metals

Article Revolution and Control of Fe-Al-(Mg, Ti)-O Oxide Inclusions in IF Steel during 260t BOF-RH-CC Process

1,2 2 2 2 1 1 Rijin Cheng , Renchun Li , Di Cheng , Junshan Liu , Qing Fang , Jian0an Zhou , Wenliang Dong 3, Hua Zhang 1,* and Hongwei Ni 1,* 1 The State Laboratory of Refractories and , Wuhan University of Science and Technology, Wuhan 430081, China; [email protected] (R.C.); [email protected] (Q.F.); [email protected] (J.Z.) 2 Technical Center, HBIS Group Hansteel Company, Handan 056001, China; [email protected] (R.L.); [email protected] (D.C.); [email protected] (J.L.) 3 Research Institute of Technology, Shougang Group Co., Ltd., Beijing 100043, China; [email protected] * Correspondence: [email protected] (H.Z.); [email protected] (H.N.); Tel.: +86-27-68862811 (H.Z. & H.N.)  Received: 1 April 2020; Accepted: 16 April 2020; Published: 19 April 2020 

Abstract: The evolution of inclusions that contain Al, Mg, and Ti was studied through industrial-grade experiments. Field emission scanning electron microscopy, energy dispersive spectrometry, inductively coupled atomic emission spectrometry, and FactSage software were used to analyze the evolution mechanisms of inclusions in Al-killed alloyed interstitial free (IF) steel. The research found that the evolution of inclusions during the smelting process of IF steel is results in ‘large sphere-like SiO2-CaO-FeO-MgO-MnO’ and ‘small cluster spherical FeO-MnO’ change to cluster-like Al O and irregular MgO Al O , then change to Al O TiOx and Al O , and finally change 2 3 · 2 3 2 3· 2 3 to Al O . It is difficult for Al O TiOx to stably exist in the IF molten steel. It is the key to extend 2 3 2 3· the holding time properly after Ruhrstahl Heraeus (RH) to ensure the removal of Al2O3 inclusion. With the increase of Mg content, the change path of MgAl2O4 inclusion in IF steel is that Al2O3 changes to MgO Al O , and finally changes to MgO. It is difficult to suppress MgO Al O spinel · 2 3 · 2 3 formation by controlling the in the steel, but Ca can modify part of the MgO Al O spinel · 2 3 inclusions during RH refining. In order to ensure the removal of 6–10 µm inclusions, the holding time is suitable for 19–42 min.

Keywords: IF steel; inclusion evolution; thermodynamics; MgAl O inclusion; Al O TiOx 2 4 2 3·

1. Introduction Ultra-low interstitial-free (IF) steel is widely used in automobile plate production because of its excellent deep draw ability and uniform mechanical properties. With the increasing quality requirements of cold rolling sheet, the control requirements of inclusions in ultra-low are increasingly strict [1,2]. In the smelting process of ultra-low carbon IF steel, a certain amount of titanium, niobium, and other elements should be added, and the interstitial atoms, such as carbon and nitrogen, in ultra-low carbon steel should be completely fixed as carbon nitrogen compounds, so as to obtain clean ferritic steel without interstitial atoms [3]. According to the difference in the chemical composition of the inclusions, the inclusions in ultra-low carbon steel can be divided into Al2O3 inclusions, Al2O3-TiOx inclusions, and CaO-Al2O3-MgO inclusions [4–7]. Such inclusions can easily block immersion nozzles during continuous casting, and can also cause defects in the product. The study shows that Al2O3-TiOx inclusions are closely related to the clogging of the nozzle and the final product defect [8].

