5 µm. The type 5 inclusions CeAlO3, which are mostly block-shaped and have a small size of about 2~5 μm. The type 5 inclusions are Ce2O2S, which are mostly spherical and have a size of about 3~8 µm. are Ce2O2S, which are mostly spherical and have a size of about 3~8 μm.

Figure 2. Elemental mapping of typical inclusions in steel: (a) Al O ;(b) (Mn,Mg)O Al O ; 2 3 · 2 3 ( c) MgO Al O ;(d) CeAlO ;(e) Ce O S. · 2 3 3 2 2 Metals 2019, 9, x FOR PEER REVIEW 4 of 13

Metals 2019, 9, 804 4 of 13 Figure 2. Elemental mapping of typical inclusions in steel: (a) Al2O3; (b) (Mn,Mg)O·Al2O3; (c)

MgO·Al2O3; (d) CeAlO3; (e) Ce2O2S. After adding Ce, in addition to the above inclusions with uniform composition, there was a After adding Ce, in addition to the above inclusions with uniform composition, there was a small small amount of complex inclusions in the steel that underwent modiﬁcation, as shown in Figure3. amount of complex inclusions in the steel that underwent modification, as shown in Figure 3. In In Figure3a, the inclusion core is Mg-Al-O, and the outer layer is Ce-Al-O. In Figure3b, the inclusion Figure 3a, the inclusion core is Mg-Al-O, and the outer layer is Ce-Al-O. In Figure 3b, the inclusion core is Mg-Al-O, and the outer layer is Ce-O-S. In Figure3c, the inclusion is divided into three layers. core is Mg-Al-O, and the outer layer is Ce-O-S. In Figure 3c, the inclusion is divided into three layers. The core is Mg-Al-O, the middle layer is Ce-Al-O, and the outermost layer is Ce-O-S. In Figure3d, the The core is Mg-Al-O, the middle layer is Ce-Al-O, and the outermost layer is Ce-O-S. In Figure 3d, inclusion core is Ce-O-S, and the outer layer is Ce-Al-O. the inclusion core is Ce-O-S, and the outer layer is Ce-Al-O.

Figure 3. Elemental mapping of typical complex inclusions: (a) Mg-Al-O + Ce-Al-O; (b) Mg-Al-O + Figure 3. Elemental mapping of typical complex inclusions: (a) Mg-Al-O + Ce-Al-O ; (b) Mg-Al-O + Ce-O-S; (c) Mg-Al-O + Ce-Al-O + Ce-O-S; (d) Ce-O-S + Ce-Al-O. Ce-O-S; (c) Mg-Al-O + Ce-Al-O + Ce-O-S; (d) Ce-O-S + Ce-Al-O . The types of uniform inclusions and complex inclusions for each sampling period are listed The types of uniform inclusions and complex inclusions for each sampling period are listed in in Table3. It can be seen that the inclusions in the three experimental steels were mainly spinel Table 3. It can be seen that the inclusions in the three experimental steels were mainly spinel after after aluminum deoxidation and magnesium treatment, and there was also a small amount of Al2O3 aluminum deoxidation and magnesium treatment, and there was also a small amount of Al2O3 inclusions. The R1 experimental steel mainly contained (Mn,Mg)O Al2O3 inclusions. The spinel inclusions. The R1 experimental steel mainly contained (Mn,Mg)O·Al· 2O3 inclusions. The spinel inclusions in the R2 and R3 experimental steels were mainly MgO Al2O3. inclusions in the R2 and R3 experimental steels were mainly MgO·Al· 2O3. Table 3. Types of inclusions at diﬀerent stages. Table 3. Types of inclusions at different stages. Chemical Formula of Samples in R1 Steel Samples in R2 Steel Samples in R3 Steel Type Samples in R1 Samples in R2 Samples in R3 ChemicalInclusions Formula of 012301230123 Type Steel Steel Steel Inclusions 1 Al2O3 √0 1 2 3 √0 1 2 3 √ 0 1 2 3 2 (Mn,Mg)O Al O √√ · 2 3 13 MgOAlAl2OO3 √√ √√√ √ √ · 2 3 24 (Mn,Mg)O·Al CeAlO3 2O3 √√ √√ √√ √√ √√ √ √ 35 MgO·Al Ce2O2S 2O3 √ √√ √ √√ √√ √√ √√ √ √ √ √ a Mg-Al-O + Ce-Al-O √ √ √ √ 4 CeAlO3 √√ √√ √√ √√ √ √ b Mg-Al-O + Ce-O-S √ √ √ √ 5c Mg-Al-O + Ce-Al-OCe2O2S+ Ce-O-S √√√ √ √√√ √√ √√ √ √ √ √ ad Mg-Al-O Ce-O-S + Ce-Al-O + Ce-Al-O √ √ √ √ √ b Note:Mg-Al-O√√ represents + Ce-O-S the main type of inclusions,√ and √ represents a small√ number√ of inclusions. √ Mg-Al-O + Ce-Al-O + Ce-O- c √ √ √ After Ce treatmentS for 1 min, the inclusions in R1 experimental steel were mainly CeAlO3 and Ce2dO 2S, and thereCe-O-S was + Ce-Al-O a small amount of complex √ inclusions of type a (Mg-Al-O + Ce-Al-O), b (Mg-Al-ONote:+ Ce-OS), √√ represents and c the (Mg-Al-O main type+ Ce-Al-Oof inclusions,+ Ce-OS). and √ represents After Cetreatment a small number for 10 of min, inclusions. the inclusions

