Origin of the Inclusions in Production-Scale Electrodes, ESR Ingots, and PESR Ingots in a Martensitic Stainless Steel

metals

Article Origin of the Inclusions in Production-Scale Electrodes, ESR Ingots, and PESR Ingots in a Martensitic Stainless Steel

Ewa Sjöqvist Persson 1,*, Andrey Karasev 2, Alec Mitchell 3 and Pär G. Jönsson 2

1 Uddeholms AB, Uddeholm, SE-68385 Hagfors, Sweden 2 Materials Science and Engineering, KTH, Royal Institute of Technology, SE-10044 Stockholm, Sweden; [email protected] (A.K.); [email protected] (P.G.J.) 3 Department of Materials Engineering, University of British Columbia, Vancouver, BC V6T 1Z4, Canada; [email protected] * Correspondence: [email protected]; Tel.: +46-070-697-7186

Received: 26 October 2020; Accepted: 27 November 2020; Published: 2 December 2020

Abstract: The focus of the study was to define the origin of the inclusions in production-scale electro-slag remelting, (ESR) and electro-slag remelting under a protected pressure controlled atmosphere, (PESR), ingots. The inclusion characteristics in production samples were studied using both polished sample surfaces (two-dimensional (2-D) investigations) and inclusions extracted from steel samples by electrolytic extraction (three-dimensional (3-D) investigations) using SEM in combination with EDS. The results were compared to results from previously reported laboratory-, pilot-, and production-scale trials including electrode, remelted, and conventional ingots. The results show that primary, semi-secondary, and secondary inclusions exist in the remelted ingots. The most probable inclusion to survive from the electrode is a MgO-Al2O3 (spinel). It was also found that the ESR/PESR process slag acts in a similar way to a calcium treatment modification of alumina inclusions. On the whole, the most significant finding is that the overall cleanliness of the electrode including the inclusions in the electrode has an influence on the inclusion content of the ESR and PESR ingots.

Keywords: ESR; PESR; inclusions; electrolytic extraction; SEM; stainless steel

1. Introduction Due to increased requirements on the final properties of high-quality steels, the limitations on the content of non-metallic inclusions (NMIs) and impurity elements in these steels are continually being tightened. The remelting techniques of electro-slag remelting (ESR) and electro-slag remelting under a protected pressure controlled atmosphere (PESR) produce clean steels with respect to non-metallic inclusion (NMI) content. Previous studies on non-metallic inclusions and cleanliness in ESR or PESR remelted ingots are presented in Table1[ 1–25]. It can be seen that the majority of them are executed in laboratory- or pilot-scale furnaces (from 0.8 to 50 kg experimental trials) and with a focus on steel grades other than martensitic stainless tool steels, which is the focus of this investigation. The inclusions found in laboratory-scale trials are usually in the size range of 1–5 µm, but more often 2 µm. However, ≤ in industrial size ingots, significantly larger inclusions can be found. Therefore, for reliable evaluations of the characteristics of NMI (especially larger size inclusions) in industrial-scale ingots, investigations of NMIs carried out only in laboratory and pilot experiments are not sufficient.

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Table 1. Previous studies on inclusions in electro-slag remelting (ESR) or electro-slag remelting under protective pressure controlled atmosphere (PESR) remelted.

Inclusions Year Author Steel Scale Diameter/Width Ref. Size 1971 D.A.R. Kay et al. N.A. N.A. N.A. N.A. [1] 1980 Z.B. Li et al. N.A. N.A. N.A. N.A. [2] 2012 C-B. SHI et al. NAK80 die steel N.A. N.A. N.A. [3] 2012 C-B. SHI et al. Die steel Laboratory N.A. N.A. [4] 2012 C-B. SHI et al. High-al steel N.A. N.A. <5µm [5] 2012 X.C. Chen et al. Inconel 718 N.A. N.A. CN 5–15 µm [6] 1969 B.C. Burel steel, iron Laboratory Mould 77 mm <15 µm [7] oxygen containing 1974 A. Mitchell et al. iron, iron-OFHC Laboratory Mould 76.2 mm 1–5 µm [8] copper 1976 J.C.F. Chan et al. Stainless steel Laboratory Electrode 35 mm N.A. [9] Die steel, 2013 C-B. SHI et al. Laboratory N.A. N.A. [10] superallloys ... . Cold rolls steel Laboratory 2013 Y. Dong et al. N.A. N.A. [11] MC5 800 gr Laboratory 2014 Y-W. Dong et al. Die steel CR-5A N.A. N.A. [12] 800 gr Laboratory, 2013 C-B. SHI et al. H13 die steel Electrcode 90 mm abt 2 µm [13] 50 kg Hot work steel abt Electrode 2015 R. Scheinder et al. Pilot N.A. [14] H11 101.5 mm High-Carbon 17% PESR mould 2016 C-B. SHI et al. Pilot abt 5 µm [15] Cr Tool Steel 170 mm ESR mould 0–15 µm, 2017 G. Du et al. H13 die steel Pilot [16] 300 mm >15 µm Mould abt 2014 L.Z. Cang et al. N.A. Pilot 15 kg [17] 105 mm Si-Mn killed steel 1–3 µm, few 2019 C-B SHI et al. ≈ Pilot PESR 95 mm [18] H13 > 3µm abt H11, H13 die ASTM E45 2013 G. Reiter et al. steel, martensitic Industrial N.A. [19] Heavy Cr-Ni steel Martensitic Moulds 2014 E.S. Persson et al. Industrial 8–30 µm [20] stainless steel 400–1050 mm Martensitic Moulds 2016 E.S. Persson et al. Industrial N.A. [21] stainless steel 400–1050 mm 2017 H. Wang et al. H13 die steel Laboratory Electrode 25 mm abt 1–2 µm [22] Martensitic Moulds 2017 E.S. Persson et al. Industrial 8–45 µm [23] stainless steel 400–500 mm Martensitic ESR mould 2017 E.S. Persson et al. Industrial 8–20 µm [24] stainless steel 300 mm Martensitic ESR mould 2018 E.S. Persson et al. Industrial 8–20 µm [25] stainless steel 300 mm

The earlier and most generally accepted theories assume that the inclusions in the electrode will be rejected to the surface of the electrode tip to be incorporated into the liquid process slag [8,9,18,26–28]. The steel melt will then seek equilibrium with the process slag, so that new, different oxygen levels in the liquid steel pool will be obtained. With respect to the inclusion solution during ESR, in laboratory trials, Mitchell et al. [29] showed that the rate of solution of Al2O3 inclusions of 100 µm in a common ESR and PESR process slag (CaF2 + 20–30 wt % Al2O3) should be dissolved within the predicted exposure time to the liquid slag at the electrode/slag interface. Li et al. [30] investigated the dissolution rate of Al2O3 into molten CaO-Al2O3-CaF2 flux. In this study, it was seen that when the Al2O3 rod was immersed into molten flux, an intermediate compound of CaO-2Al2O3 was formed initially, before being dissolved in the flux. The dissolution rate of Al2O3 increased with an increased CaO/Al2O3 relation in the flux, a higher rotation speed of the rod in the flux, and a higher temperature. Metals 2020, 10, 1620 3 of 16