Metals 2020, 10, 528; doi:10.3390/met10040528 www.mdpi.com/journal/metals Metals 2020, 10, 528 2 of 13

Al2O3-TiOx inclusions are easily formed when the content of FeO is high [4], and the inclusions are unstable [6]. The [Al] and [Ti] react with the [O] from the self-dissociation of SiO2 in the and form inclusions changing from solid Al2O3 to Al2O3-TiOx inclusions. The inclusion chemistry composition is mainly dependent on the w([Al]) and w([Ti]), while the number of inclusions can be reduced by increasing the ratios of CaO to SiO2 and CaO to Al2O3 in the slag [9]. Rare earth Ce is usually used to modify spinel inclusions in steel [10]. The irregular Al2O3 inclusions will be modified by the rare earth element Ce in Al-killed titanium-alloyed IF steel. The irregular Al2O3 inclusions of 10–15 µm are wrapped by rare earth, and gradually transformed into spherical CeAlO3, Ce2O3, and Ce2O2S inclusions of 5 µm, and finally dispersed into IF slabs [11]. When T[O] in steel is reduced to 0.001%, the spinel inclusions in molten steel become the most important factor affecting the purity of steel [12]. The MgO Al O spinel will be modified by Ca · 2 3 treatment. The inclusion modification route is Al O change to MgO Al O , and finally change to 2 3 · 2 3 liquid complex inclusions [13]. Mg migrates to the molten steel and MgAl2O4 spinel inclusion is formed due to a reaction between Mg and Al2O3 inclusions. The spinel inclusion changes entirely into liquid oxide inclusion via the transfer of Ca from slag to metal. The modification reaction is more efficient as the SiO2 content in the slag decreases [14]. When the content of Al in steel is constant, with the increase of Mg content in steel, the inclusions in steel continuously precipitate MgAl2O4 spinel and finally changes into liquid MgO [15]. It is very important to control the content of Mg and Al in the alloy and to prevent the secondary oxidation of molten steel [16]. In summary, the inclusions have an important impact on the blockage of immersion nozzles and product quality of ultra-low carbon steel. In actual production, the molten steel in the basic oxygen furnace (BOF)-Ruhrstahl Heraeus (RH)-continuous casting (CC) process was sampled, and the characteristics, formation and evolution process, formation mechanism, and influencing factors of the inclusions were analyzed in detail to provide a theoretical basis for the control of inclusions in ultra-low carbon steel. Finally, the mechanical properties of DC06 IF steel were tested, and it was confirmed that the improved IF steel met the requirements of relevant steel grades. The experimental results have a complete understanding of the evolution process of inclusions in IF steel, provide guidance for the evolution and control of MgO Al O spinel inclusions and MgO TiOx inclusions in steel, and put · 2 3 · forward the process parameters to reduce the number and size of inclusions in steel. This is of great significance for improving the control level of non-metallic inclusions in automobile plate steel.

2. Materials and Methods

2.1. Materials DC06 IF steel was produced by the 260 t BOF-RH-Holding-CC process at the Hanbao steel plant. During tapping, 600 kg lime was added into the BOF, and 680 kg aluminum slag deoxidant was added onto the top slag after tapping. RH vacuum treatment includes , followed by aluminum deoxidization, then 338 kg Ti-Fe containing 70% Ti alloying and keeping RH pure circulation time for 8–10 min. After RH treatment, molten steel remained unstirred in the ladle for 25–45 min before casting. The standard for judging the chemical composition of DC06 IF steel is shown in Table1.

Table 1. The standard for judging the chemical composition of IF steel, wt%.

C Si Mn P S Als Ti B N 0.0020 0.010 0.08–0.14 0.012 0.009 0.020–0.050 0.065–0.075 0.0003–0.0008 0.003 ≤ ≤ ≤ ≤ ≤

In order to investigate the evolution of inclusions in the IF steel smelting process, samples were taken by samplers of Φ 30 mm 10 mm during the industrial experiment process, and the × whole industrial experiments were carried out over 6 heats in total. A schematic diagram of charging and taking specimens during the steelmaking processes is shown in Figure1. Metals 2020, 10, x FOR PEER REVIEW 3 of 13

Metalsthe whole2020, 10 industrial, 528 experiments were carried out over 6 heats in total. A schematic diagram3 of 13of charging and taking specimens during the steelmaking processes is shown in Figure 1.