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in steel were mainly CeAlO3, while there was still a small amount of Ce2O2S inclusions as well as type a (Mg-Al-O + Ce-Al-O) and d (Ce-O-S + Ce-Al-O) complex inclusions. After Ce treatment for 30 min, the inclusions in the steel were almost all CeAlO3. It indicated that Ce-Al-O and Ce-O-S inclusions would be directly formed in the steel after Ce addition. Meanwhile, spinel inclusions would also be modified by Ce. As time prolonged, spinel inclusions were gradually modified completely, and Ce-O-S would be transformed into Ce-Al-O. After Ce treatment for 1 min, the inclusions in R2 experimental steel were mainly CeAlO3 and Ce2O2S, and there was a small amount of complex inclusions of type a (Mg-Al-O + Ce-Al-O), b (Mg-Al-O + Ce-OS), and c (Mg-Al-O + Ce-Al-O + Ce-OS). After Ce treatment for 10 min, the inclusions in steel were mainly CeAlO3, while there was still a small amount of Ce2O2S inclusions as well as type a (Mg-Al-O + Ce-Al-O) and b (Mg-Al-O + Ce-O-S) complex inclusions. After Ce treatment for 30 min, both CeAlO3 and Ce2O2S were present in the steel. It indicated that Ce-Al-O and Ce-O-S inclusions would directly form in the steel after Ce addition. Meanwhile, spinel inclusions would also be modified by Ce. As time prolonged, spinel inclusions would gradually be transformed into Ce-Al-O and Ce-O-S. After Ce treatment for 1 min, the inclusions in R3 experimental steel were mainly Ce2O2S, and there was a small amount of complex inclusions of type a (Mg-Al-O + Ce-Al-O) and c (Mg-Al-O + Ce-Al-O + Ce-OS). After Ce treatment for 10 min, the inclusions in steel were mainly Ce2O2S. It indicated that Ce-O-S inclusions would directly form in the steel after Ce addition. Meanwhile, spinel inclusions would also be modified by Ce. As time prolonged, spinel inclusions would be gradually transformed into Ce-O-S.

3.2. The Number Density and Size Change of Inclusions In order to investigate the influence of Ce treatment on the number and size of inclusions in steel, Image pro plus software (Version 6.0, Media Cybernetics, Shanghai, China) was used to count the number density (number of inclusions per square millimeter) and size (equivalent diameter) of inclusions for each sampling period in the three experimental steels. Each sample counted 150 fields of view at 2000 magnification, for a total area of 1.5 mm2. The number density and size change of inclusions during the experiment are shown in Figure4. It can be seen from Figure4 that the variation law of the size and number density of inclusions in the three experimental steels is basically identical. After Ce treatment for 1 min, the proportion of inclusions with a size 0.5–2.0 µm (see Figure4a) and the number density of inclusions (see Figure4b) increased significantly. With the prolongation of treatment time, the number density of inclusions showed a decreasing trend, and the size of inclusions showed a slight increasing trend. However, inclusions with a size less than 5 µm were still dominant after 30 min. Metals 2019, 9, x FOR PEER REVIEW 6 of 13 Metals 2019, 9, 804 6 of 13

FigureFigure 4.4. SizeSize distributiondistribution ((aa)) andand numbernumber densitydensity ((bb)) ofof inclusionsinclusions inin threethree steelssteels atat didifferentﬀerent stages. stages.