In contrast to the accepted postulate of total inclusion removal by slag dissolution, an investigation using La2O3 as a tracer in the process slag showed that only 50% of the inclusions contained La [24,25]. This finding illustrated that only 50% of the inclusions had been in contact with the slag, either by total or partial solution. The remaining inclusions, which did not contain La, either survived from the electrode, precipitated without containing La, or without a detectible amount of La. The above examination shows that the most likely inclusion path from an electrode to an ingot involves two distinct routes, i.e., a direct transfer without a change of the inclusion characteristics and a solution/re-precipitation reaction. It was seen that the inclusions containing Mg spinels (MgO-Al2O3) were the ones most likely to be found without La. The share of the total number of oxide inclusions, consisting of Al inclusions containing >3% Mg of the total number of oxide inclusions, did not depend on the specific melting rate (melting rate per area) [25]. However, neither did it depend on the exposure time at the electrode/slag interface. The spinel proportion increases with increased ingot and inclusion sizes. Thus, spinel inclusions found without La were assumed to have been trapped within steel drops falling from the liquid film on the electrode tip [25]. Burel [7] already proposed this entrapment mechanism, even though it could not be seen in his experimental work. Another reason for primary inclusions could be that they were contained in solid steel fragments falling directly into the pool from the porous central region of the cast electrode. This latter effect would not be seen in small-scale experiments using rolled or forged electrodes and may account for the absence of large or unaltered inclusions in those results. The two-dimensional (2-D) and three-dimensional (3-D) investigations of NMI on the electrode and remelted ingots [23], as well as the tracer element investigations, showed that many of the inclusions contained an inner core of spinel, covered by a layer corresponding to the chemical composition of the process slag. This result supports a more recent theory [24], that proposes that two ESR ingot inclusion types exist; first, secondary inclusions, which have gone through some form of solution/re-precipitation reaction where they reacted with the slag and finally acted as nucleates to grow in the ingot pool and solidifying region; second, primary inclusions from the electrode, which have not been influenced by the melting. Studies indicate that the inclusions in the second category that are most likely to survive are pure Mg spinels [23–25]. Some of the spinel inclusions were found to contain a layer corresponding to the ESR/PESR process slag composition (denoted below as semi-secondary inclusions). They are assumed to have survived due to their higher melting point and hence lower solution rate in either the slag or metal; therefore they may have acted as nucleus during cooling and solidification in the liquid steel pool [25]. With nuclei present, the semi-secondary inclusions would have the possibility to grow larger as the liquid metal pool cooled compared to if they were simply re-precipitated during the solidification. According to Wang et al. [22], the original oxide inclusions in a Mg-free H13 consumable electrode are Al2O3 and the original oxide inclusions in a Mg-containing H13 steel consumable electrode are Mg spinels. After an ESR remelting, the oxide inclusions in the Mg-free ESR ingot are still Al2O3, while both Al2O3 and MgO-Al2O3 inclusions exist in the Mg-containing ESR ingot. Related studies [31] on the removal of large inclusions from a cast steel electrode made with a deliberately-high content of inclusions larger than 500 µm have also indicated that not all large inclusions were removed by an ESR remelting, identifying some large inclusions that have survived into the ingot without change. A similar study reported by Paton et al. [32] indicates that approximately 20% of the large inclusions in the ESR ingot have the same composition as those found in the electrode. Mg spinel inclusions can be formed by reactions during the ESR process. Shi et al. [18] remelted vacuum-induction remelting (VIM)-produced electrodes (Si-Mn killed steel) containing only MnO-SiO2-Al2O3 inclusions in a pilot PESR using a process slag containing 3–4% MgO. The inclusions found in the remelted ingot (Ø95 mm), were only Mg spinel inclusions containing 3 mass % Mg, ≈ readily formed in the liquid pool as a result of the reactions between the alloying elements and the dissolved oxygen that dissociated from the MnO-SiO2-Al2O3 inclusions in the liquid steel. Metals 2020, 10, 1620 4 of 16

Laboratory results, obtained by Dong et al. [12] indicate that most non-metallic inclusions in an ESR remelted Cr-5A die steel, using a multi-component process slag, are MgO-Al2O3 inclusions. Based on a laboratory study of H13 steel remelting, Shi et al. [14] also found that MgO-containing inclusions survive from the electrode. Specifically, they found that all the inclusions in the consumable electrode were Mg spinels, occasionally surrounded by an outer (Ti, V)N or MnS layer. Their study showed that when only Al-based deoxidant additions or no deoxidants are used, all the oxide inclusions remaining in the PESR ingots are Mg spinels. After a PESR refining combined with a proper calcium treatment, the ingot inclusion population was modified to mainly consist of CaO-MgO-Al2O3 inclusions but also including some CaO-Al2O3 inclusions. The size range of the inclusions in the PESR ingots of this laboratory study was approximately <2 µm, suggesting that the inclusions were predominantly precipitates formed during solidification or cooling. Many research studies [33,34], have reported on the physical removal of inclusions during steelmaking processes, with a focus on the influence of buoyancy forces on the removal. The ESR ingot pool presents a similar case, but with significant differences in the liquid flow patterns and residence times. The terminal velocity of a Mg spinel inclusion 50 µm in diameter rising in a liquid steel is approximately 5 10 4 m/s given the computed ingot pool pattern [35]. This implies that × − any inclusion removal is determined by a balance between the probabilities of being exposed to the slag/ingot interface or being trapped in the solidifying metal as dictated by the bulk metal flow of the liquid ingot pool. Inclusions (depending on their size and density) will be trapped in the flow pattern, many never reaching the steel/slag interface. According to this model, the contribution of inclusion refining though flotation is much smaller than what has previously been estimated. The oxygen content in ESR and PESR remelted steels is discussed in several articles [5,9,16,18,36–41]. The overall finding is that the system moves towards equilibrium directed by the slag/metal equilibria. However, the reported oxide inclusion contents contributes for only a very small fraction (<1%) of the total analyzed oxygen content. Therefore, the reduction in the oxygen level often reported between the electrode and ingot cannot be allocated to a removal of inclusions to the slag. The oxygen level in the investigated high-chromium steel is normally between 5–10 ppm. While oxygen reductions during ESR are indicative of a refining process, they are not in themselves indicators of whether inclusion removal or formation of new inclusions have taken place. Higher amounts of oxygen in the electrode (about 30–100 ppm) lead to a decreased oxygen content in a remelted steel [5,36,37,40]. In contrast, a lower amount of oxygen (about <10–15 ppm) leads to an oxygen pick-up in the remelted ingot [18,19,37–40]. The current study, as well as some conference presentations by the same author, focused on a different steel grade than ones presented in previously reported studies in the literature, see Table1, namely a martensitic stainless steel. A production-scale electrode, an ESR ingot, and a PESR ingot, all from the same electrode heat, were investigated in order to verify the results from earlier theoretical, laboratory, and pilot studies presented above. This study focused on investigations of larger inclusions ( 8 µm) and their behavior during the ESR and PESR remelting processes, since this size range is more ≥ meaningful with respect to the influence of inclusions on various mechanical properties compared to the smaller inclusion sizes.