Vacuum End Al Mn-Fe、 Vacuum Al slag Tundish start De-C add Ti-Fe add end

BOF RH CC

Sample Sample Sample Sample Sample Sample Sample 1 2 3 4 5 6 7

Figure 1. Schematic of charging and taking specimens during steelmaking processes. Figure 1. Schematic of charging and taking specimens during steelmaking processes 2.2. Mechanical Property Experiment 2.2. MechanicalDuring the Property tensile Exper experiment,iment the DC06 IF steel coil transverse sample was taken, and the steelDuring plate was the tensile processed experiment, into a dumbbell-shaped the DC06 IF steel samplecoil transverse by using sample the Zwickwas taken, 2Z50 and (Ennepeta, the steel Nordrhein-Westfalen,plate was processed Germany)into a dumbbell sample preparation-shaped sample machine. by Theusing initial the gauge Zwick lengths 2Z50 of ( theEnnepeta, tensile samplesNordrhein l0 -andWestfalen b0 are, 80 Germany mm and) sample 20 mm, preparation respectively. machine. Then the The XKA714C initial (BYJC,gauge Beijing,lengths China)of the automatictensile samples numerical l0 and control b0 are machine80 mm and was 20 used mm, to respectively. mill the edge Then of the the sample XKA7 to14C make (BYJC, it meet Beijing, the standardChina) automatic GB/T228.1-2010. numerical According control machine to the inspection was used standard to mill the GB edge/T5213-2019, of the sample the Zwick to make automatic it meet tensilethe standard testing machineGB/T228.1 (Model-2010. Z150roboAccording Test to L, the Ennepeta, inspection Nordrhein-Westfalen, standard GB/T5213 Germany)-2019, the was Zwick used toautomatic test the tensiletensile test testing samples machine to obtain (M mechanicalodel Z150robo performance Test L data,, Ennepeta, whereby 700Nordrhein sets of tensile-Westfalen tests, wereGermany carried) was out used on DC06 to test IF the steel, tensile and test the averagesamples valueto obtain of mechanical mechanical properties performance was data obtained., whereby 700 sets of tensile tests were carried out on DC06 IF steel, and the average value of mechanical 2.3. Methods of Chemical Analysis properties was obtained. Each steel sample on the cross-sectional had been ground and polished by SiC papers and w1.52.3. M diamondethods of C suspensions.hemical Analysis The sample should be ground and polished in the way of transverse andEach longitudinal steel sample intersections, on the cross and-sectional inclusions had been of each ground steel and sample polished were by SiC detected papers using and w1.5 FEI Novadiamond NanoSEM400 suspensions (FEI,. The Hillsboro, sample sh OR,ould USA) be ground scanning and electron polished microscopy in the way (SEM) of transverse coupledwith and energy-dispersivelongitudinal intersection X-ray spectroscopys, and inclusions (EDS, of FEI, each Hillsboro, steel sample USA). were The number, detected size, using and FEI chemical Nova compositionNanoSEM40 of0 inclusions(FEI, Hillsboro were analyzed, OR, USA automatically) scanning usingelectron an ASPEX microscopy scanning (SEM) electron coupled microscope with (ASPEXenergy-dispe SEM,rsive FEI, X- Delmont,ray spectroscopy PA, USA). (EDS The, FEI, concentration Hillsboro, USA of). the The Al, number Ti, and, size Ca, and in steel chemical was determinedcomposition byofIRIS inclusions advantage were radial analyzed inductively automatically coupled plasmausing an atomic ASPEX emission scanning spectrometry electron (ICP-AES,microscope Thermo (ASPEX Elemental, SEM, FEI, MA, PA USA)., USA). The The [O] concentration concentration of in the steel Al, was Ti,measured and Ca in online steel bywas a MSO-3690oxygendetermined by IRIS sensor advantage (MINCO, radial Harland, inductively WI, USA). coupled plasma atomic emission spectrometry (ICP-AES, Thermo Elemental, MA, USA). The [O] concentration in steel was measured online by a 3. Results and Discussion MSO-3690oxygen sensor (MINCO, Harland, WI, USA). 3.1. Morphology and Evolution of Typical Non-Metallic Inclusions in Molten Steel 3. Results and Discussion Figure2 shows the morphology and evolution of typical inclusions in molten steel. Sample 1 was taken3.1. Morphology from BOF and before Evolution tapping. of Typical The morphology Non-Metallic of Inclusions inclusions in before Molten tapping Steel is shown in Figure2a. Inclusions in BOF steel are spherical. The inclusions in the BOF process are SiO2-CaO-FeO-MgO-MnO Figure 2 shows the morphology and evolution of typical inclusions in molten steel. Sample 1 type multi-phase composite inclusions. Due to the deep decarburization by oxygen blowing in BOF,was taken the molten from steelBOFhas before strong tapping. oxidizability. The morphology Therefore, thereof inclusions are many before cluster tapping spherical is FeO-MnOshown in Figure 2a. Inclusions in BOF steel are spherical. The inclusions in the BOF process are inclusions below 5 µm in the molten steel, and other large sphere-like inclusions below 50 µm are SiO2-CaO-FeO-MgO-MnO type multi-phase composite inclusions. Due to the deep decarburization SiO2-CaO-FeO-MgO-MnO. by oxygen blowing in BOF, the molten steel has strong oxidizability. Therefore, there are many Sample 2 was taken from ladle before vacuum start. Figure2b shows typical inclusions in molten cluster spherical FeO-MnO inclusions below 5 μm in the molten steel, and other large sphere-like steel before vacuum start during RH refining. The spherical and lump inclusions in the steel further inclusions below 50 μm are SiO2-CaO-FeO-MgO-MnO. grow up and float up, and many CaO-SiO2-FeO-Al2O3-MgO-MnO inclusions are in the 50 µm size. Sample 2 was taken from ladle before vacuum start. Figure 2b shows typical inclusions in Due to the addition of Al slag modifier, large particles of Al2O3 above 50 µm appear in the steel. molten steel before vacuum start during RH refining. The spherical and lump inclusions in the steel Sample 3 was taken from ladle after decarburization. After decarburization by RH vacuum refining, further grow up and float up, and many CaO-SiO2-FeO-Al2O3-MgO-MnO inclusions are in the 50 μm typical inclusions in molten steel are shown in Figure2c. Irregular FeO-MnO-MgO and MgO-Al 2O3 size. Due to the addition of Al slag modifier, large particles of Al2O3 above 50 μm appear in the steel. inclusions combine to form approximately 15 µm spherical inclusions or irregular FeO-MnO-MgO and CaO-Al2O3-FeO-SiO2 inclusions combine to form approximately 50 µm nearly spherical inclusions.