4. ThermodynamicsIt can be seen from Analysis Figure 4 that the variation law of the size and number density of inclusions in theAfter three Ce experimental treatment, the steels oxygen is basically activity identica in the R1l. experimentalAfter Ce treatment steel decreasedfor 1 min, (seethe proportion Table2), and of theinclusions inclusions with in a the size steel 0.5–2.0 were μ mainlym (see Figure CeAlO 4a)3 at and 30 min the afternumber Ce treatmentdensity of (seeinclusions Table3 ).(see The Figure oxygen 4b) activityincreased in R2significantly. and R3 steels With before the prolongation Ce treatment of had treatment dropped time, to a verythe number low level density due to of the inclusions high Al content.showed Therefore,a decreasing the trend, oxygen and activity the size would of inclus not beions further showed decreased a slight after increasing Ce addition trend. (see However, Table2). Theinclusions inclusions with present a size inless the than steels 5 μ werem were mainly still dominant Ce2O2S at after 30 min 30 aftermin. Ce treatment, and only a small amount of CeAlO3 inclusions were found in the R2 experimental steel (see Table3). It indicates that Ce treatment4. Thermodynamics can modify Analysis spinel inclusions into rare earth inclusions. The deoxidation reaction between Ce andAfter O will Ce preferentiallytreatment, the proceed oxygen afteractivity the in addition the R1 ofexperimental Ce to steel, andsteel Ce decreased will react (see with Table S when 2), and the Othe content inclusions is at in avery the steel low were level. mainly The experimental CeAlO3 at 30 phenomenon min after Ce is treatment consistent (see with Table previous 3). The research oxygen resultsactivity [11 in, 12R2]. and R3 steels before Ce treatment had dropped to a very low level due to the high Al content.To assess Therefore, the evolution the oxygen mechanism activity would of inclusions not be further in the decreased steel, an inclusionafter Ce addition stability (see diagram Table TM of2). Fe-10%Mn-0.001%Mg-0.015%S-0.015%Ce-Al-O The inclusions present in the steels were mainly system Ce2O2S at at 1873 30 min K was after calculated Ce treatment, by FactSage and only a programsmall amount (Version of CeAlO 7.1, developed3 inclusions by were Thermfact found in/CRCT the R2 (Montreal, experimental Canada) steel (see and Table GTT-Technologies 3). It indicates (Achen,that Ce treatment Germany)), can as modify shown spinel in Figure inclusions5. The compositionsinto rare earth of inclusions. the three The experimental deoxidation steels reaction are alsobetween marked Ce and in it. O In will the preferentially calculation, in proceed addition after to FactPS,the addition FToxid, of Ce and to FSstel steel, databases,and Ce will a react privately with createdS when databasethe O content MYDT is at containing a very low thermodynamiclevel. The experimental data of phenomenon Ce2O2S[13] wasis consistent also selected. with previous As can be seen from the ﬁgure, the stable inclusion region shows a change of CeAlO CeAlO + Ce O S research results [11,12]. 3 → 3 2 2 Ce O S as the oxygen content decreases within a fairly large range of Al content in the steel. The → To2 2assess the evolution mechanism of inclusions in the steel, an inclusion stability diagram of Fe-10%Mn-0.001%Mg-0.015%S-0.015%Ce-Al-O system at 1873 K was calculated by FactSageTM