2. Materials and Methods The material studied was from one ingot-cast consumable electrode 300 mm 300 mm (denoted × as the CE-300 sample), one ESR remelted ingot 400 mm 400 mm (denoted as the ESR-400 sample), × and one PESR remelted ingot Ø500 mm (denoted as the PESR-500 sample). All electrodes were cast from the same initial steel charge. A typical composition of martensitic stainless steel used in this study is as follows: C 0.38%, Si 0.9%, Mn 0.45%, Cr 13.6%, and V 0.28%. The diﬀerence between the ESR and PESR processes is that the ESR process here represents a multiple-electrode remelting process (involving electrode changes), which is performed in a moving mold, in an open furnace under an air atmosphere and using a continuous aluminum deoxidation. In contrast, the PESR process is a Metals 2020, 10, x FOR PEER REVIEW 5 of 17 and one PESR remelted ingot Ø 500 mm (denoted as the PESR-500 sample). All electrodes were cast from the same initial steel charge. A typical composition of martensitic stainless steel used in this study is as follows: C 0.38%, Si 0.9%, Mn 0.45%, Cr 13.6%, and V 0.28%. The difference between the ESR and PESR processes is that the ESR process here represents a multiple-electrode remelting process (involving electrode changes), which is performed in a moving mold, in an open furnace under an air atmosphere and using a continuous aluminum deoxidation. In contrast, the PESR process is aMetals single2020, -10electrode, 1620 remelting process using a static mold and an inert pressure5 of 16 controlled atmosphere. In the ESR and PESR trials, a common process slag was used containing about one third each of CaO,single-electrode CaF2, and Al remelting2O3, and process including using a static ≈3% mold MgO and and an inert ≈1.5% pressure SiO2 controlled. atmosphere. The steelIn the samples ESR and PESRwere trials, taken a common from horizontal process slag was slice/slices used containing of the about electrode one third and each ofingots CaO, as follows: CaF , and Al O , and including 3% MgO and 1.5% SiO . 2 2 3 ≈ ≈ 2 top (T), middleThe (M) steel, and samples bottom were (B) taken for from the horizontal electrode, slice /twoslices samples of the electrode taken and at ingots positions as follows: in between the top and bottomtop (T), for middle the (M),ESR and ingot bottom and (B) one for themiddle electrode, sample two samples for the taken PESR at positionsingot, as in betweenshown in Figure 1. The samplesthe for top andthe bottomSEM investigations for the ESR ingot were and one taken middle from sample corner for the to PESR corner ingot, on as the shown electrode in (3 × 5 Figure1. The samples for the SEM investigations were taken from corner to corner on the electrode samples), the(3 central5 samples), sample the central as sampleclose to as closethe center to the center as possible as possible due due toto the the secondary secondary pipe/dense pipe/dense area × in the electrode),area in the from electrode), the ESR from ingot the ESR (2 ingot × 7 samples), (2 7 samples), and and from from side side toto side side from from the PESRthe PESR ingot ingot (1 × 9 × (1 9 samples). samples). ×

(a) (b) (c)

Figure 1. SchematicFigure 1. Schematic illustration illustration of horizontal of horizontal slices slices and and sample sample positions positions on the consumableon the consumable electrode electrode (a) CE-300, (b) ESR, and (c) PESR remelted ingots. (a) CE-300, (b) ESR, and (c) PESR remelted ingots. The characteristics of the non-metallic inclusions were studied using both a common The characteristicstwo-dimensional (2-D) of the investigation non-metallic of NMI inclusions on polished cross were sections studied of metal using samples both [20 ,21a ,24 common] two- and by using three-dimensional (3-D) investigations of NMIs on the surfaces of metal samples after dimensional (2-D) investigation of NMI on polished cross sections of metal samples [20,21,24] and by electrolytic extraction (EE) combined with SEM + EDS [23]. using three-dimensionalThe 2-D studies (3 were-D) performed investigations on a larger of area NMI of samples on surface. the Moresurfaces specifically, of metal the field samples of after electrolyticview extraction per sample (EE) was combined about 6500 mmwith2. TheSEM samples + EDS were [23] first. analyzed using a scanning electron The 2-microscopeD studies (FEI were Quanta performed 600 Mark II, on Thermo a larger Fisher area Scientific, of sample Waltham, surface. MA, USA). More The number, specifically, size, the field and chemical composition of the inclusions larger than 8 µm were analyzed using the “Inca features” 2 of view persoftware sample from was Oxford about Instruments. 6500 mm Afterwards,. The samples the inclusions were first were dividedanalyzed into using four size a classes,scanning electron microscopenamely (FEI Quanta 8–11.2 µm, 600 11.2–22.4 Markµm, II, 22.4–44.8 Thermoµm, Fisher and larger Scientific than 44.8, Waltham,µm. The observed MA, inclusionsUSA). The number, size, and chemical composition of the inclusions larger than 8 µm were analyzed using the “Inca features” software from Oxford Instruments. Afterwards, the inclusions were divided into four size classes, namely 8–11.2 µm, 11.2–22.4 µm, 22.4–44.8 µm, and larger than 44.8 µm. The observed inclusions were also classified, according to their composition, as follows: (1) Duplex oxysulfides (OSs)—oxide containing MnS and/or CaS sulfide, (2) Type AM—Al2O3-MgO oxides with 10–35%

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Metals 2020, 10, x FOR PEER REVIEW 6 of 17 were also classified, according to their composition, as follows: (1) Duplex oxysulfides (OSs)—oxide MgO and <10% CaO, (3) Type A—almost pure Al2O3 oxides with <10% MgO and <10% CaO, (4) Type containing MnS and/or CaS sulfide, (2) Type AM—Al2O3-MgO oxides with 10–35% MgO and <10% AC—Al2O3-CaO oxides with 50–90% Al2O3, 10–50% CaO, and <10% MgO, (5) Type ACM—Al2O3- CaO, (3) Type A—almost pure Al2O3 oxides with <10% MgO and <10% CaO, (4) Type AC—Al2O3-CaO CaO-MgO oxides containing 45–80% Al2O3, 10–40% CaO, and 10–25% MgO, and (6) MnS inclusions— oxides with 50–90% Al2O3, 10–50% CaO, and <10% MgO, (5) Type ACM—Al2O3-CaO-MgO oxides almost pure Mn sulfides. containing 45–80% Al2O3, 10–40% CaO, and 10–25% MgO, and (6) MnS inclusions—almost pure MnIn sulfides. the 3-D studies, 0.1–0.3 g of steel samples were dissolved in a 10% AA electrolyte (10%w/v acetylacetoneIn the 3-D—1 studies,%w/v tetramethylammonium 0.1–0.3 g of steel samples chloride were— dissolvedmethanol). in aThereafter, 10% AA electrolyte the solution (10%w was/v filtratedacetylacetone—1%w and the inclusions/v tetramethylammonium were collected on a film chloride—methanol). filter. After electrolytic Thereafter, extraction, the the solution NMIs were was investigatedfiltrated and on the a inclusions surface of were metal collected sample on. a However, film filter. d Afterue to electrolytic the high extraction, amount of the intermetallic NMIs were inclusionsinvestigated (IMI ons) a in surface the martensi of metaltic sample. stainless However, ingot, the due NMIs to the could high amountnot be investigated of intermetallic precisely inclusions on the(IMIs) film infilter the under martensitic a layer stainless of extracted ingot, IMI thes NMIsafter the could filtration not be of investigated electrolytes. precisely Therefore, on the the non film- metallicfilter under inclusions, a layer ofwhich extracted appeared IMIs aftercompletely the filtration on a s ofur electrolytes.face of metal Therefore, sample after the non-metallic electrolytic extraction,inclusions, were which also appeared investigated completely on the sample on a surface surfaces. of metal sample after electrolytic extraction, were also investigated on the sample surfaces. 3. Results 3. Results The total number of inclusions per unit area (NA) for each size class and ingot type obtained from theThe 2 total-D studies number is ofdisplayed inclusions in per Figure unit 2. area Additionally (NA) for each, the sizeNA values class and for ingot different type inclusion obtained compofrom thesitions 2-D are studies displayed is displayed in Figure in Figure3a–c. The2. Additionally,most common the inclusion N A values types for in di thefferent size inclusionrange 8- 44.80compositions µm in the areelectrode displayed (field in of Figure view 3ofa–c. 312, The300 mm most2) are common almost inclusion pure MnS types inclusions in the followed size range by 2 almost8-44.80 pureµm inAl the2O3 electrodeand Al2O3 (field-CaO ofoxides view ofcontaining 312,300 mm<10%) areMgO almost (Types pure A MnSand AC inclusions inclusions). followed For 2 theby ESR almost and pure PESR Al ingots2O3 and (using Al2O a3 -CaOfield oxidesof view containing of 36,380 and<10% 19 MgO,800 mm (Types, respectively), A and AC inclusions).Al2O3 and 2 AlFor2O3 the-CaO ESR oxides and PESR followed ingots (using by Al a2O field3-CaO of- viewMgO of oxides 36,380 and (Type 19,800 ACM), mm , a respectively),re the most Al common2O3 and inclusions.Al2O3-CaO In oxides addition, followed approximately by Al2O3-CaO-MgO 30% fewer oxides inclusions (Type are ACM), present are in the the most PESR common ingot compared inclusions. toIn the addition, ESR ingot. approximately The compositions 30% fewer of various inclusions types are of presentoxide inclusions in the PESR in different ingot compared samples to observed the ESR byingot. 2-D investigaThe compositionstions on polished of various sample types of surface oxides inclusions are shown in in di Figurefferent samples 4 in an Al observed2O3-MgO by-CaO 2-D diagrinvestigationsam. Since on the polished sulfide sampleinclusions surfaces are fully are shown dissolved in Figure during4 in electrode an Al 2O 3 melting-MgO-CaO and diagram. mostly accumulatedSince the sulfide by the inclusions liquid technological are fully dissolved slag, the during presen electrodet investigation melting focu andsed mostly primar accumulatedily on the oxide by components.the liquid technological slag, the present investigation focused primarily on the oxide components.

