The Research of Low-Oxygen Control and Oxygen Behavior During RH Process in Silicon-Deoxidization Bearing Steel

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Article The Research of Low-Oxygen Control and Oxygen Behavior during RH Process in Silicon-Deoxidization Bearing Steel

Wei Xiao, Min Wang and Yanping Bao *

State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, Beijing 100083, China * Correspondence: [email protected]; Tel.: +86-010-8237-6593

Received: 26 June 2019; Accepted: 19 July 2019; Published: 24 July 2019

Abstract: The variation of total oxygen (T.O) content, characterization of inclusions, slag composition, and oﬀ-gas behavior during the smelting process of silicon-deoxidization bearing steel were investigated with industrial experiments. The change of content of combined oxygen during RH (Ruhrstahl–Hereaeus vacuum degassing furnace) process was calculated and compared with T.O content change. It is found that the decrease of oxygen content is mainly caused by the removal of dissolved oxygen rather than the removal of oxides during RH process. Carbon was found to be a strong deoxidizer (stronger than aluminum) in high vacuum degree. Top slag is an oxygen source of the deoxidization process, leading to the re-oxidization of liquid steel, even though the FeO content is low in top slag. During the RH process, the change of oxygen mainly exists in three processes: 1) Deoxidization reaction in vacuum chamber, 2) oxygen mass transfer process between liquid steel out from a vacuum chamber and in ladle, and 3) oxygen mass transfer between ladle slag and liquid steel. It depends mainly on the mass transfer of the oxygen in the liquid steel.

Keywords: RH process; bearing steel; deoxidization; T.O content

1. Introduction Bearing steel is often used for roller and ball bearings in engineering structures as bearing, for example, metallurgy, wind power, mining machinery, and aerospace [1]. The life of bearing is significant for security in engineering. However, non-metal inclusion as a kind of defect is one of the most important causes of life reduction of bearing [2–4]. In order to ensure the quality of bearing, there are two kinds of control conception: One controls the cleanliness of bearing steel, meaning very low total oxygen (T.O) content in the steel, and the other controls the type and morphology of inclusions by reducing inclusions unfavorable to fatigue life, such as TiN and large-sized calcium aluminate (Ca–Al–O) [5]. The first control conception is widely applied in the production of bearing steel and the T.O content can be controlled below 5 ppm, even 3–4 ppm, due to the development of production technology. The production process for bearing steel is usually EAF (electric arc furnace)/BOF (basic oxygen furnace) LF (ladle furnace) VD (vacuum degasser) CC (continuous casting) [6], where Al is → → → used for deoxidization as a kind of strong deoxidizer and the oxygen content can be reduced to 3–4 ppm. However, in this production process, the controlling of the type and morphology of inclusions is difficult. Types of inclusions are mainly calcium aluminate (Ca–Al–O), spinal (Al–Mg–O), silicate (Si–Al–O), and a few TiN, in which calcium aluminate, spinal, and TiN are considered as the most hazardous to the fatigue life of bearing steel [7]. Recently, in order to control the type and morphology of inclusions—reducing the content of calcium aluminate and spinal—a kind of production method of

Metals 2019, 9, 812; doi:10.3390/met9080812 www.mdpi.com/journal/metals Metals 2019, 9, 812 2 of 15 bearing steel by using Si instead of Al as deoxidizer was put forward [8]. The production process is BOF LF RH (Ruhrstahl–Hereaeus) CC. The main non-metal inclusion in the steel is silicate. → → → The effect of the oxide inclusions generated in Si deoxidization on the fatigue life is lower than that in Al deoxidization, even if the T.O content is slightly higher. However, in this production process, the T.O content after the LF process is more than the Al deoxidization method because of the weak deoxidization ability of Si. In order to further deoxidization, the RH process is adopted to forced deoxidization to ensure the T.O content in the controlling range (less than 12 ppm). In recent years, the RH process is gradually applied in the production of bearing steel instead of VD. The RH degassing furnace is vacuum refining equipment and commonly used in decarburization and degassing [9,10]. Due to the vacuum and intense stirring condition, the RH process is beneficial for low dissolved gas content and the removal of smaller inclusions because of more collision chance that can further reduce the T.O content in steel. Much research has been conducted to understand the decarburization in the RH process. Some researchers [11–15] have established the model for prediction of decarburization reaction during the RH refining process. Liu [16] investigated the decarburization behaviors of ultra-low-carbon steel. The effects of the initial carbon content, initial oxygen content, and pressure in the vacuum chamber on decarburization rate were discussed and Liu found that enhancing the initial carbon content, the initial oxygen content, and reducing the pressure increased decarburization rate. However, when it comes to deoxidization in the RH process, it will be more complicated. In this paper, the deoxidization process in bearing steel will be discussed. The bearing steel was produced by BOF LF RH CC, and silicon was used for deoxidization after the converter → → → tapping process. There was no deoxidizer added in the RH process. The T.O content after LF refining in the Si-deoxidization bearing steel was about 20–30 ppm, which was much higher than 15 ppm after LF refining in the Al-deoxidization bearing steel [8]. However, after deoxidization in the RH process and continuous casting, the T.O content in billets can be 8–10 ppm. Therefore, the aim of the current study is to clarify the low-oxygen control and oxygen behavior during the RH process in Si-deoxidization bearing steel. Both the industrial experiments and lab detection were conducted. The T.O content, characteristic of inclusions, composition of slag, and off-gas were obtained for further analysis. The relationship between smelting time and content of combined and dissolved oxygen was calculated to clarify the oxygen removal method. The reaction of [C] and [O] in the vacuum condition was discussed and the deoxidization abilities of several deoxidizer were compared to understand the deoxidization ability of carbon in the vacuum condition. Finally, the behavior of [O] during the RH process was obtained, which can be a guideline for Si-deoxidization bearing steel production to further control the T.O content in the steel.