Metals 2019, 9, x FOR PEER REVIEW 7 of 13

program (Version 7.1, developed by Thermfact/CRCT (Montreal, Canada) and GTT-Technologies (Achen, Germany)), as shown in Figure 5. The compositions of the three experimental steels are also marked in it. In the calculation, in addition to FactPS, FToxid, and FSstel databases, a privately created database MYDT containing thermodynamic data of Ce2O2S [13] was also selected. As can be Metals 2019, 9, 804 7 of 13 seen from the figure, the stable inclusion region shows a change of CeAlO3 → CeAlO3 + Ce2O2S → Ce2O2S as the oxygen content decreases within a fairly large range of Al content in the steel. The composition of R1 experimental steel was located in the CeAlO3 stable region, and the stable composition of R1 experimental steel was located in the CeAlO3 stable region, and the stable inclusions inclusions in the R2 experimental steel were CeAlO3 + Ce2O2S, which was consistent with the in the R2 experimental steel were CeAlO3 + Ce2O2S, which was consistent with the experimental resultsexperimental (see Table 3results). The (see thermodynamic Table 3). The calculation thermodynamic results calculation of R3 experimental results of steelR3 experimental showed that steel thereshowed were nothat inclusions, there were which no inclusions, was inconsistent which withwas theinconsistent experimental with results.the experimental The main reasonresults. is The thatmain thermodynamic reason is that calculations thermodynamic by Factsage calculations have large by Factsage deviations have from large the deviations actual situation from the when actual thesituation Al content when is higher. the Al This content phenomenon is higher. isThis also phenom provedenon by others is also [14 proved], and aby large others amount [14], and of data a large indicateamount that of the data actual indicate equilibrium that the oxygen actual content equilibrium in steel oxygen is lower content than the in calculated steel is valuelower underthan the highcalculated aluminum value conditions under high in steel. aluminum Therefore, conditions the activity in steel. relationship Therefore, the will activi be usedty relationship to express thewill be criticalused condition to express of the inclusion critical transformation condition of inclusion as follows. transformation as follows.

Figure 5. Stability diagram in the Fe-10%Mn-0.001%Mg-0.015%S-0.015%Ce-Al-O system at 1873 K. Figure 5. Stability diagram in the Fe-10%Mn-0.001%Mg-0.015%S-0.015%Ce-Al-O system at 1873 K. According to the experimental results, the chemical reactions involved in the Fe-Mn-Al-Mg-Ce-O-S system andAccording related standardto the experimental Gibbs free results, energy [the15– chem20] areical shown reactions in Table involved4. in the Fe-Mn-Al-Mg-Ce- O-S system and related standard Gibbs free energy [15–20] are shown in Table 4. Table 4. Possible chemical reactions in steel and its standard Gibbs free energy. Table 4. Possible chemical reactions in steel and its standard Gibbs free energy. θ No. Reaction Equations in Molten Steel ∆G (J/mol) θ No. Reaction equations in molten steel ΔG (J/mol) 1 2[Al] + 3[O] = Al2O3(s) 867,370 + 222.5T [20] − 1 2 2[Al] [Mg] + 3[O]+ [O] == AlMgO(s)2O3(s) 89,960 −82.0T867370 [19 ]+ 222.5T [20] − − 2 3 [Mg] [Mn] ++ [O][O] == MgO(s)MnO(s) 287,900 + 125T−89960 [17 −] 82.0T [19] − 4 [Mn] + 2[Al] + 4[O] = MnO Al O (s) 1,545,000 + 524T [17] 3 [Mn] + [O] = MnO(s)· 2 3 − −287900 + 125T [17] 5 [Mg] + 2[Al] + 4[O] = MgO Al2O3(s) 978,182 + 128.93T [18] 4 [Mn] + 2[Al] + 4[O] = MnO·Al· 2O3(s) − −1545000 + 524T [17] 6 2[Ce] + 2[O] + [S] = Ce O S(s) 1,353,592.4 + 331.6T [16] 2 2 − 5 7 [Mg] [Ce] + 2[Al]+ [Al] + 4[O]+ 3[O] = =MgO·AlCeAlO 2O(s)3(s) 1,366,460−+978182364T [15 + ]128.93T [18] 3 − 6 2[Ce] + 2[O] + [S] = Ce2O2S(s) −1353592.4 + 331.6T [16] 7 [Ce] + [Al] + 3[O] = CeAlO3(s) −1366460 + 364T [15] The deoxidation equilibrium relationship of the molten steel includes Al-O, Al-Mg-O, Al-Ce-O, and Ce-S-O,The anddeoxidation the formation equilibrium conditions relationship of the corresponding of the molten inclusionssteel includes at 1873 Al-O, K areAl-Mg-O, as follows. Al-Ce-O, andThe Ce-S-O, formation and conditionthe formation of Al conditions2O3 inclusions of the is co thatrresponding oxygen activity inclusions in steel at 1873 is greater K are thanas follows. that limited by the Al–O equilibrium, as shown in Equation (1) The formation condition of Al2O3 inclusions is that oxygen activity in steel is greater than that

limited by the Al–O equilibrium, as2 /shown3 1/ 3in Equation (1) 5 2/3 aO > a− K− = 6.468 10− a− , (1) Al · Al2O3 × · Al where K represents the equilibrium constant of the corresponding reaction, and a represents the activity of the corresponding component (the same as below). Metals 2019, 9, 804 8 of 13