(a) (b)

FigureFigure 2. 2. NumberNumber of of (a ()a )oxide oxide and and (b (b) )sulfide sulﬁde inclusions inclusions per per unit unit area area (N (NAA)) for for the the C C-300,-300, ESR ESR-400-400 and and PESRPESR-500-500 samples. samples. The The amounts amounts of sulfursulfur ininthe the steel steel [%S] [%S] in in the the ESR-400 ESR-400 and and in thein the PESR-500 PESR-500 are 68%are 68%respectively respectively 76% 76% of the of amountthe amount of sulfur of sulf [%S]ur [%S] in the in CE-300.the CE-300.

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0.02 0.04 22.4-44.8 µm

) 11.2-22.4 µm ) 0.03

2 8-11.2 µm 0.01 0.02

(incl./mm

N 0.01

0 0 OS AM ACM A+AC MnS OS AM ACM A+AC MnS Type of NMI Type of NMI (a) (b)

0.04

) 0.03

0.02

(incl./mm

N 0.01

0 OS AM ACM A+AC MnS Type of NMI (c)

Figure 3. FigureNumber 3. Number of di ofﬀ erentdifferent non-metallic non-metallic inclusions inclusions (NMI (NMIs)s) per unit per area unit(NA) observed area (N byA) two observed- by two-dimensionaldimensional (2-D) (2-D investigations) investigations on on different diﬀerent polished polished steel samples: steel samples: (a) CE-300 (Ma )(b CE-300) ESR-400 M, and (b ) ESR-400, and (c) PESR-500.(c) PESR-500.

(a) (b)

(c) 1 Figure 4. Compositions of oxide inclusions in different samples observed by two-dimensional (2-D) Figure 4. Compositions of oxide inclusions in diﬀerent samples observed by two-dimensional (2-D) investigations on polished sample surfaces, (a) CE-300 M (b) ESR-400, and (c) PESR-500. investigations on polished sample surfaces, (a) CE-300 M (b) ESR-400, and (c) PESR-500.

Examples of the typical largest oxide inclusions observed in diﬀerent investigated samples are shown in Figures5 and6[ 23]. The results show that neither of the ESR-400 and PESR-500 samples contain large sulﬁdes. The Mg spinel core that is found in many of the inclusions, is about 1–10 µm. In addition, it was found that most of the inclusions in the ESR-400 and PESR-500 samples have spherical (or almost spherical) morphologies. It can be concluded that the total composition or the composition of the surface layer of those inclusions corresponds to the liquid CaO_Al2O3 phase, as can be seen in Figure7. Metals 20202020,, 1010,, xx FORFOR PEERPEER REVIEW 88 of 1717

Examples of thethe typicaltypical largestlargest oxide inclusionsinclusions observedobserved inin differentdifferent investigatedinvestigated samplessamples areare shownshown inin FiguresFigures 55 andand 66 [23][23].. TheThe resultsresults showshow thatthat neitherneither ofof thethe ESR--400 and PESR--500 samplessamples contain large sulfides. The Mg spinelspinel corecore thatthat isis foundfound inin manymany ofof thethe inclusiinclusionss,, isis aboutabout 11--10 µm.. IIn addition, itt waswas foundfound thatthat mostmost ofof thethe inclusionsinclusions inin thethe ESR--400 and PESR--500 samples have sphericalspherical (or(or almostalmost spherical)spherical) morphologies.morphologies. IItt cancan bebe concludedconcluded thatthat thethe totaltotal compositioncomposition oror thethe Metals 2020, 10, 1620 8 of 16 composition of thethe surfacesurface layerlayer ofof thosethose inclusionsinclusions correspondcorrespondss toto thethe liquidliquid CaO_CaO_Al22O33 phase, as can be seen in Figure 7..

((a)) ((b))

((c)) ((d))

Figure 5. TypicalFigure 5. large-sizedTypical large--size oxidesized oxide inclusions inclusions in in di differentﬀerent samples samples observed observed by twotwo by--dimensional two-dimensional ((2-- (2-D) D)) investigatinvestigationsions onon polishedpolished samplesample surface;surface; ((a)) Types A and AM: Al22O33,, AlAl22O33--MgO; ((b)) Type ACM: investigations on polished sample surface; (a) Types A and AM: Al2O3, Al2O3-MgO; (b) Type ACM: Al22O33--MgO + CaO--Al22O33 + (CaS);(CaS); ((cc)) Type AC: Al22O33--CaO + (CaS); ((d)) Type AC: Al22O33--CaO + CaF22.. Al2O3-MgO + CaO-Al2O3 + (CaS); (c) Type AC: Al2O3-CaO + (CaS); (d) Type AC: Al2O3-CaO + CaF2.

Metals 2020, 10, x FOR PEER REVIEW 9 of 17 ((a)) ((b))

Figure 6. TypicalFigure 6.oxide Typical inclusions oxide inclusions in different in differentsamples samples observed observed by three-dimensional by three-dimensional (3-D) (3 investigations-D) investigations on surfaces of steel samples after electrolytic extraction; (a) Type A: Al2O3; (b) Type on surfaces of steel samples after electrolytic extraction; (a) Type A: Al2O3;(b) Type AM: Al2O3-MgO; AM: Al2O3-MgO; (c) Type ACM: Al2O3-MgO + CaO-Al2O3 + CaF2; (d) Type AC: CaO-Al2O3 + (CaF2 + (c) Type ACM:CaS). Al2O3-MgO + CaO-Al2O3 + CaF2;(d) Type AC: CaO-Al2O3 + (CaF2 + CaS).