2. Experimental

2.1. Production Process In this steelmaking route, most sulphur was removed in KR (Kambara Reactor) pretreatment of hot metal. After BOF smelting, silicon was added to remove oxygen in the steel, and refining flux materials were added to slag during tapping process. At the beginning of the LF process, CaO was added to adjust the basicity of slag, carbon was added to adjust the carbon content in the steel, and silicon was added to reduce oxygen content. In the middle of LF refining, quartz sand was added to adjust the basicity of slag. Liquid steel was then processed with an RH degassing system for further removal of oxygen with no alloy addition. The ladle was finally shifted to the tundish for the continuous casting. The chemical composition of the steel is listed in Table1. The steelmaking route and sampling locations are shown in Figure1. Metals 2019, 9, 812 3 of 15

Table 1. Compositions of the silicon-deoxidization bearing steel (wt%).

Component T.O C Si Cr Mn P S Al

Metals 2019, 9 FORConcentration PEER REVIEW 0.0008 1 0.27 1.38 0.35 0.015 0.0065 0.002 3

FigureFigure 1. 1. ProcessProcess of of industrial industrial tria trialsls and and sampling sampling locations. locations. 2.2. Detection and Analysis Method 2.2. Detection and Analysis Method 2.2.1. Inclusion and Slag Composition Detection 2.2.1. Inclusion and Slag Composition Detection Steel samples were taken with pail-samplers (interior diameter 60 mm, height 100 mm): One after BOFSteel smelting, samples three were in the taken LF process, with pail-samplers three in the RH (interior process, anddiameter one in 60 continuous mm, height casting, 100 asmm): shown One in afterFigure BOF1. These smelting, samples three were in the machined LF process, to rectangular three in the pieces RH (10process, mm and10 mmone in10 continuous mm), and polishedcasting, × × asfor shown characterization in Figure 1. of These the inclusions samples were by using machined an automatic to rectangular inclusion pieces analysis (10 mm system × 10 mm (EVO18, × 10 mm), Zeiss, andOberkochen, polished Germany)for characterization and scanning of the electron inclusions microscopy-energy by using an automatic dispersive inclusion spectroscopy analysis (SEM-EDS). system (EVO18,Cylinders Zeiss, (φ 5 mmOberkochen,50 mm) Germany) were machined and forscan thening measurement electron microscopy-energy of the oxygen contents dispersive by the × spectroscopyinfrared absorption (SEM-EDS). method. Cylinders The compositions (ϕ 5 mm × 50 of themm) steel were were machined determined for the by measurement inductively coupled of the oxygenplasma contents atomic emission by the infrared spectrometer. absorption method. The compositions of the steel were determined by inductivelyTwo slag samplescoupled inplasma the RH atomic process emission were taken spectrometer. out and ground to powder. The composition of the slagTwo was slag analyzed samples byin the an X-rayRH process fluorescence were taken spectrometer. out and ground to powder. The composition of the slag was analyzed by an X-ray fluorescence spectrometer. 2.2.2. Off-Gas Analysis 2.2.2. Off-Gas Analysis During the RH process, the off-gas flow rate was analyzed and the content of CO in the off-gas wasDuring monitored the continuouslyRH process, the by usingoff-ga ans flow off-gas rate analysis was analyzed system. and The the off -gascontent was of taken CO in from the theoff-gas tube wascontinuously monitored and continuously sent to the by o ffusing-gas analysisan off-gas system, analysis after system. removing The off-gas dust andwas water,taken from where the its tube CO continuouslycontent was measuredand sent to by the infrared off-gas analyzer analysis and system, its flow after rate removing was measured dust and by gaswater, flowmeter. where its CO content was measured by infrared analyzer and its flow rate was measured by gas flowmeter. 3. Results 3. Results 3.1. T.O Content Change 3.1. T.OFigure Content2 shows Change the changes in T.O during the whole smelting process. It can be read that T.O contentFigure was 2 about shows 20 the ppm changes at the beginning in T.O during of the the LF processwhole smelting and barely process. changed It can during be read the LF that process. T.O contentHowever, was T.O about content 20 beforeppm at the the RH beginning process was of aboutthe LF16–28 process ppm and, and barely rapidly changed decreased during to 8–10 the ppm LF process.at the end However, of the RH T.O process, content whichbefore meansthe RH that process8–20 was ppm aboutoxygen 16–28 was ppm removed, and rapidly in the RH decreased process. toAlthough 8–10 ppm the at T.O the content end of beforethe RH the process, RH process which varied, means thethat T.O 8–20 content ppm oxygen after the was RH removed process reached in the RHthe process. same low Although level. the T.O content before the RH process varied, the T.O content after the RH process reached the same low level.

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Figure 2. Change of total oxygen (T.O) content in the steel.

3.2. The Change of Oxides During RH Process Figure 2. Change of total oxygen (T.O) content in the steel. The main oxides in this steel are spinal, silicate, and calcium aluminate, where the content of silicate3.2. The is Change the highest. of Oxides The During number RH density Process of the oxides indicates remarkable changes, and oxides of the size of 1–6 µm were effectively reduced as shown in Figure3, where the number density of the silicateThe decreased main oxides while in the this number steel are density spinal, of spinalsilicate, and and calcium calcium aluminate aluminate, increased. where the According content toof analysis,silicate is before the highest. the RH The process number was density about 5.76of the N /oxidesmm2, amongindicates which remarkable the number changes, density and of oxides oxides of smallerthe size than of 1–6 5 µ μmm was were about effectively 5.35 N /reducedmm2. After as shown 10 min in of Fi treatmentgure 3, where in RH, the the number number density density of the of oxidessilicate decreased decreased to while 3.65 the N/mm number2 with density the value of spinal of oxides and calcium smaller aluminate than 5 µm increased. at about 3.24According N/mm 2to. Aboutanalysis, 23 minbefore later, the the RH number process density was about of all 5.76 detected N/mm oxides2, among and which those smallerthe number than density 5 µm decreased of oxides tosmaller 2.76 N /thanmm 25and μm 2.35was N about/mm2 5.35, respectively. N/mm2. After 10 min of treatment in RH, the number density of oxidesIt can decreased be seen to that 3.65 RH N/mm has a2 strongwith the ability value to of remove oxides oxidessmaller from than liquid5 μm steel.at about The 3.24 small N/mm size2. oxidesAbout gathered 23 min later, and grewthe number to oxides density with biggerof all detected size. The oxides bigger-size and those oxides smaller floated than and 5 were μm decreased captured byto the2.76 slag N/mm layer,2 and and 2.35 were N/mm then2, removed respectively. due to the strong stirring in the RH process. TheIt can microstructure be seen that RH of complexhas a strong inclusion ability in to the remove RH processoxides from is shown liquid in steel. Figure The3. small In this size Si-deoxidizationoxides gathered steel,and thegrew Si contentto oxides in thewith complex bigger inclusionsize. The wasbigger-size more than oxides Al content. floated However,and were incaptured Al-deoxidization by the slag steel, layer, the and Al were content then inremove the complexd due to inclusionthe strong wasstirring more in the than RH Si process. content [8]. This inclusionThe microstructure with more Siof content complex had inclusion less effect in on the the RH fatigue process life is of shown bearing in steel. Figure 3. In this Si- deoxidization steel, the Si content in the complex inclusion was more than Al content. However, in Al-deoxidization steel, the Al content in the complex inclusion was more than Si content [8]. This inclusion with more Si content had less effect on the fatigue life of bearing steel.