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Sample 3 was taken from ladle after decarburization. After decarburization by RH vacuum refining, typical inclusions in molten steel are shown in Figure 2c. Irregular FeO-MnO-MgO and

MgOMetals-2020Al2O, 103 ,inclusions 528 combine to form approximately 15 μm spherical inclusions or irregular4 of 13 FeO-MnO-MgO and CaO-Al2O3-FeO-SiO2 inclusions combine to form approximately 50 μm nearly spherical inclusions. Large inclusions have been partially removed, and the single Al2O3 has been combinedLarge inclusions with other have inclusions been partially to form removed, complex and inclusions. the single Al2O3 has been combined with other inclusionsSample to 4 form was complextaken from inclusions. ladle after adding aluminum. Typical inclusions in molten steel after addingSample aluminum 4 was takenfor 2 frommin are ladle shown after addingin Figure aluminum. 2d. After Typical decarburization inclusions inin moltenthe RH steel refining after process,adding aluminum aluminum for is added 2 minare to the shown molten in Figure steel 2ford. deep After deoxidization. decarburization Due in theto the RH reaction refining between process, aluminum isand added oxygen, to the a large molten number steel for of deepcluster deoxidization.-like or coral- Duelike Al to2 theO3 inclusions reaction between are generated aluminum by reactionand oxygen, (1). aIn large additi numberon, the of inclusions cluster-like in orthe coral-like steel aggregate Al2O3 inclusions and grow are, and generated the lump by MgO·Al reaction2 (1).O3 In addition, the inclusions in the steel aggregate and grow, and the lump MgO Al O spinel inclusions spinel inclusions are wrapped by compact coral-like Al2O3 inclusions. Other ·Ca2-based3 and Si-based inclusionsare wrapped are byrelatively compact rare coral-like after floating Al2O3 andinclusions. removal. Other Ca-based and Si-based inclusions are relatively rare after floating and removal.