Type of NMI Mapping of main elements in NMI Type AM, Al2O3-MgO

Type ACM, Al2O3-MgO + CaO-Al2O3 + CaF2

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Figure 6. Typical oxide inclusions in different samples observed by three-dimensional (3-D) investigations on surfaces of steel samples after electrolytic extraction; (a) Type A: Al2O3; (b) Type Metals 2020, 10, 1620 9 of 16 AM: Al2O3-MgO; (c) Type ACM: Al2O3-MgO + CaO-Al2O3 + CaF2; (d) Type AC: CaO-Al2O3 + (CaF2 + CaS).

Type of NMI Mapping of main elements in NMI Type AM, Al2O3-MgO

Type ACM, Al2O3-MgO + CaO-Al2O3 + CaF2

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Type AC, Al2O3-CaO-SiO2 + CaS

Type AC, CaO-Al2O3 + CaF2 + CaS

FigureFigure 7. Mapping 7. Mapping of ofmain main elements elements in in di differentﬀerent inclusions observed observed by bythree three-dimensional-dimensional (3-D) (3-D) investigationsinvestigations on theon the surfaces surfaces of of steel steel samples samples afterafter electrolytic extraction. extraction.

4. Discussion4. Discussion

4.1. Contrast4.1. Contrast between between a Conventional a Conventional Cast Cast Electrode Electrode and Remelted Ingots Ingots The resultsThe results show show that that the the number number of ofsmaller smaller oxide inclusions inclusions is isless less in the in theelectrode electrode,, (CE-300) (CE-300),, than in the remelted ingots, (ESR-400 and PESR-500), see Figure 2a. The number of larger oxide than in the remelted ingots, (ESR-400 and PESR-500), see Figure2a. The number of larger oxide inclusions is approximately similar in the electrode and the remelted ingots. However, it should be noted that the electrodes have a loose central structure, which contains most of the large-sizes inclusions. This population of large inclusions in the center of the electrode cannot be fully counted by the conventional NMIs determination due to the large porosity of the central region of the electrode. In this study, the determination of NMIs in the central region was done in dense steel obtained as close to the center as practical. As a possible result, apparently anomalous effects can be observed in Figure 2a. The number of sulfide inclusions, mainly MnS, is more than ten times larger in the electrode than in the remelted ingots, see Figue 2b. Altogether, the electrode contains approximately 50% more inclusions than the remelted ingots. An explanation for the larger number of small oxide inclusions in the ESR and PESR ingots compared to the electrode is that primary, semi- secondary, and secondary inclusions are present in the ESR and PESR ingots. The primary inclusions, often Al2O3-MgO inclusions containing >10% MgO and <10% CaO (Type AM), are assumed to be electrode inclusions that have been trapped in steel drops or particles that have fallen from the electrode tip, through the slag bath to the steel pool, without having been overheated and dissolved, as can be seen in Figure 8. According to the literature, the diameter of the droplets leaving the electrode is generally in the range of 1–10 mm but usually about 5 mm [42,43], i.e. much larger than the individual inclusion size range.

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inclusions is approximately similar in the electrode and the remelted ingots. However, it should be noted that the electrodes have a loose central structure, which contains most of the large-sizes inclusions. This population of large inclusions in the center of the electrode cannot be fully counted by the conventional NMIs determination due to the large porosity of the central region of the electrode. In this study, the determination of NMIs in the central region was done in dense steel obtained as close to the center as practical. As a possible result, apparently anomalous eﬀects can be observed in Figure2a. The number of sulﬁde inclusions, mainly MnS, is more than ten times larger in the electrode than in the remelted ingots, see Figue 2b. Altogether, the electrode contains approximately 50% more inclusions than the remelted ingots. An explanation for the larger number of small oxide inclusions in the ESR and PESR ingots compared to the electrode is that primary, semi-secondary, and secondary inclusions are present in the ESR and PESR ingots. The primary inclusions, often Al2O3-MgO inclusions containing >10% MgO and <10% CaO (Type AM), are assumed to be electrode inclusions that have been trapped in steel drops or particles that have fallen from the electrode tip, through the slag bath to the steel pool, without having been overheated and dissolved, as can be seen in Figure8. According to the literature, the diameter of the droplets leaving the electrode is generally in the range of 1–10 mm but usually about 5 mm [42,43], Metals 2020, 10, x FOR PEER REVIEW 11 of 17 i.e., much larger than the individual inclusion size range.

(a) (b)

FigureFigure 8. 8. SchematicSchematic illustrations illustrations of different of different oxide oxide inclusions inclusions in the in melted the melted metal layer metal on layer the surface on the ofsurface electrode, of electrode, liquid slag, liquid melted slag, metal melted bath, metal and bath, solidified and solidifiedingot during ingot the during ESR process. the ESR (a process.) Solid electrode(a) Solid electrode—Liquid—Liquid slag; (b) Liquid slag; (b slag;) Liquid (c) Liquid slag; (c )slag Liquid—Melted slag—Melted metal bath; metal (d) bath; Melted (d) metal Melted bath metal— Solidifiedbath—Solidified ingot. ingot.

TheThe semisemi-secondary-secondary inclusions, which often are almostalmost pure pure Al Al22OO33 and Al Al22O3--CaOCaO oxides containingcontaining <10% MgO (Types(Types A A+ +AC), AC) are, are assumed assumed to to be be small small Mg Mg spinel spinel inclusions inclusions (<8 µ(<8m) µm) that havethat havesurvived surv fromived thefrom electrode. the electrode. The Mg The spinel Mg spinel inclusions inclusions already already exist in exist the electrode,in the electrode, either as either a solitary as a solitary spinel inclusion or as an inclusion with a core of a Mg-spinel and an outer layer corresponding to the top slag composition in the final treatment ladle. When the coated Mg spinel reaches the electrode tip, the outer layer melts, leaving the spinel. Thereafter, the corresponding ESR or PESR process slags potentially coat the spinel inclusions (Type ACM). This explanation is also confirmed by the detection of some amounts of CaF2 in the outer CaO-Al2O3 layer of inclusions (see Figures 6c,d and 7), since the ESR or PESR process slags contain CaF2 and the ladle slag does not. Moreover, the slag inclusions (Type AC) can be carried into the liquid steel pool with metal droplets, which passed down through the ESR or PESR process slags, as shown in Figure 8c. An alternative is the novel evolution mechanism using both thermodynamic and kinetic techniques, presented by Xuan et al. [44]. In this case, this type of inclusion can have an origin from a coalescence-collision between a solid Mg spinel and a liquid (Ca-Al oxide) inclusion in the liquid steel, that is (1) at the electrode tip, (2) in the process slag or, (3) in the molten steel pool, which in cases (2) and (3) would be the ESR/PESR process slag acting as an infinitive source of a Ca-Al oxide inclusion. However, further studies are needed to evaluate this alternative inclusion mechanism in the ESR/PESR processes. The secondary inclusions, for example, Al2O3-MgO (with a low content of MgO) and Al2O3, are formed in the liquid metal pool as a result of the reactions between alloying elements and dissolved oxygen [41]. Calcium aluminates are also assumed to precipitate in the molten steel pool. The generation of these oxide inclusions during solidification of liquid steel in the mold is assumed to