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FigureFigure 3. 3. ChangesChanges in number in number density density and size and distribu sizetion distribution of oxide inclusions of oxide during inclusions Ruhrstahl– during Ruhrstahl–HereaeusHereaeus (RH) treatment: (RH) treatment: (a) Spinal, (a )(b Spinal,) silicon, (b )and silicon, (c) calcium and (c) calciumaluminate aluminate inclusions inclusions before beforetreatment; treatment; (d) spinal, (d) spinal, (e) silicon, (e) silicon, and (f)and calcium (f) calcium aluminate aluminate inclusions inclusions after being after treated being for treated 10 min; for 10and min; (g and) spinal, (g) spinal, (h) silicon, (h) silicon, and (i) andcalcium (i) calcium aluminate aluminate inclusions inclusions after being after treated being for treated 23 min; for (j) 23SEM min; and (k) EDS results of inclusion in the RH process. (j) SEM and (k) EDS results of inclusion in the RH process.

3.3.3.3. Change Change of of FeO FeO Content Content in in Reﬁning Refining Slag Slag TheThe compositions compositions of of toptop slag before before and and after after th thee RH RH process process were were analyzed analyzed and the and results the results are areshown shown in inTable Table 2. 2The. The basicity basicity of the of slags the slags (R) were ( R) nearly were kept nearly the kept same the value same during value the during RH process. the RH process.The main The change main change of compositions of compositions were FeO were and FeO MnO, and where MnO, the where content the contentof FeO decreased of FeO decreased by 0.441 by 0.441wt% wt% and and the thecontent content of MnO of MnO increased increased by 0.023 by 0.023wt%. wt%.

Table 2. The composition of top slag (wt%).

Stage CaO SiO2 Al2O3 MgO FeO MnO R SRH1 37.2 41.7 4.67 12.5 1.386 0.859 1.072 SRH2 37.6 41.8 4.76 12.4 0.945 0.882 1.074

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Table 2. The composition of top slag (wt%).

Stage CaO SiO2 Al2O3 MgO FeO MnO R SRH1 37.2 41.7 4.67 12.5 1.386 0.859 1.072 SRH2 37.6 41.8 4.76 12.4 0.945 0.882 1.074

Note: R = w(CaO + MgO)/w(SiO2 + Al2O3).

3.4. Component Analysis of Off-Gas Figure4a shows the variation of the pressure in the RH vacuum chamber and the CO content in the off-gas during the RH process. It can be seen that the pressure of the RH vacuum chamber decreased to about 1000 Pa at 180 s and directly decreased to ultimate vacuum degree without a pressure drop platform while the CO concentration rapidly increased to the peak of about 3.25% within 100 s and subsequently decreased to a certain value where the final operation time as 23 min. Figure4b shows the off-gas flow rate during the RH process. During the period of vacuum decreasing, the off-gas flow rate reached the peak value and was kept steady for about 3 min. Subsequently, it dropped to a relatively steady value rapidly until the end of the RH process. Metals 2019, 9 FOR PEER REVIEW 7

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Figure 4. Variation in pressure of the RH vacuum chamber and CO content in the off-gas (a), off-gas Figureflow rate 4. Variation of (b) during in pressure the RH of process. the RH vacuum chamber and CO content in the off-gas (a), off-gas 4. Discussionflow rate of (b) during the RH process.

4.4.1. Discussion The Relationship of Smelting Time and Content of Combined and Dissolved Oxygen µ 4.1. TheFrom Relationship the results, of Smelting the diameters Time an ofd Content oxides areof Combined mainly in and a range Dissolved of 0.5 Oxygen to 5 m in the steel while the types of oxides are mainly silicate, spinal, and calcium aluminate. Detailed data were obtained by anFrom automatic the results, inclusion the diameters analysis of system, oxides and are themainly number in a range of oxides of 0.5 per to unit 5 μm area in the was steel calculated while theaccording types of to oxides these are detected mainly data. silicate, In order spinal, to and evaluate calcium the aluminate. oxygen content Detailed existing data aswere oxides obtained in the