(a)-1 (a)-2 (a)-3 Atomic% Atomic% Atomic% O:53.08 O:55.17 O:40.63 Mg:5.36 Mg:1.42 Mn:6.56 Al:1.11 Al:2.71 Fe:52.81 Si:3.41 Si:15.96 Ca:14.35 Ca:12.93 Mn:2.53 Mn:3.78 Fe:20.16 Fe:8.03

(a1) (a2) (a3) (b)-1 (b)-2 Atomic% Atomic% O:67.43 O:62.76 Mg:2.43 Al:37.24 Al:7.09 Si:10.67 Ca:8.99 Mn:0.83 Fe:2.56

(b1) (b2) (c)-1 (c)-2

Atomic% Atomic% O:45.24 O:45.72 Mg:4.36 Mg:30.06 Mn:4.16 Mn:3.09 Atomic% Fe:46.24 Fe:21.24 Atomic% O:53.64 O:56.34 Mg:12.82 Mg:9.15 Al:29.67 Mn:5.16 Mn:1.11 Ca:20.31 Fe:2.76 Fe:9.03

(c1) (c2) (d)-1 (d)-2 (d)-3

Atomic% Atomic% Atomic% O:58.42 O:56.65 O:56.14 Al:41.58 Mg:7.88 Al:38.57 Al:35.47 Fe:5.29

Atomic% O:23.80 Al:53.38 Fe:22.82

(d1) (d2) (d3)

Figure 2. Cont. Metals 2020, 10, 528 5 of 13 Metals 2020, 10, x FOR PEER REVIEW 5 of 13

(e)-1 (e)-2

Atomic% Atomic% O:49.14 O:63.64 Al:20.15 Atomic% Al:36.36 O:61.98 Ti:20.83 Atomic% Fe:9.88 Al:19.44 Ti:18.58 O:56.04 Al:43.96

(e1) (e2) (f)-1 (f)-2 (f)-3

Atomic% Atomic% Atomic% O:59.69 O:56.93 O:34.56 Al:27.64 Atomic% Al:39.54 Al:30.31 Ti:11.53 Fe:3.53 Atomic% O:64.32 Ti:2.34 Fe:1.14 O:62.99 Al:17.42 Fe:32.79 Al:17.07 Ti:17.24 Ti:19.94 Fe:1.01

Atomic% (f1) O:62.99 (f2) (f3) Al:17.07 (g)-1 (Tgi:)1-92.94

Atomic% Atomic% Atomic% O:59.15 O:59.62 O:60.08 Al:40.85 Al:40.38 Al:39.92

(g1) (g2)

FigureFigure 2 2.. TypicalTypical inclusions inclusions in molten in molten steel:steel: (a) Before (a) Beforetapping; tapping; (b) Before (b )vacuum Before start; vacuum (c) A start;fter decarburization;(c) After decarburization; (d) After adding (d) After aluminum adding for aluminum 2 min; (e) for After 2 min;adding (e) titanium After adding for 2 min; titanium (f) After for RH2 min; treatment; (f) After (g RH) From treatment; tundish. (g ) From tundish.

[Al] + [O] = (Al2O3)inclusion (1) [Al ][O]  (Al 2O3)inclusion (1) Sample 5 was taken from ladle after adding titanium alloy. Figure2e shows typical inclusions in moltenSample steel 5 after was RH taken refining from andladle adding after adding titanium titanium alloy. After alloy. adding Figurealuminum 2e shows typical for deoxidization inclusions in molten steel after RH refining and adding titanium alloy. After adding aluminum for during RH refining, a large number of cluster-like Al2O3 inclusions are generated in the steel. With the deoxidization during RH refining, a large number of cluster-like Al2O3 inclusions are generated in floating of Al2O3 inclusions, the total oxygen content in the steel is greatly reduced. Subsequently, theafter steel. adding With Ti-Fe the floating alloying, of someAl2O3 inclusions, [Ti] reacts withthe total [Al] oxygen and [O] content by reaction in the steel (2) in is the grea steeltly reduced. to form Subsequently, after adding Ti-Fe alloying, some [Ti] reacts with [Al] and [O] by reaction (2) in the Al2O3 TiOx inclusions below 10 µm, and a part of [Ti] reacts with [O] to generate TiOx, and the TiOx steel to· form Al2O3·TiOx inclusions below 10 μm, and a part of [Ti] reacts with [O] to generate TiOx, inclusion wraps around the outside of the Al O inclusions to produce irregular lump Al O TiOx 2 3 2 3· andinclusions the TiO byx inclusion reaction (3).wraps around the outside of the Al2O3 inclusions to produce irregular lump Al2O3·TiOx inclusions by reaction (3).