Metals 2020, 10, 1620 11 of 16

spinel inclusion or as an inclusion with a core of a Mg-spinel and an outer layer corresponding to the top slag composition in the final treatment ladle. When the coated Mg spinel reaches the electrode tip, the outer layer melts, leaving the spinel. Thereafter, the corresponding ESR or PESR process slags potentially coat the spinel inclusions (Type ACM). This explanation is also confirmed by the detection of some amounts of CaF2 in the outer CaO-Al2O3 layer of inclusions (see Figure6c,d and Figure7), since the ESR or PESR process slags contain CaF2 and the ladle slag does not. Moreover, the slag inclusions (Type AC) can be carried into the liquid steel pool with metal droplets, which passed down through the ESR or PESR process slags, as shown in Figure8c. An alternative is the novel evolution mechanism using both thermodynamic and kinetic techniques, presented by Xuan et al. [44]. In this case, this type of inclusion can have an origin from a coalescence-collision between a solid Mg spinel and a liquid (Ca-Al oxide) inclusion in the liquid steel, that is (1) at the electrode tip, (2) in the process slag or, (3) in the molten steel pool, which in cases (2) and (3) would be the ESR/PESR process slag acting as an infinitive source of a Ca-Al oxide inclusion. However, further studies are needed to evaluate this alternative inclusion mechanism in the ESR/PESR processes. The secondary inclusions, for example, Al2O3-MgO (with a low content of MgO) and Al2O3, Metals 2020, 10, x FOR PEER REVIEW MetalsareMetals 2020 formed 2020, 10, ,10 x ,FORin x FOR the PEER PEER liquid REVIEW REVIEW metal pool as a12 result of 17 of the reactions between alloying elements12 12of andof17 17 dissolved oxygen [41]. Calcium aluminates are also assumed to precipitate in the molten steel pool. give a small contribution to the total amount of inclusionsgiveThegive a generation smalla smallof >1 contribution µmcontribution of in these the remeltedoxideto toth the inclusions totale totalingot amount amount [18 during] since of of inclusions solidification inclusions of of>1 of >1 µm liquid µm in in steelthe the remelted inremelted the mold ingot ingot is assumed[18 [18] since] since to the time for particle growth or agglomeration is small.thegivethe Thetime time a population smallfor for pa pa contributionrticlerticle of growth extremelygrowth to or theor agglomeration agglomerationsmall total amountprecipitated is of issmall. inclusions small. The The population of population>1 µm in of the of extremely extremely remelted small ingot small precipitated [ 18precipitated] since the inclusions of <1 µm is probably high (accountinginclusions fortimeinclusions the for greater particleof of <1 <1 partµm growthµm isof isprobably the probably or inclusion agglomeration high high- accountable(accounting (accounting is small. for for Thethe the populationg reatergreater part part of of extremelyof the the inclusion inclusion small-accountable- precipitatedaccountable oxygen content), but has not been determined in thisoxygeninclusionsoxygen study. content) content) Their of < ,composition 1but, µbutm has ishas probablynot not beenis been driven determined high determined entirely (accounting inby in this this for study. study. the greaterTheir Their composition partcomposition of the inclusion-accountableis isdriven driven entirely entirely by by the ESR slag/metal reactions. A schematic over thethe threeoxygenthe ESR ESR inclusion slag/metal content), slag/metal types but reactions. reactions. hasis displayed not A been Aschematic schematic in determined Table over 2 over. inthe the this three three study. inclusion inclusion Their compositiontypes types is isdisplayed displayed is driven in inTable entirely Table 2. 2 by. the ESR slag/metal reactions. A schematic over the three inclusion types is displayed in Table2. Table 2. Characterization and a general schematic of theTable threeTable 2.described 2.Characterization Characterization inclusion and types. and a generala general schematic schematic of ofthe the three three described described inclusion inclusion types. types. Table 2. Characterization and a general schematic of the three described inclusion types. Secondary Inclusions SecondarySecondary Inclusions Inclusions Inclusion Primary Inclusions (PI) SemiInclusion-SecondaryInclusion Inclusions Primary (SSI)Primary Inclusions Inclusions (PI) (PI) SemiSemi-Secondary-Secondary Inclusions Inclusions (SSI) (SSI) Primary Inclusions Semi-Secondary Secondary Inclusions Inclusion (SI) (SI)(SI) (PI) Inclusions (SSI) (SI)

Schematic SchematicSchematic Schematic illustration illustration illustrationillustration

Type of NMI AM ACM, A and AC A, AC Type of TypeType of of AM ACM, A and AC AMAM A, AC MediumACM,ACM, A (normally andA and AC AC < A, A,AC AC NMI NMINMI Size on NMI Large, see below ≈ Small (<10 µm) 30 µm) Size on SizeSize on on Large, see below Medium (normally ≈ < 30 µm)Large, Large, se es ebelowe Smallbelow (<10 µm) MediumMedium (normally (normally ≈ < ≈ 30 < 30µm) µm) SmallSmall (<10 (<10 µm) µm) NMI NMINMI From the electrode, the size depends on the size From the electrode, the size depends FromFrom the the electrode electrodeof, the , the size inclusions size depends depends in the Primary AM oxides on the size of the inclusions in the Primary AM oxides coonveredon the the size by size of oftheelectrode the inclusions inclusions and in inthe the size ofPrimaryPrimarycovered AM AM oxides by oxides process co veredcovered slag, by by electrode and the size of the steel process slag, sometimeselectrode withelectrode CaF and2 and thethe the size steel size of droplets.ofthe the steel steel In theprocessprocesssometimes slag, slag, sometimes sometimes with with CaF with CaF2 CaF2 2 Source or SourceSource or or Precipitated in the ingot PrecipitatedPrecipitatedPrecipitated inin thein the ingot ingot ingot Source or formation ingot the spinels are attached. If CaF is droplets. In the ingot the spinels are attached. If CaF2 is found,droplets.droplets. it is proof In Inthe the ingot ingot the the spinels spinels are are attachedattached. If. CaFIf CaF2 is2 found,is found, it2 isit proofis proof during solidiﬁcation of formation formationformationmechanism oftenduring found solidification in inclusion of found, it is proof that the duringduring solidification solidification of of often found in inclusion clusters with that the inclusion oftenhasoften been found found in in ininclusion inclusion clusters clusters with with thatthat the the inclusion inclusion has has been been in in the liquid steel mechanism mechanismmechanism clustersthe liquid with complexsteel inclusion has been in thethe liquid liquid steel steel complex shapes, often together with contact with the ESR/PESRcomplexcomplex process shapes shapes , often, often together together with with contactcontact with with the the ESR/PESR ESR/PESR process process shapes, often together contact with the other inclusions and elements (C, Cr, slag. otherother inclusions inclusions withand and elements other elements inclusions (C, (C, Cr, Cr, and ESR/PESRslag.slag. process slag. Si, Mn, S, Ca). Si, Si,Mn, Mn,elements S, Ca).S, Ca). (C, Cr, Si, Mn, S, Ca). In the slag/metal reaction scheme, Ca and Mg can beIn introducedI then the slag/metal slag/metal into reaction the reaction metal scheme, duescheme, to Caa reductionCa and and Mg Mg ca can ben be introduced introduced into into the the metal metal due due to toa reductiona reduction with an Al deoxidant according to the following reactions:withwith an an Al Al deoxidant deoxidant according according to tothe the following following reactions: reactions:

3(MgO) + 2[Al] = 3[Mg] + (Al2O3) 3(MgO)3(MgO) + 2[Al]+ 2[Al](1) = 3[Mg]= 3[Mg] + (Al+ (Al2O23O) 3) (1)( 1)