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Metalsby an2019 automatic, 9, 812 inclusion analysis system, and the number of oxides per unit area was calculated8 of 15 according to these detected data. In order to evaluate the oxygen content existing as oxides in the steel,steel, it it was was calculated calculated by by Equations Equations (1)–(4) (1)–(4) [ 17[17],], in in which which DeHo DeHoff’sﬀ’s formulas formulas were were expressed expressed by by EquationsEquations (1) (1) and and (2) (2),, the the volume volume fraction fraction of of oxides oxides was was calculated calculated by by Equation Equation (3), (3), and and the the oxygen oxygen contentcontent existing existing as as oxides oxides in in the the steel steel was was calculated calculated by by Equation Equation (4). (4). 2 N N = · a , (1) V2 πN d N = a , (1) V π · 1 1 d1 = ∑ , (2) d n d 1 1 X 1 i = , (2) n π 3d d V = d ·Ni , (3) 6 V π 3 V = d− NV, (3) 6 ρ · [O] = ox! ·V·(O) , (4) ox ρ ρ ox [O] = ox FeV (O) , (4) ox ρ · · ox where NV is the number of oxides per unit volumeFe in specimen, m−3; Na is the number of oxides per −2 i 3 𝑑̅ whereunit areaNV isin thespecimen, number m of; oxides d is apparent per unit particle volume size in specimen,of i-th oxide m −among; Na is n the oxides, number m; of is oxides harmonic per 2 [𝑂] unitmean area of inoxide specimen, particle m size,− ; d im;is V apparent is volume particle fraction size of of oxides;i-th oxide among is then oxygenoxides, content m; d is harmonicexisting as oxides in the steel; ρox is the oxide density, kg·m−3; ρFe is steel density, kg·m−3; (𝑂) is the oxygen mean of oxide particle size, m; V is volume fraction of oxides; [O]ox is the oxygen content existing as 3 3 oxidescontent in of the oxides. steel; ρox is the oxide density, kg m ; ρ is steel density, kg m ; (O) is the oxygen · − Fe · − ox contentThe of change oxides. of the oxygen content existing as oxides and dissolved oxygen in the steel is shown in FigureThe change 5. At the of thebeginning oxygen of content the RH existing process, as th oxidese oxygen and dissolvedcontent existing oxygen as in oxides the steel in the is shown steel was in Figureabout5 3. Atppm the and beginning decreased of the to 1.4 RH ppm process, after the 10 oxygenmin of contenttreatment existing in RH. as When oxides the in treatment the steel was time aboutreached 3 ppm 23 min, and decreasedthe oxygen to content 1.4 ppm existing after 10 as min oxides of treatment in the steel in RH.barely When changed. the treatment However, time the reachedchange 23of min,the oxygen the oxygen content content existing existing as oxides as oxides in the in steel the steel at the barely beginning changed. and However,end of the theRH change process ofwas the about oxygen 1.7 content ppm, which existing is much as oxides smaller in the than steel the at change the beginning of the T.O and content end of before the RH and process at the was end aboutof the 1.7 RH ppm, process, which which is much was smaller 10–20 ppm. than theThus, change most ofof thethe T.Ooxygen content in the before steel and was at not the removed end of the by RHthe process, removal which of oxides, was but 10–20 due ppm. to the Thus, removal most of ofdissolved the oxygen oxygen in the during steel wasthe RH not process. removed by the removal of oxides, but due to the removal of dissolved oxygen during the RH process.

Figure 5. Change of the content of oxygen existing as oxides and dissolved oxygen in the steel. 4.2. CarbonFigure Deoxidization 5. Change duringof the content the RH of Process oxygen existing as oxides and dissolved oxygen in the steel.

4.2. InCarbon order Deoxidizatio to explain then during removal the of RH dissolved Process oxygen in steel, the component of oﬀ-gas was detected. The existence of CO in oﬀ-gas in Figure4 indicates that there is a C–O reaction that occurred in the RH

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Metals 2019, 9,In 812 order to explain the removal of dissolved oxygen in steel, the component of off-gas9 of 15 was detected. The existence of CO in off-gas in Figure 4 indicates that there is a C–O reaction that occurred in the RH process. However, the small peak value of CO concentration indicates a weak C–O reaction process.during However, the RH the process, small peak which value is ofcaused CO concentration by the very indicateslow content a weak of dissolved C–O reaction oxygen during in the the liquid RH process,steel. which is caused by the very low content of dissolved oxygen in the liquid steel.

4.2.1. The4.2.1. Deoxidization The Deoxidization Ability Ability of Carbon of Carbon during during Diﬀerent Different Vacuum Vacuum Condition Condition OxidesOxides were partlywere partly removed removed due to due the to agitation the agitation and dissolvedand dissolved oxygen oxygen reacting reacting with with carbon carbon in in thethe liquid liquid steel steel where where its its reaction reaction product, product, CO, CO, was was exhausted exhausted out out ofof the RH RH system system to to remove remove oxygen oxygenfrom from liquid liquid steel steel during during the RH process.process. AccordingAccording toto the the detected detected results, results, it it can can be be found found that that the the mainmain oxidization oxidization process process was was the reactionthe reaction of dissolved of dissolved oxygen oxygen with carbonwith carbon when when there wasthere high was high carboncarbon content content in the steelin the during steel during the RH the process. RH process. The C–O The equilibrium C–O equilibrium and deoxidization and deoxidization ability ability of of the RHthe process RH process in high-carbon in high-carbon steel will steel be will discussed be discussed here. here. DuringDuring the RH the process, RH process, the deoxidization the deoxidization reactions reactions can be can represented be represented by by

[C][C] + [O][O] == COCO (g) (g), , (5) (5)

∆GѲ = –23642364 – 39.6339.63TT , ,(6) (6) − − where where∆G is ∆ theGѲ standardis the standard Gibbs Gibbs free energy, free energy, J/mol; andJ/mol;T isand the T temperature, is the temperature,K. K. The equilibriumThe equilibrium constant constantK is expressed K is expressed by by P P K = CO = CO , PCOa ·a f ·f ·[C%][O%]PCO (7) K = C O= C O , (7) aC aO fC fO [C%][O%] where PCO is the partial pressure of ·CO, Pa; a·C is· the activity of element C in the liquid steel; aC is the where activityPCO is the of partialelement pressure O in the of liquid CO, Pa; steel;aC is [C%] the activity is the mass of element fraction C inof theelem liquident C steel; in theaC liquidis the steel; activity[O%] of element is the Omass in the fraction liquid of steel; element [C%] isO thein the mass liquid fraction steel; of elementfC and fO C are in thethe liquid activity steel; coefficients [O%] of is the masselements fraction C and of elementO in the O liquid in the steel, liquid respectively. steel; f C and f O are the activity coeﬃcients of elements C and O in theWhen liquid the steel, concentration respectively. of carbon is 0.02–2wt%, fC and fO can be approximately equal to 1. Thus, WhenEquation the concentration (7) can be expressed of carbon by is 0.02–2wt%, f C and f O can be approximately equal to 1. Thus,