[Ti][Ti]+[Al][Al ]+[[O]O]  (Al(Al2OO3TiOTiOX))inclusion (2) → 2 3 · x inclusion (2) [Ti] + [O] + (Al2O3)inclusion (Al2O3 TiOX)inclusion (3) [Ti][O] (Al 2O3)inclusion →(Al 2O3 TiO· x )inclusion (3) Sample 6 was taken from ladle after RH treatment. After RH refining is completed, typical Sample 6 was taken from ladle after RH treatment. After RH refining is completed, typical inclusions in molten steel are shown in Figure2f. At this time, large-scale Al 2O3 TiOx inclusions in the inclusions in molten steel are shown in Figure 2f. At this time, large-scale Al2·O3·TiOx inclusions in molten steel above 20 µm have floated and removed, but there are still newly generated Al O TiOx the molten steel above 20 μm have floated and removed, but there are still newly generated2 3· Al2O3·TiOx inclusions below 20 μm in the molten steel, and Al2O3·TiOx grows with Al2O3 as the core and wraps Al2O3, and the inclusions are mainly Al2O3·TiOx, except for a few clusters of Al2O3 in the molten steel.

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inclusions below 20 µm in the molten steel, and Al O TiOx grows with Al O as the core and wraps 2 3· 2 3 Al O , and the inclusions are mainly Al O TiOx, except for a few clusters of Al O in the molten steel. 2 3 2 3· 2 3 Sample 7 was taken from tundish during continuous casting. Figure2g shows typical inclusions in the molten steel of tundish. After the molten steel remains unstirred in the ladle for 25–45 min during the holding process, most of the large-particle Al2O3 inclusions have been removed by floating. Since the molten steel is sufficiently homogenized, the Al O TiOx inclusions cannot stably exist in the 2 3· steel, so the Al O TiOx inclusions have decomposed into stable Al O inclusions before entering the 2 3· 2 3 continuous casting mold, and Ti re-enters the molten steel. It can be proven from Figure2g that the inclusions in the final molten steel are mainly Al2O3. Therefore, it is key to ensure the quality of IF steel to extend the holding time properly after RH to ensure the removal of Al2O3 inclusions.

3.2. Thermodynamics of Evolution and Control of Typical Oxide Inclusions during Refining The predominance area diagram of Fe-Al-(Mg, Ti)-O system was calculated by using the thermodynamic software FactSage 7.0 (ThermFact Inc., Montreal, QC, Canada). The evolution and control of typical Fe-Al-(Mg, Ti)-O system inclusions in IF steel smelting process are analyzed by using these diagrams.

3.2.1. Typical Spinel Inclusions in Molten Steel before RH Refining Figure3 shows the predominance area diagram of the Fe-Al-Mg-O system with di fferent oxygen contents at 1600 ◦C. The Fe-Al-Mg-O system is used to study the MgO, MgAl2O4 spinel and Al2O3 phase diagrams obtained from the change of Al content from 1 10 6 to 1% and the change of Mg × − content from 0.1 10 6 to 100 10 6. When the Mg content is lower than the critical line of the Al O × − × − 2 3 and MgO Al O phases, an Al O phase is formed. At this time, the [Mg] content in the molten steel is · 2 3 2 3 veryMetals low,2020, and10, x itFOR is dissolvedPEER REVIEW in the molten steel in the form of elemental Mg. The Mg content is higher7 of 13 than the boundary line of MgO Al O and MgO, and Mg exists as MgO inclusions phase. · 2 3