The formation condition of MgO Al O inclusions is that the actual oxygen activity in steel is · 2 3 greater than that limited by the Mg-Al-O equilibrium, as shown in Equation (2)

1/2 1/4 1/4 = 6 1/2 1/4 aO > aAl− aMg− KMgO− Al O 7.303 10− aAl− aMg− . (2) · · · 2 3 × · ·

The formation conditions of CeAlO3 inclusions is that the actual oxygen activity in steel is greater than that limited by Ce-Al-O equilibrium, as shown in Equation (3)

1/3 1/3 1/3 7 1/3 1/3 aO > a− a− K− = 4.31 10− a− a− . (3) Al · Ce · CeAlO3 × · Al · Ce

The formation conditions of Ce2O2S inclusions is that the actual oxygen activity in steel is greater than that limited by Ce-O-S equilibrium, as shown in Equation (4)

1 1/2 1/2 11 1 1/2 aO > a− a− K− = 6.1 10− a− a− . (4) Ce· S · Ce2O2S × · Ce· S The transformation reaction between the inclusions can be obtained by associating the reactions in Table2, as follows. Conditions for Al O transformation into MgO Al O : 2 3 · 2 3 4Al O (s) + 3[Mg] = 2[Al] + 3MgO Al O (s) ∆Gθ = 534, 934 503.21T. (5) 2 3 · 2 3 − Let ∆G < 0, at 1873 K: 4 2/3 a > 1.625 10− a . (6) Mg × · Al Conditions for MgO Al O transformation into CeAlO : · 2 3 3 4[Ce] + 3MgO Al O (s) = 4CeAlO (s) + 3[Mg] + 2[Al] ∆Gθ = 2, 531, 294 + 1069.21T. (7) · 2 3 3 − Let ∆G < 0, at 1873 K: 4 3/4 1/2 a > 2.061 10− a a . (8) Ce × · Mg · Al Conditions for CeAlO3 transformation into Ce2O2S:

[Ce] + [S] + CeAlO (s) = Ce O S(s) + [Al] + [O] ∆Gθ = 12, 867.6 32.4T. (9) 3 2 2 − Let ∆G < 0, at 1873 K: 1 a > 0.0464 a a a− . (10) Ce · O· Al· S Conditions for MgO Al O transformation into Ce O S: · 2 3 2 2 8[Ce] + 4[S] + 3MgO Al O (s) = 4Ce O S(s) + 3[Mg] + 6[Al] + 4[O] · 2 3 2 2 (11) ∆Gθ = 2, 479, 823.6 + 939.61T. − Let ∆G < 0, at 1873 K: 3 3/8 3/4 1/2 1/2 a > 3.09 10− a a a a− . (12) Ce × · Mg · Al · O · S According to the above analysis, the inclusion formation conditions can be obtained as follows. (1) The formation condition of MgO Al O in steel at 1873 K after magnesium treatment needs · 2 3 to be considered from two diﬀerent formation mechanisms, as above mentioned. One is that Al2O3 can transform into MgO Al O (as shown in Equation (6)), which will provide a limit on magnesium · 2 3 activity. The other one is that MgO Al O can directly form from steel (as shown in Equation (2)), which · 2 3 requires the oxygen activity to be higher than the equilibrium oxygen activity limited by Mg-Al-O, as follows. 4 2/3 6 1/2 1/4 a > 1.625 10− a , a > 7.303 10− a− a− . (13) Mg × · Al O × · Al · Mg Metals 2019, 9, 804 9 of 13

(2) There are also two considerations for the formation condition of single CeAlO3 in steel at 1873 K. One is that MgO Al O can transform into stable CeAlO (as shown in Equation (8)), which means · 2 3 3 CeAlO3 cannot spontaneously transform into Ce2O2S (as shown in Equation (10)); this will provide both the upper and lower limits of Ce activity. The other consideration is that CeAlO3 can directly form from steel (as shown in Equation (3)), but Ce2O2S should not spontaneously precipitate (as shown in Equation (4)). This will also limit the upper and lower limits of oxygen activity, which means the oxygen activity should be higher than the equilibrium oxygen activity limited by Ce-Al-O but lower than the equilibrium oxygen activity limited by Ce-O-S, as follows.