Equation (7) can be expressed by PCO PCO K=P = P . (8) K = aCOC·aO =[C%][O%]CO . (8) a a [C%][O%] When T = 1873 K, there is C· O When T = 1873 K, there is [C%][O%] = 0.0025PCO . (9) [C%][O%] =0.0025PCO. (9) Extra carbon in liquid steel can react with oxygen and CO as the reaction product will be Extra carbon in liquid steel can react with oxygen and CO as the reaction product will be exhausted exhausted out of the vacuum chamber during the RH process. It means that carbon becomes out of the vacuum chamber during the RH process. It means that carbon becomes deoxidizer in a deoxidizer in a vacuum environment and its deoxidization ability increases with the increase of vacuum environment and its deoxidization ability increases with the increase of vacuum degree. vacuum degree. Figure6 shows the equilibriums of [C], [Al], [Si], and [O] in liquid steel at 1873 K during di ﬀerent Figure 6 shows the equilibriums of [C], [Al], [Si], and [O] in liquid steel at 1873 K during different vacuum degrees. The reaction of [Al] and [O] can be expressed by vacuum degrees. The reaction of [Al] and [O] can be expressed by

2[Al] + 3[O]3[O] == AlAl22OO3(s)3(s), , (10) (10)

∆∆GGѲ == –1218799-1,218,799 ++ 394.13394.13TT. .(11) (11) The reactionThe reaction of [Si] andof [Si] [O] and can [O] be can expressed be expressed by by

[Si] + 2[O] = SiO2(s) , (12) [Si] + 2[O] = SiO2(s), (12) ∆∆G Ѳ = –594285-594,285 ++ 229.76229.76TT. .(13) (13)

ConsideringConsideringf Al = f SifAl= = ffSiO == fO1, = and1, and when whenT = T1873 = 1873 K, K, there there are are

1.5 −7 Al%O%1.5 =2.00×10 , 7 (14) [Al%][O%] = 2.00 10− , (14) × 2 5 [Si%][O%] = 2.45 10− . (15) × Metals 2019, 9 FOR PEER REVIEW 10

[ ][ ]2 −5 Metals 2019, 9, 812 Si% O% =2.45×10 . (15) 10 of 15 It can be seen from Figure 6 that the content of deoxidizer in liquid steel determined the oxygen content in liquid steel when the reaction reached equilibrium, where the oxygen content was less It can be seen from Figure6 that the content of deoxidizer in liquid steel determined the oxygen when the content of deoxidizer was more. The deoxidization ability of aluminum, which is a kind of content in liquid steel when the reaction reached equilibrium, where the oxygen content was less strong deoxidizer, was obviously better than the deoxidization ability of Si. When it comes to C, its when the content of deoxidizer was more. The deoxidization ability of aluminum, which is a kind deoxidization ability was related to the value of PCO, or vacuum degree in the RH process. When the of strong deoxidizer, was obviously better than the deoxidization ability of Si. When it comes to C, vacuum degree was less than 1.013 × 104 Pa, the deoxidization capability of carbon was better than its deoxidization ability was related to the value of P , or vacuum degree in the RH process. When silicon; when the vacuum degree reached 67 Pa, thCOe deoxidization capability of carbon was even the vacuum degree was less than 1.013 104 Pa, the deoxidization capability of carbon was better than better than aluminum. As for this high-carbon× steel, of which carbon content is about 1 wt%, the silicon; when the vacuum degree reached 67 Pa, the deoxidization capability of carbon was even better oxygen content in liquid steel can even reach to 10−6 wt%. than aluminum. As for this high-carbon steel, of which carbon content is about 1 wt%, the oxygen 6 content in liquid steel can even reach to 10− wt%.

Figure 6. The equilibriums of [C], [Al], [Si], and [O] in liquid steel at 1600 ◦C during different vacuum degrees. Figure 6. The equilibriums of [C], [Al], [Si], and [O] in liquid steel at 1600 °C during different vacuum However,degrees. the T.O content at the end of the RH process, which was measured from pail-samplers, 6 is 8–10 ppm, which is much more than 10− wt%. The reasons might be (1) the small content of oxygen However, the T.O content at the end of the RH process, which was measured from pail-samplers, reacted with carbon, leading to the remaining oxygen content, and (2) the existence of other oxygen is 8–10 ppm, which is much more than 10−6 wt%. The reasons might be (1) the small content of oxygen source and transfer oxygen to liquid steel. reacted with carbon, leading to the remaining oxygen content, and (2) the existence of other oxygen In Figure4, it can be seen that the flow rate remained stable when the vacuum chamber reached source and transfer oxygen to liquid steel. minimal value and the removal rate of CO (vRCO, mol/s) can be calculated by the off-gas flow rate and In Figure 4, it can be seen that the flow rate remained stable when the vacuum chamber reached CO content (CO%) in the off-gas, shown in Figure7. The removal weight of oxygen ( wRO, ppm) can be minimal value and the removal rate of CO (vRCO, mol/s) can be calculated by the off-gas flow rate and expressed by CO content (CO%) in the off-gas, shown in FigureR t 7. The removal weight of oxygen (wRO, ppm) can 16 vRCOdt be expressed by w = 0 , (16) RO w t steel 16 0 vRCOdt w = 8 ,R t (16) where w is the weight of steel in ladle,RO 1.2 w10 g; v dt is the cumulative CO removal amount, steel × steel 0 RCO 10,132 mol. 8 where wsteel is the weight of steel in ladle, 1.2 × 10 g; 𝑣𝑑𝑡 is the cumulative CO removal amount, The calculated removal weight of oxygen is 1351 ppm, which is much more than the T.O content 10,132 mol. in the steel, meaning that there is no possibility of reason (1). Thus, the existence of other oxygen source should be discussed.