Figure 3. Predominance area diagram of Fe-Al-Mg-O system with different oxygen contents at 1600 C. Figure 3. Predominance area diagram of Fe-Al-Mg-O system with different oxygen contents at 1600 °C◦ . The steel samples were taken before RH, after RH, and in the tundish, and the samples were taken 3.2.2. Typical Mg-Al Spinel Inclusions in Molten Steel after RH Refining from six heats industrial experiment. The chemical compositions of Al, Ti, and Ca elements in the steel are analyzed,Figure 4 shows and the the average predominance compositions areaare diagram shown of in Fe Table-Al-2Mg. -Ca-O system with 1 × 10−6 Ca and different oxygen contents at 1600 °C. As can be seen from Figure 4, when the content of [Mg] is fixed, the type of inclusions generated in the molten steel is always fixed within a certain range regardless of the change in the [Al] content. Therefore, in the Fe-Al-Mg-O-Ca molten steel system, the type of inclusions is mainly affected by the [Mg] content. In addition, as the [O] content in the molten steel increases, the range of Al corresponding to the formation of MgAl2O4 spinel and Al2O3 phases increases. In other words, the probability of brittle inclusions such as MgO·Al2O3 spinel and Al2O3 increases with increase the [O] content in the steel. However, it is difficult to suppress MgO·Al2O3spinel formation by controlling the oxygen in the steel. In the Figure 4, ‘liquid oxide’ represents CaO-Al2O3-MgO series liquid inclusions, and ‘CxAy’ represents (CaO)x(Al2O3)y. When 1 × 10−6 [Ca] is added to the molten steel, the MgO·Al2O3 spinel area is significantly reduced, a part of the MgO·Al2O3 spinel area is replaced by the ‘liquid oxide’, and the original Al2O3 phase area is also replaced by a part of the ‘liquid oxide’. For the IF steel containing 0.042% Al after RH refining in this experiment, when the content of [Mg] in the steel is less than 0.48 × 10−6, Mg is dissolved in the molten steel and the inclusions in the molten steel are Al2O3 and CxAy type aluminates. The overall area ratio is larger than that without Ca. When content of [Mg] in steel increases to 0.48 × 10−6, MgO·Al2O3 spinel inclusions begin to generate in the molten steel and it also contains Al2O3 and CxAy type calcium aluminates. When the content of [Mg] in the molten steel increases to 1.85 × 10−6, the Al-containing inclusions in the molten steel all exist as MgO·Al2O3 spinel. As the content of [Mg] in the molten steel increases to 10.15 × 10−6, the inclusions in the molten steel include MgO inclusions besides MgO·Al2O3 spinel. When the content of [Mg] in the molten steel increases to 16.39 × 10−6, the Mg-containing inclusions in the molten steel all exist as MgO.

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Table 2. The average chemical compositions of Al, Ti, and Ca elements in the steel, wt%

Process Al Als Ca Ti Before RH 0.472 0.363 – – After RH 0.044 0.042 0.0001 0.077 Ladle holding 0.036 0.033 0.0001 0.070

The ladle slag is modified by Al slag modifier after tapping. Although the Al in the molten steel is about 0.363%, the oxygen in the molten steel is still high. When the content of Mg in the molten steel is 0.54 10 6, MgAl O can stably exist in the steel. When the Mg content in the steel is greater than × − 2 4 18.36 10 6, Mg will coexist as MgAl O and MgO inclusions. When the Mg content in the steel is × − 2 4 larger than 45.79 10 6, Mg exists only as MgO inclusion. Therefore, as long as the Mg concentration × − in the molten steel is (0.54–45.79) 10 6, the MgAl O spinel inclusions will exist in the molten steel. × − 2 4 3.2.2. Typical Mg-Al Spinel Inclusions in Molten Steel after RH Refining Figure4 shows the predominance area diagram of Fe-Al-Mg-Ca-O system with 1 10 6 Ca and × − different oxygen contents at 1600 ◦C. As can be seen from Figure4, when the content of [Mg] is fixed, the type of inclusions generated in the molten steel is always fixed within a certain range regardless of the change in the [Al] content. Therefore, in the Fe-Al-Mg-O-Ca molten steel system, the type of inclusions is mainly affected by the [Mg] content. In addition, as the [O] content in the molten steel increases, the range of Al corresponding to the formation of MgAl2O4 spinel and Al2O3 phases increases. In other words, the probability of forming brittle inclusions such as MgO Al O spinel · 2 3 andMetals Al 20202O, 310increases, x FOR PEER with REVIEW increase the [O] content in the steel. However, it is difficult to suppress8 of 13 MgO Al O spinel formation by controlling the oxygen in the steel. · 2 3