3(CaO) + 2[Al] = 3[Ca] + (Al2O3) 3(CaO)3(CaO) + 2[Al]+ 2[Al](2) = 3[Ca]= 3[Ca] + (Al+ (Al2O23O) 3) (2)(2) The driving force for Mg and Ca in the steel (reacTheThetions driving driving (1) and force force (2 )), for forshould Mg Mg and leadand Ca Ca to in ain the higher the steel steel (reac (reactionstions (1 )( 1and) and (2 )(),2) ),should should lead lead to toa a higher higher amount of [Mg] and a lower content of [Ca] in theamount conventionalamount of of [Mg] [Mg] ingot and and anda lowera lowera higher content content amount of of [Ca] [Ca] of in[Ca] in the the conventional conventional ingot ingot and and a highera higher amount amount of of [Ca] [Ca] and a lower content of [Mg] in the ESR/PESR ingot.andand The a lowera reason lower content iscontent that of the of [Mg] content[Mg] in inthe ofthe ESR/PESRsilica ESR/PESR (SiO 2ingot.) isingot. The The reason reason is isthat that the the content content of of silica silica (Si (SiO2O) 2is) is higher in the ladle slag compared to the processhigherhigher slag in inin the ESR/PESR. the ladle ladle slag slagAccording compared compared to to laboratory to the the process process slag slag in in ESR/PESR. ESR/PESR. According According to to laboratory laboratory investigations by Shin et al. [45] the direction of theinvestigationsinvestigations driving force by inby Shin equations Shin et etal. al. [45] 1 [45] and the the 2 direction is direction controlled of of the the driving driving force force in in equations equations 1 and1 and 2 is2 iscontrolled controlled by the amount of silica in the process slag. However,byby thethe the amountdifferenc amount ofe of silicain silica [Mg] in in andthe the process [Ca] process contents slag. slag. However, could However, the the differenc difference ine in [Mg] [Mg] and and [Ca] [Ca] contents contents could could not be determined, or was considered to be in the marginnotnot be be determined, of determined, error, from or or thewas was total considered considered composition to tobe be resultsin in the the margin margin of of error, error, fr omfrom the the total total composition composition results results of the samples from the conventional ingots, electrodesofof the the, andsamples samples ESR/PESR from from the ingots. the conventional conventional ingots, ingots, electrodes electrodes, and, and ESR/PESR ESR/PESR ingots. ingots. The discussion above assumes that the Mg spinel,The bothThe discussion discussionin the electrode above above assumes and assumes in thethat that ESR/PESR the the Mg Mg spinel, spinel, both both in inthe the electrode electrode and and in inthe the ESR/PESR ESR/PESR ingots is formed due to a reaction taking place iningots theingots liquid is isformed formedsteel/slag. due due toAccording toa reactiona reaction to takingKiessling taking place place [46 in] , inthe the liquid liquid steel/slag. steel/slag. According According to toKiessling Kiessling [46 [46], ], non-metallic inclusion nucleus with MgO as onenon componentnon-metallic-metallic mayinclusion inclusion be formed nucleus nucleus as witha withproduct MgO MgO dueas as one toone component component may may be be formed formed as as a aproduct product due due to to reactions between the furnace refractories and the furnacereactionsreactions or between ladle between slag, the theor furnace with furnace the refractories refractoriesmolten steel and and itself. the the furnace furnace or or ladle ladle slag, slag, or or with with the the molten molten steel steel itself. itself. Magnesium has both a very low solubility in iron MagnesiumandMagnesium a Mg-spinel has has both has both a highverya very meltinglow low solubility solubility point ofin in 2135iron iron and and a Mga Mg-spinel-spinel has has a higha high melting melting poin point oft of 2135 2135 °C [46]. According to the low amount of MgO in°C the°C [46 ESR[46]. ].According processAccording slag to to the(about the low low 3 amount%) amount and theof of MgO lack MgO ofin inthe the ESR ESR process process slag slag (about (about 3%) 3%) and and the the lack lack of of refractory in the remelting processes, these resultsrefractoryrefractory indicate in in thatthe the remelting the remelting primary processes, processes, MgO containing these these results results indicate indicate that that the the primary primary MgO MgO containing containing

Metals 2020, 10, 1620 12 of 16

In the slag/metal reaction scheme, Ca and Mg can be introduced into the metal due to a reduction with an Al deoxidant according to the following reactions:

3(MgO) + 2[Al] = 3[Mg] + (Al2O3) (1)

3(CaO) + 2[Al] = 3[Ca] + (Al2O3) (2) The driving force for Mg and Ca in the steel (reactions (1) and (2)), should lead to a higher amount of [Mg] and a lower content of [Ca] in the conventional ingot and a higher amount of [Ca] and a lower content of [Mg] in the ESR/PESR ingot. The reason is that the content of silica (SiO2) is higher in the ladle slag compared to the process slag in ESR/PESR. According to laboratory investigations by Shin et al. [45] the direction of the driving force in equations 1 and 2 is controlled by the amount of silica in the process slag. However, the diﬀerence in [Mg] and [Ca] contents could not be determined, or was considered to be in the margin of error, from the total composition results of the samples from the conventional ingots, electrodes, and ESR/PESR ingots. The discussion above assumes that the Mg spinel, both in the electrode and in the ESR/PESR ingots is formed due to a reaction taking place in the liquid steel/slag. According to Kiessling [46], non-metallic inclusion nucleus with MgO as one component may be formed as a product due to reactions between the furnace refractories and the furnace or ladle slag, or with the molten steel itself. Magnesium has both a very low solubility in iron and a Mg-spinel has a high melting point of 2135 ◦C[46]. According to the low amount of MgO in the ESR process slag (about 3%) and the lack of refractory in the remelting processes, these results indicate that the primary MgO containing inclusions from the electrode should survive the ESR/PESR remelting processes. As a comparison, the melting points for common inclusions are presented in Table3, the data of which are taken from [46].

Table 3. Melting points of inclusions occurring in electrode charges and ESR/PESR ingots, data from [46].

Type of Inclusion Melting Point

MgO-Al2O3 spinels 2135 ◦C CaO-Al2O3 calcium aluminates 1455–1850 ◦C CaO-SiO2 calcium silicates 1475–2070 ◦C MnS manganese sulﬁde 1610 ◦C CaS calcium sulﬁde about 2500 ◦C

Yang et al. calculated stability diagrams of the Mg-Al-O system in molten steel at 1873 K and 1773 K [47]. According to them, as well as from our observations the measured amounts of Mg, O and Al, Mg spinel is stable both in the electrodes as well as in the ESR/PESR ingots. The working temperature on the electrode tip is assumed to be about 25 ◦C higher than the melting point of the steel, which is approximately 1798 K [13,18,41,48]. Yang et al. [47], (as well as Shi et al. [13]) also presented a mechanism that could explain the modiﬁcation of Mg-spinel inclusions into MgO-Al2O3-CaO-CaS inclusions, during calcium treatment. However, the inclusions obtained by calcium treatment are very similar to the inclusions seen in this and previous investigations [12,23–25]. This indicates, to a certain level, that the ESR process slag itself acts in the same way as a conventional calcium treatment of a steel melt. Xuan et al.´s theory regarding attachment behavior on evolution mechanism of Mg-Al oxides particles in steel [44] describes two possibilities to obtain an inclusion with one or more Mg spinel particles as a core and with a coating layer of Ca-Al-(Mg) oxide. The inclusion evolution can take place by either a chemical reaction or a coalescence-collision behavior. This idea may explain the appearance of this kind of inclusions in the electrode, and possibly also in the ESR/PESR processes. The results presented above correspond to the results from the trials using a tracer in the ESR process slag [24,25]. More speciﬁcally, approximately 50% of the inclusions in an ESR ingot are primary inclusions, with an origin from the electrode, and the most common type is a Mg spinel inclusion. Metals 2020, 10, 1620 13 of 16