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Metals 2019, 9 FOR PEER REVIEW 9 Figure 7. Removal rate of CO and cumulative CO removal amount. 4.2.2.In Eff ectorder of OxidizingtoFigure explain 7. Removal RH the Slag removal rate on Deoxidizationof ofCO dissolved and cumulative oxygen CO inremoval steel, amount.the component of off-gas was detected. The existence of CO in off-gas in Figure 4 indicates that there is a C–O reaction that occurred inThe theUsually, calculated RH process. the removal cleanliness However, weight of the steel of small oxygen is significantly peak is value1351 ppm, of aff COected which concentration by theis much composition moreindicates than of a topthe weak T.O slag, C–O content especially reaction in thetheduring steel,w(FeO) themeaning inRH the process, top that slag there which during is nois the causedpossibility RH process by the of thatreasonvery a lowff ects(1). content theThus, T.O theof content. dissolvedexistence In order oxygenof other to considerin oxygen the liquid the sourceexistencesteel. should of be other discussed. oxygen source, the top slag is considered as the most possible source [18]. w(FeO) is the index to evaluate the oxidability of slag. When w(FeO) is more, the oxidability of slag is higher and 4.2.2.the4.2.1. Effect liquid The of steel DeoxidizationOxidizing is easier RH to AbilitySlag be oxidized, on ofDeoxidization Carbon which during will result Different in the Vacuum deterioration Condition of cleanliness of steel. In the RH process, the following reactions will occur between top slag and steel. Usually,Oxides the were cleanliness partly ofremoved steel is significantlydue to the agitation affected and by thedissolved composition oxygen of reactingtop slag, with especially carbon in the liquid steel where its reaction product, CO, was exhausted out of the RH system to remove oxygen the w(FeO) in the top slag during the RH process(FeO) that= [O] affects+ [Fe], the T.O content. In order to consider the (17) existencefrom ofliquid other steel oxygen during source, the RH the process.top slag Accordingis considered to theas the detected most possible results, itsource can be [18]. found w(FeO) that the is themain index oxidization to evaluate process the oxidability was the ofreaction slag.x[M] When +ofy [O]di wssolved(FeO)= (Mx O isoxygeny more,). thewith oxidability carbon when of slag there is higher was (18)high carbon content in the steel during the RH process. The C–O equilibrium and deoxidization ability of and the Oxygenliquid steel will is get easier into to the be liquid oxidized, steel which and oxidize will result elements in the through deterioration these reactions. of cleanliness Thus, of T.O the RH process in high-carbon steel will be discussed here. steel.content in steel will increase. It can be seen from Table2 that most components of the top slag were During the RH process, the deoxidization reactions can be represented by unchanged,In the RH process, while w (FeO)the following decreased reaction 0.441s wt% will indicatingoccur between that Equationstop slag and (17) steel. and (18) occurred and the oxygen entering the liquid steel(FeO)[C] led = +[O] to [O] the+ =[Fe] re-oxidizationCO , (g) , of liquid steel. (17) (5) The equilibrium relationship between FeO and liquid steel can be expressed as Equation (17). The equilibrium constants K andx[M]∆ +G yѲ[O] =are –2364 = expressed (M x–O 39.63y) . byT , (18) (6) whereOxygen ∆G willѲ is theget standardinto the liquid Gibbs steelfree energy,and oxidiz J/mol;a e aelements and T is throughthe temperature, these reactions. K. Thus, T.O [O]· [Fe] content inThe steel equilibrium will increase. constant It can K be is seenexpressed fromK = Tableby 2 that, most components of the top slag were (19) aFeO unchanged, while w(FeO) decreased 0.441PCO wt% indicatingPCO that Equations (17) and (18) occurred and K = = , (7) the oxygen entering the liquid steel led toaC∆· aGtheO =re-oxidizationf 117,733.7·f ·[C%][O%] 49.85 of liquidT. steel. (20) C O − The equilibrium relationship between FeO and liquid steel can be expressed as Equation (17). whereAccording PCO is the to partial Equation pressure (19), the of CO, activity Pa; ofaC dissolvedis the activity oxygen of element is related C in to the the liquid activity steel; of [Fe] aC is and the The equilibrium constants K and ∆GѲ are expressed by FeOactivity in slag of element at a certain O in temperature. the liquid steel; However, [C%] is in the mass bulk offraction liquid of steel, elem theent activity C in the of liquid [Fe] can steel; be a[O]·a[Fe] considered[O%] is the as mass 1. Thus, fraction the activity of element ofK = dissolved O in the, oxygen liquid is steel; only relatedfC and f toO are the activitythe activity of FeO coefficients in slag at of a a (19) certainelements temperature. C and O in According the liquid tosteel, the respectively. thermodynamicFeO data of Equation (20), the relationship between the activityWhen ofthe FeO concentration and the activity∆G ofѲ =carbon 117733.7 of oxygen is 0.02–2wt%, − 49.85 canT be. calculatedfC and fO can as be shown approximately in Figure8 equal(20). As shown to 1. Thus, in TableEquation2, there (7) wascan 0.9%–1.4%be expressed FeO by in refining slag, while the w(FeO) in slag is usually in the range of According to Equation (19), the activity of dissolved oxygen is related to the activity of [Fe] and 0.5–1.5 wt% meaning the mole fractionP of FeOP is in the range of 0.0044–0.013. K= CO = CO . (8) FeO in slag at a certain temperature. However,aC·aO [C%][O%] in the bulk of liquid steel, the activity of [Fe] can be When T = 1873 K, there is

[C%][O%] = 0.0025PCO . (9) Extra carbon in liquid steel can react with oxygen and CO as the reaction product will be exhausted out of the vacuum chamber during the RH process. It means that carbon becomes deoxidizer in a vacuum environment and its deoxidization ability increases with the increase of vacuum degree. Figure 6 shows the equilibriums of [C], [Al], [Si], and [O] in liquid steel at 1873 K during different vacuum degrees. The reaction of [Al] and [O] can be expressed by

2[Al] + 3[O] = Al2O3(s) , (10)

∆GѲ = –1218799 + 394.13T. (11) The reaction of [Si] and [O] can be expressed by

[Si] + 2[O] = SiO2(s) , (12)