1 4 3/4 1/2 0.0464 aO aAl a− > aCe > 2.061 10− a− a− , · · · S × · Mg · Al (14) 11 1 1/2 7 1/3 1/3 6.1 10 a a− > a > 4.31 10 a− a− . × − · Ce− · S O × − · Al · Ce

(3) There are also two considerations for the formation condition of CeAlO3 and Ce2O2S in steel at 1873 K. One is that MgO Al O can transform into stable CeAlO (as shown in Equation (8)), and · 2 3 3 CeAlO3 can transform into Ce2O2S (as shown in Equation (10)); this will provide both the lower limits of Ce activity. The other consideration is that CeAlO3 and Ce2O2S can directly form from steel (as shown in Equations (3) and (4)). This will also limit the lower limits of oxygen activity, which means the oxygen activity should be higher than the equilibrium oxygen activity limited by Ce-Al-O and Ce-O-S, as follows. 4 3/4 1/2 3 3/8 3/4 1/2 1/2 aCe > max 2.061 10− a a , 3.09 10− a a a a− , × · Mg · Al × · Mg · Al · O · S (15) n 7 1/3 1/3 11 1 1/2o a > max 4.31 10 a− a− , 6.1 10 a a− . O × − · Al · Ce × − · Ce− · S

(4) There are also two considerations for the formation condition of single Ce2O2S in steel at 1873 K. One is that MgO Al O can transform into stable Ce O S (as shown in Equation (10)), which · 2 3 2 2 means CeAlO3 cannot spontaneously transform into CeAlO3 (as shown in Equation (8)); this will provide both the upper and lower limits of Ce activity. The other consideration is that Ce2O2S can directly form from steel (as shown in Equation (4)), but CeAlO3 should not spontaneously precipitate (as shown in Equation (3)). This will also limit the upper and lower limits of oxygen activity, which means the oxygen activity should be higher than the equilibrium oxygen activity limited by Ce-O-S but lower than the equilibrium oxygen activity limited by Ce-Al-O, as follows.

3 3/8 3/4 1/2 1/2 7 1/3 1/3 11 1 1/2 a > 3.09 10− a a a a− , 4.31 10− a− a− > a > 6.1 10− a− a− . (16) Ce × · Mg · Al · O · S × · Al · Ce O × · Ce· S In summary, the modification mechanism of Ce to spinel inclusions can be shown in Figure6. Ce can modify the spinel inclusions into CeAlO3 or Ce2O2S. There are three main modification paths: O1 Spinel can directly transform into CeAlO by Ce, namely spinel Ce-Al-O + Mg-Al-O CeAlO , 3 → → 3 which can be proved by the complex inclusion shown in Figure3a. O2 After Ce treatment, spinel can be preferentially transformed to CeAlO3. After a while, the resulting CeAlO3 product layer will be transformed to Ce2O2S. The first-step reaction was considered to be difficult to perform completely because we observed some inclusions with a three-layer structure in the experiment, as shown in Figure3c. If treatment time is enough, it will eventually transform into uniform Ce 2O2S. The above path can be summarized as spinel Ce-Al-O + Mg-Al-O Ce-O-S + Ce-Al-O + Mg-Al-O Ce O S. → → → 2 2 O3 Spinel can be directly transformed into Ce O S by Ce, the modification path is spinel Ce-O-S + 2 2 → Mg-Al-O Ce O S. The related modification mechanism is as follows: After Ce addition, Mn and Mg → 2 2 in the spinel inclusions are first replaced by Ce, and the spinel structure will be destroyed in the above process to form CeAlO3. If the O content in the steel is low, which means that the S content is relatively high, S in the steel will participate in the above displacement reaction to replace the O in the inclusion. In general, CeAlO3 and spinel inclusions will transform into Ce2O2S through paths 2 and 3. Under the experimental steel conditions, the modification sequence of Ce to spinel inclusions is spinel CeAlO → 3 Ce O S. → 2 2 Metals 2019, 9, 804 10 of 13 MetalsMetals 2019 2019, 9, x, 9FOR, x FOR PEER PEER REVIEW REVIEW 10 of10 13 of 13

FigureFigure 6. ModificationModiﬁcation 6. Modification mechanism mechanism of spinelof spinel inclusions. inclusions.