4.2. Difference between ESR and PESR Remelted Ingots The difference in inclusion characteristics (morphology, composition, size, and number) between the ingots from the open-air furnace and the ingots from the PESR process is probably both due to the difference, which comes from electrode changing, the extra chemical reactions taking place between air and slag bath, and the aluminum deoxidation during the ESR process. The aluminum deoxidant, dissolved in the slag pool reacts with FeO in the process slag as follows [10,49]:

2[Al] in slag + 3(FeO) = (Al2O3) + 3[Fe] (3)

Schematic illustrations of the mechanisms of oxygen transfers and Al deoxidation in an open ESR process are shown in Figure9. Due to the reactions, the feasibility for nucleation of a larger number of inclusions exists. According to Li et al. [30] an Al2O3 rod immersed in a CaO-Al2O3-CaF2 ﬂux led to the formation of an intermediate coating compound of CaO-2Al2O3. This could explain why more Metalsthan a2020 double, 10, x FOR amount PEER REVIEW of Al-Ca oxides (Type AC) were found in the ESR ingot as compared14 to of the 17 PESR ingot.

(a) (b)

Figure 9. ((aa)) Schematic Schematic illustration illustration of of the the mech mechanismsanisms of of oxygen oxygen transfers transfers and and A All deoxidation deoxidation in in ESR ESR process (lsp (lsp-liquid-liquid steel pool) pool).. ( b)) Numbering of reactions in the figure ﬁgure is given according toto [[10].10].

Another critical critical point point is is that, in in order to prevent a liquid break breakout,out, a smaller electrode/ingot electrode/ingot ratioratio is is often often used used in in ESR ESR furnaces furnaces with with movable movable molds molds or or baseplates. baseplates. A A smaller smaller electrode/ingot electrode/ingot ratio ratio concentrates thethe heatheat moremore towards towards the the center center of of the the ingot, ingo whicht, which leads lead tos a to deeper a deeper pool pool of molten of molten steel steelin the in ingot. the ingot. A deeper A deepe poolr results pool results in a longer in a longestlonger solidificationlongest solidification time (LST), time i.e. (LST a longer), i.e. residencea longer residencetime in the time liquid in pool. the liquid This, pool.in turn, This, promotes in turn, more promotes nucleation more and nucleation inclusion and growth. inclusion According growth. to AccordingCao et al. [43 to], C aao larger et al. filling [43], ratioa larger (electrode filling/ ingotratio diameter)(electrode/ingot also leads diameter) to more also and leads smaller to dropletsmore and as smallercompared droplets to a smaller as compared filling ratio. to a smaller More and filling smaller ratio droplets. More and increase smaller the droplets contact area increase between the thecontact slag areaand steel,betwee whichn the promotes slag and the steel, removal which of promote non-metallics the inclusionsremoval of from non the-metallic original inclusions electrode from material. the original electrode material. 5. Conclusions 5. ConclusionsThe following conclusions can be drawn based on the results from this study:

TheBoth following larger inclusions conclusions (up tocan 44.8 beµ drawnm) and based an approximatively on the results from two timesthis study: higher number of oxide •  Bothinclusions larger per inclusions area were (up found to 44.8 in µm) the ESR and ingot an approximatively compared to the two PESR times ingot. higher This number is believed of oxide to be inclusionscaused by theper remeltingarea were under found air in atmosphere,the ESR ingot the compared Al deoxidation, to the PESR the lower ingot. ﬁlling This ratiois believed between to bethe caused electrode by andthe theremelting ingot, andunder an unintentionalair atmosphere, uneven the Al melting deoxidation, during thethe electrodelower filling changes. ratio betweenAccording the to electrode the recent and classiﬁcation the ingot, and of an inclusions, unintentional primary, uneven semi-secondary melting during and the secondary electrode • changes.inclusions exist in the remelted ingots as explained below.  According to the recent classification of inclusions, primary, semi-secondary and secondary inclusions exist in the remelted ingots as explained below.  The primary inclusions (such as Al2O3-MgO oxides, Type AM) are assumed to have survived from the electrode because they were trapped inside a steel drop or a fallen steel fragment, without having contact with the ESR/PESR process slag.  The most common inclusion type in the remelted ingots has a core of MgO_Al2O3 with an outer layer that corresponds to their corresponding process slag (Type ACM). However, inclusions containing MgO are characterized by having an exogenous origin, where the main sources of MgO are refractories or the furnace or ladle slags. Due to the low solubility of Mg in iron and the high melting point of MgO, it is more likely that at least the majority of the Mg spinel inclusions in the remelted ingots are primary inclusions, which survived from the electrode. This kind of inclusion is denoted as semi-secondary inclusions.  The secondary inclusions are assumed to be formed in the liquid steel pool as a result of the reactions between alloying elements and the dissolved oxygen.  According to the literature, the inclusions after a calcium treatment modification, (used to modify MgO-Al2O3 to softer inclusions with a lower melting point of the outer layer of inclusions) are very similar to the inclusions found in the ESR/PESR ingots. This observation

Metals 2020, 10, 1620 14 of 16

The primary inclusions (such as Al O -MgO oxides, Type AM) are assumed to have survived • 2 3 from the electrode because they were trapped inside a steel drop or a fallen steel fragment, without having contact with the ESR/PESR process slag. The most common inclusion type in the remelted ingots has a core of MgO_Al O with • 2 3 an outer layer that corresponds to their corresponding process slag (Type ACM). However, inclusions containing MgO are characterized by having an exogenous origin, where the main sources of MgO are refractories or the furnace or ladle slags. Due to the low solubility of Mg in iron and the high melting point of MgO, it is more likely that at least the majority of the Mg spinel inclusions in the remelted ingots are primary inclusions, which survived from the electrode. This kind of inclusion is denoted as semi-secondary inclusions. The secondary inclusions are assumed to be formed in the liquid steel pool as a result of the • reactions between alloying elements and the dissolved oxygen. According to the literature, the inclusions after a calcium treatment modification, (used to modify • MgO-Al2O3 to softer inclusions with a lower melting point of the outer layer of inclusions) are very similar to the inclusions found in the ESR/PESR ingots. This observation indicates that the ESR process slag acts in some way like a calcium treatment modification. This hypothesis is supported by the observed appearance of the inclusions described above. The results presented above correspond to the results from previous trials, made using a tracer • (La2O3) in the ESR process slag, which indicates that the most probable inclusion to survive from the electrode is a Mg-spinel. On the whole, we find that the overall cleanliness of the electrode (and also the composition • of the inclusions in the electrode) has a direct bearing on the inclusion content of the ESR and PESR ingots.

Author Contributions: Conceptualization, E.S.P. and A.K.; methodology, E.S.P. and A.K.; validation, E.S.P., A.K. and A.M.; formal analysis, E.S.P. and A.K.; investigation, E.S.P.; resources, E.S.P.; data curation, E.S.P.; writing—original draft preparation, E.S.P.; writing—review and editing, E.S.P., A.M., A.K. and P.G.J.; visualization, E.S.P.; supervision, A.M., A.K. and P.G.J.; project administration, E.S.P.; funding acquisition, E.S.P. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Conﬂicts of Interest: The authors declare no conﬂict of interest.

References

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