∆GѲ = –594285 + 229.76T. (13)

Considering fAl = fSi = fO = 1, and when T = 1873 K, there are

Al%O%1.5=2.00×10−7 , (14)

Metals 2019, 9 FOR PEER REVIEW 12 considered as 1. Thus, the activity of dissolved oxygen is only related to the activity of FeO in slag at a certain temperature. According to the thermodynamic data of Equation (20), the relationship between the activity of FeO and the activity of oxygen can be calculated as shown in Figure 8. As shown in Table 2, there was 0.9%–1.4% FeO in refining slag, while the w(FeO) in slag is usually in the range of 0.5–1.5 wt% meaning the mole fraction of FeO is in the range of 0.0044–0.013. Some scholars [19,20] have investigated the activity of FeO in slag when the basicity of slag is 0.8–1.3, as shown in Figure 8. When the mole fraction of FeO is in the range of 0.0044–0.013, the activity values is in the range of 0.01–0.08. Thus, it can be seen in Figure 8 that when the activity of FeO was 0.01, the activity of oxygen was 21 ppm and it increased with the increase of the mole fraction of FeO, indicating that the activity of oxygen was more than 21 ppm in this paper. However, the activity of dissolved oxygen in liquid steel was less than 10 ppm according to detected results, which is much smaller than the oxygen controlled by the equilibrium relationship between FeO and liquid steel. Therefore, the top slag would be the source of oxygen and transfer the oxygen into liquid steel. Metals 2019, 9, 812 12 of 15

Figure 8. Relationship between activity of FeO and activity of oxygen and FeO content in the CaO–SiO –Al O –FeO–MgOsat slag system (B = 0.8 to 1.3 and T = 1823 to 1873 K), data from [19]. Figure 8. Relationship2 2 3 between activity of FeO and activity of oxygen and FeO content in the CaO– SiOSome2–Al2 scholarsO3–FeO–MgOsat [19,20] slag have system investigated (B = 0.8 to the 1.3 activityand T = 1823 of FeO to 1873 in slag K), data when from the [19]. basicity of slag is 0.8–1.3, as shown in Figure8. When the mole fraction of FeO is in the range of 0.0044–0.013, the activity 4.3. Behavior of [O] during RH Process values is in the range of 0.01–0.08. Thus, it can be seen in Figure8 that when the activity of FeO wasIn 0.01 the, theRH activityprocess, of the oxygen change was of oxygen 21 ppm mainly and it increased exists in three with theprocesses, increase as of shown the mole in Figure fraction 9a: of (1)FeO, Deoxidization indicating that reaction the activity and the of removal oxygen wasof CO(g more) in than a vacuum 21 ppm chamber, in this paper. (2) oxygen However, massthe transfer activity processof dissolved between oxygen liquid in steel liquid out steel from was a vacuum less than ch 10amber ppm accordingand in ladle, to detectedand (3) oxygen results, mass which transfer is much betweensmaller ladle than slag the oxygenand liquid controlled steel. by the equilibrium relationship between FeO and liquid steel. Therefore,(1) Deoxidization the top slag reaction would be mainly the source takes of place oxygen in a andvacuum transfer chamber the oxygen that involves into liquid the following steel. three reaction sites: (a) Spontaneous CO bubbles formed in the bulk steel; (b) Ar bubbles from the Ar blowing4.3. Behavior in the of upper [O] during snorkel; RH Process(c) bath surface exposed in the ultra-low-pressure environment. The reactionIn product, the RH process, CO (g), thewill change be exhausted of oxygen due mainly to the vacuum exists in conditio three processes,n promoting as shown the reaction. in Figure 9a: (1) Deoxidization(2) There is part reaction of the and liquid the st removaleel in the of ladle CO(g) that in ais vacuum nearly immobile chamber, when (2) oxygen the circulation mass transfer of RHprocess agitates between liquid liquidsteel, which steel out is called from athe vacuum dead zone chamber in ladle. and The in ladle,oxygen and content (3) oxygen of the mass liquid transfer steel outbetween from the ladle vacuum slag and chamber liquid steel.is relatively lower than that in the dead zone in the ladle because of the deoxidization reaction that occurred in the vacuum chamber. The gradient of oxygen results in the(1) oxygenDeoxidization mass transfer reaction between mainly dead takes zone place and stirring in a vacuum zone. chamber that involves the following three reaction sites: (a) Spontaneous CO bubbles formed in the bulk steel; (b) Ar bubbles

from the Ar blowing in the upper snorkel; (c) bath surface exposed in the ultra-low-pressure environment. The reaction product, CO (g), will be exhausted due to the vacuum condition promoting the reaction. (2) There is part of the liquid steel in the ladle that is nearly immobile when the circulation of RH agitates liquid steel, which is called the dead zone in ladle. The oxygen content of the liquid steel out from the vacuum chamber is relatively lower than that in the dead zone in the ladle because of the deoxidization reaction that occurred in the vacuum chamber. The gradient of oxygen results in the oxygen mass transfer between dead zone and stirring zone. (3) The oxygen in the slag significantly affects the deoxidization effect in the RH process as explained above. The oxygen mass transfer between the ladle slag and liquid steel mainly occurred due to the reactions of Equations (17) and (18). Some researchers [21] used double-film theory to explain the re-oxidization process, which is shown in Figure9c. According to the theory, the activity Metals 2019, 9 FOR PEER REVIEW 13

(3) The oxygen in the slag significantly affects the deoxidization effect in the RH process as explainedMetals 2019, 9above., 812 The oxygen mass transfer between the ladle slag and liquid steel mainly occurred13 of 15 due to the reactions of Equations (17) and (18). Some researchers [21] used double-film theory to explain the re-oxidization process, which is shown in Figure 9c. According to the theory, the activity of oxygen at the boundary layer depends on the Fe–O equilibrium at the steel slag interface, of oxygen at the boundary layer depends on the Fe–O equilibrium at the steel slag interface, while while the concentration of oxygen in liquid steel depends on the M–O equilibrium. The M–O the concentration of oxygen in liquid steel depends on the M–O equilibrium. The M–O equilibrium equilibrium cannot be established because of the deoxidization of RH process with the vacuum cannot be established because of the deoxidization of RH process with the vacuum condition. Thus, condition. Thus, a new M–O equilibrium can be established only when [M] in steel reacts with O a new M–O equilibrium can be established only when [M] in steel reacts with O in slag continuously, in slag continuously, which results in the mass transfer of oxygen from ladle slag to liquid steel. which results in the mass transfer of oxygen from ladle slag to liquid steel.