HuangHuang et al.et [[7,9]al.7, 9[7,9]] and and Wang Wang et al.et [[8]al.8] reported[8]reported reported thatthat that thethe the sequencesequence sequence ofof Ce-modiﬁedCe-modifiedof Ce-modified spinel spinel is spinel is spinel

→ CeAlO→CeAlO33 →3 Ce→Ce22OO332 →3 Ce→Ce22OO222S or2 spinel → Ce→Ce2O2O332 →3 Ce→Ce22OO22S.2S. However,2However, nono CeCe22OO332 inclusions3 were → CeAlO→ Ce →O Ce O S or spinel→ Ce →O Ce O S. However, no Ce O inclusions were foundfound in thein the presentpresent present experimentalexperimental experimental steel.steel. steel. After After comparingcomp comparingaring the the experimental experimental conditions, conditions, it was it was found found thatthat the the contents contents contents of of Alof Al Aland and and S S in S in thein the the steel steel steel were were were di different. diﬀfferent.erent. Therefore, Therefore,Therefore, the the stability stability diagram diagram diagram in thein in the theFe- Fe- Fe-10%Mn-0.001%Mg-0.015%Ce-0.005%O-Al-S10%Mn-0.001%Mg-0.015%Ce-0.005%O-Al-S10%Mn-0.001%Mg-0.015%Ce-0.005%O-Al-S system system system at at1873 18731873 K KwasK was was calculated calculated calculated using using using FactSage FactSage FactSage 7.1 7.1 (databases(databases FactPS, FactPS, FToxid, FToxid, FSstel,FSstel, FSstel, MYDT), MYDT), as shownas shown in Figurein Figure7 7.. AsAs 7. canAscan can bebe seenseenbe seen fromfrom from thethe the redred red lineline line inin in FigureFigure 77,, as as7, the theas the SS contentcontent S content inin thethein the steelsteel steel increased,incr increased,eased, the the precipitation precipitation condition condition for for forthe the the Ce Ce 2CeO2O32 Orequired3 3required required a a alower lowerlower Al Al Alcontent. content. content. The The The S Scontent S content content in in ourin our our experimental experimental experimental steels steels steels was was was as as highas high as 0.015%.as 0.015%. Therefore, Therefore, the the formationformation condition condition of Ceof 2CeO32 Owaswas3 was not not not reached reached reached when when when the the the Al Al Alcontent content content was was was higher higher than than 0.02%. 0.02%.

Figure 7. FigureFigure 7. Stability 7. Stability diagram diagram in the in theFe-10%Mn-0.001%Mg Fe-10%Mn-0.001%Mg-0.015%Ce-0.005%O-Al-S Fe-10%Mn-0.001%Mg-0.015%Ce-0.005%O-Al-S-0.015%Ce-0.005%O-Al-S system system at 1873 at 1873 K. K. 5. Ce Content Required for Complete Modification of Spinel Inclusions 5. Ce5. CeContent Content Required Required for for Complete Complete Modification Modification of Spinelof Spinel Inclusions Inclusions There are complex interactions between different elements, such as Ce, Al, O, and S, in the steel There are complex interactions between different elements, such as Ce, Al, O, and S, in the steel during theThere modification are complex process. interactions Under between present different experimental elements, conditions, such as Ce, spinel Al, O, is and preferentially S, in the steel duringduring the the modification modification process. process. Under Under present present ex perimentalexperimental conditions, conditions, spinel spinel is preferentiallyis preferentially transformed to CeAlO3. In order to obtain the minimum Ce content required for complete modification transformed to CeAlO3. In3 order to obtain the minimum Ce content required for complete of spineltransformed inclusions, to CeAlO the effect. In of aluminumorder to andobtain oxygen the contentminimum on theCe relatedcontent Ce required content infor steel complete was modification of spinel inclusions, the effect of aluminum and oxygen content on the related Ce calculatedmodification by FactSage. of spinel The inclusions, content of eachthe effect element of inaluminum the steel discussed and oxygen in this content section on is totalthe related content, Ce content in steel was calculated by FactSage. The content of each element in the steel discussed in this andcontent the calculation in steel was results calculated are shown by FactSage. in Figure 8The. content of each element in the steel discussed in this sectionsection is total is total content, content, and and the the calculation calculation results results are are shown shown in Figurein Figure 8. 8.