Figure 9. (a) Schematic drawing of the reaction mechanisms of the decarburization process in the RH; Figure(b) oxygen 9. (a) mass Schematic transfer drawing from dead of the zone reaction to stirring mechanisms area inthe of the ladle; decarburization (c) double film process theory: in The the massRH; (transferb) oxygen from mass (FeO) transfer to [O]. from dead zone to stirring area in the ladle; (c) double film theory: The mass transfer from (FeO) to [O]. In the RH process, the change of oxygen, including the deoxidization rate, mainly depends on the mass transfer of the carbon and oxygen in the liquid steel. It has been reported [11] that the value of In the RH process, the change of oxygen, including the deoxidization rate, mainly depends on the[C] /mass[O] a transferffects the of rate-determining the carbon and oxygen step of in mass the liquid transfer. steel. While It has the been [C] reported content in [11] the that steel the is value about of1wt% [C]/[O] and affects the [O] the content rate-determining is about 20~30 step ppmof mass after transfer. the LF While refining the process,[C] content the in value the steel of [C] is /about[O] in 1wt%the steel and isthe much [O] content more than is about 0.52. 20~30 Thus, ppm the after mass the transfer LF refining of the process, oxygen the in value the liquid of [C]/[O] steel in is the the steelrate-determining is much more step. than 0.52. Thus, the mass transfer of the oxygen in the liquid steel is the rate- determiningAt the beginningstep. of RH, as the pressure in the vacuum chamber decreased, the C–O reaction startedAt the in the beginning vacuum of chamber, RH, as the and pressure the oxygen in th contente vacuum of the chamber liquid steeldecreased, in the the vacuum C–O chamberreaction startedkept decreasing. in the vacuum The liquid chamber, steel and with the low-oxygen oxygen co contentntent of inthe the liquid vacuum steel chamber in the vacuum entered chamber the ladle keptas the decreasing. flow cycle The progressed. liquid steel Compared with low-oxygen with the deadcontent zone, in the the vacuum oxygen contentchamber in entered the stirring the ladle zone aswas the lower, flow cycle forming progressed. a concentration Compared gradient with of the oxygen dead zone, and causing the oxygen oxygen content to transfer in the fromstirring the zone dead waszone lower, to the forming stirring a zone. concentration At the same gradient time, theof oxyg topen slag and kept causing transmitting oxygen oxygento transfer to thefrom liquid the dead steel zoneas described to the stirring above. zone. Due At to the the same strong time, agitation the top of slag the RHkept process, transmitting the mass oxygen transfer to the of liquid oxygen steel in asthe described liquid steel above. was Due accelerated, to the strong and the agitation oxygen of at the the RH slag–steel process, interface the mass was transfer continuously of oxygen di inffused the liquidto the steel low-oxygen-content was accelerated, zone, and the which oxygen accelerates at the slag–steel the oxygen interface transfer was process continuously from the diffused slag to theto theliquid low-oxygen-content steel. zone, which accelerates the oxygen transfer process from the slag to the liquidIn steel. the initial stage of the C–O reaction, oxygen in the liquid steel was the main source of oxygen in theIn reaction.the initial At stage this of time, the C–O oxygen reaction, mass transferoxygen in in the the liquid liquid steel steel was was the the main rate-determining source of oxygen step. inThe the stirring reaction. in theAt RHthis processtime, oxygen greatly mass accelerated transfer the in massthe liquid transfer steel of was oxygen. the rate-determining Thus, the reaction step. rate Thewas stirring fast at thein the beginning RH process of RH, greatly and theaccelerated CO content the mass in the transfer off-gas rapidlyof oxygen. increased. Thus, the When reaction the C–Orate wasreaction fast at proceeded the beginning to a certain of RH, extent, and the the CO oxygen conten contentt in the inoff-gas the liquid rapidly steel increased. in the stirring When zonethe C–O was significantly reduced, and the reaction rate was significantly reduced. At this time, the oxygen mass transfer in the slag–steel interface became the rate-determining step of the removal of oxygen. Metals 2019, 9, 812 14 of 15

5. Conclusions Industrial experiments were carried out to investigate the deoxidization in the RH process of GCr15 bearing steel. T.O and characterization of inclusions in whole smelting process, slag composition, and oﬀ-gas during RH process were investigated. The following conclusions were obtained:

(1) The content change of the combined oxygen during the RH process was calculated. Compared with the change of T.O, it is found that the decrease of the oxygen content is not mainly caused by the removal of oxides, but caused by the removal of dissolved oxygen during RH process. (2) Carbon deoxidization exists in the RH process in high-carbon steel with low oxygen content, and carbon is found to be a strong deoxidizer. In high vacuum degree, the deoxidizing capacity of carbon is even stronger than Al. (3) The oxygen is transferred from the top slag to the liquid steel during the deoxidization process, leading to the re-oxidization of liquid steel despite the low FeO content in top slag. (4) During the RH process, the change of the oxygen content mainly exists in three processes: 1) Deoxidization reaction in the vacuum chamber, 2) oxygen mass transfer process between liquid steel out from the vacuum chamber and in the ladle, and 3) oxygen mass transfer between the ladle slag and liquid steel. The changes in the oxygen content mainly depends on the mass transfer of the oxygen in the liquid steel.

Author Contributions: Data curation, W.X.; writing—original draft preparation, W.X.; writing—review and editing, W.X., M.W. and Y.B.; supervision, Y.B. Funding: This research was funded by the State Key Laboratory for Advanced Metallurgy Foundation (No. 41618030). Conﬂicts of Interest: The authors declare no conﬂict of interest.

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