Metals 2019, 9, 804 11 of 13

Figure 8. Eﬀect of oxygen and aluminum content on inclusions in steel, (a) wS = 0.015% and wAl = 0.02%, (b) wO = 0.0045% and wS = 0.015%.

Figure8a shows the e ffect of oxygen content on inclusions in steel, where wAl = 0.02% and wS = 0.015%. It can be seen from the red line in the figure that the minimum Ce content required for modification of spinel inclusions is linear with the oxygen content in the steel. When w = 2.99 w , the Ce · O inclusions in the steel can be controlled as CeAlO3. Wang et al. [8] also obtained a similar conclusion: that increasing the O content would increase the Ce content required for complete modification of spinel inclusions. Theoretically, the inclusions can be controlled to be CeAlO3 when the Ce content in the steel is three times that of the oxygen content. Therefore, by measuring the oxygen content in the steel, a suitable rare earth Ce addition amount required for complete modification of spinel inclusions can be obtained. Figure8b shows the e ffect of aluminum content on types of stable inclusions in steel, where wO = 0.0045% and wS = 0.015%. It can be seen from the red line in the figure that as the aluminum content in the steel increases, the minimum Ce content required for modification spinel inclusions does not change much, which indicates that the aluminum content has little effect on the minimum Ce content required for modification of spinel inclusions.

6. Conclusions In present paper, the transformation process of inclusions in medium manganese steel was studied by high-temperature simulation experiments. The modiﬁcation mechanism of inclusions in steel under diﬀerent aluminum contents was discussed by thermodynamic calculations. Under the present experimental conditions, the following conclusions were obtained. 1

Metals 2019, 9, 804 12 of 13

(1) Ce treatment can modify spinel inclusions into rare earth inclusions. After Ce treatment, the amount of inclusions in the steel will decrease, and the size of inclusions is basically less than 5 µm. (2) The modification mechanism of spinel inclusions by Ce is as follows: After Ce is added to steel, Mn and Mg in the spinel inclusions are first replaced by Ce to form CeAlO3, which will cause the destruction of the spinel structure. When the O content in the steel is at a lower level, S in the steel will take part in the replacement reaction to replace the O in the inclusion, then the CeAlO3 and spinel inclusions will be finally transformed into Ce2O2S. (3) In the Fe-Mn-Al-Mg-Ce-O-S system at 1873 K, the conditions for forming 1 4 3/4 1/2 11 1 1/2 single CeAlO are 0.0464 a a a > a > 2.061 10 a a , 6.1 10 a a− > 3 · O· Al· S− Ce × − · Mg · Al × − · Ce− · S 7 1/3 1/3 aO > 4.31 10− a− a− ; the conditions for simultaneous formation of CeAlO3 × · Al · Ce 4 3/4 1/2 3 3/8 3/4 1/2 1/2 and Ce O S are a > max 2.061 10 a a , 3.09 10 a a a a− , a > 2 2 Ce × − · Mg · Al × − · Mg · Al · O · S O n 7 1/3 1/3 11 1 1/2o max 4.31 10 a− a− , 6.1 10 a a− ; and the conditions for forming single Ce O S are × − · Al · Ce × − · Ce− · S 2 2 3 3/8 3/4 1/2 1/2 7 1/3 1/3 11 1 1/2 a > 3.09 10 a a a a− , 4.31 10 a− a− > a > 6.1 10 a a− . Ce × − · Mg · Al · O · S × − · Al · Ce O × − · Ce− · S (4) The minimum Ce content required for complete modification of spinel inclusions is mainly related to the O content. When the total Ce content is three times that of the total O content in the steel, the spinel inclusions can be transformed into CeAlO3.

Author Contributions: Z.X. and C.L. conceived and designed the experiments, analyzed the data, and conducted writing–original draft preparation, editing, and visualization. Funding: This work was financially supported by the National Key R&D Program of China (No. 2017YFC0805100), the National Natural Science Foundation of China (No. 51874082), and the Fundamental Research Funds for the Central Universities (Grant No. N180725008). Acknowledgments: Thanks to Jiyu Qiu for providing language help. Conflicts of Interest: The authors declare no conflict of interest.

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