6 Figure 4. Predominance area diagram of Fe-Al-Mg-Ca-O system with 1 10− Ca and different oxygen Figure 4. Predominance area diagram of Fe-Al-Mg-Ca-O system with× 1 × 10−6 Ca and different contents at 1600 ◦C. oxygen contents at 1600 °C . In the Figure4, ‘liquid oxide’ represents CaO-Al 2O3-MgO series liquid inclusions, and ‘CxAy’ 6 represents3.2.3. Typical (CaO) Al-Tix(Al InclusionsO )y. When in Molten 1 10 Steel[Ca] after is addedRH Refining to the molten steel, the MgO Al O spinel 2 3 × − · 2 3 area is significantly reduced, a part of the MgO Al O spinel area is replaced by the ‘liquid oxide’, Figure 5 shows the predominance area diagram· 2 3 of Fe-Al-Ti-O system with different oxygen and the original Al O phase area is also replaced by a part of the ‘liquid oxide’. contents at 1600 °C.2 It3 can be seen from the figure that Al2O3 can remain stable in the molten steel. WhenFor the the content IF steel of containing[O] in steel0.042% is less than Al after 10 × RH 10−6 refining, in addition in this to experiment,the [Al] and when[Ti] dissolved the content in the of [Mg] in the steel is less than 0.48 10 6, Mg is dissolved in the molten steel and the inclusions in molten steel, other trace Al and ×Ti in− the steel are stable in the form of Al2O3 and Ti3O5, and the molten steel are Al O and C A type calcium aluminates. The overall area ratio is larger than Al2O3·TiOx will not be stable2 3 in thex molteny steel. that without Ca. When content of [Mg] in steel increases to 0.48 10 6, MgO Al O spinel inclusions When the [O] content in steel increases to 10 × 10−6, the× stable− region· 2 of3 Al2O3 increases. begin to generate in the molten steel and it also contains Al O and C A type calcium aluminates. However, Ti exists as Ti2O3 instead of Ti3O5. As the dissolved2 [O]3 in steelx y increases above 15 × 10−6, Al2O3 may react with Ti or TiOx by reaction (3) or (4) to form Al2O3·TiOx.

[Ti] (Al 2O3 )inclusion  (Al 2O3 TiO x )inclusion [Al ] (4)

It can be seen from the phase diagram that Al2O3·TiOx may exist in steel in three situations: (1) when the dissolved [O] in the molten steel is up to 15 × 10−6. (2) When the local Ti concentration in the molten steel is up to 0.49% during the titanium alloying adjusting process. (3) When the local Al concentration in the molten steel is as low as 0.0079%. As the dissolved oxygen in ultra-low carbon steel is low, the Al-Ti inclusions in the ultra-low carbon steel liquid cannot exist stably. When the composition of Al and Ti in the steel liquid is uniform, the Al-Ti inclusions will react with the Al in the steel continuously to form Al2O3 inclusions.

Metals 2020, 10, x FOR PEER REVIEW 8 of 13

Figure 4. Predominance area diagram of Fe-Al-Mg-Ca-O system with 1 × 10−6 Ca and different oxygen contents at 1600 °C .

3.2.3. Typical Al-Ti Inclusions in Molten Steel after RH Refining Figure 5 shows the predominance area diagram of Fe-Al-Ti-O system with different oxygen contents at 1600 °C. It can be seen from the figure that Al2O3 can remain stable in the molten steel. When the content of [O] in steel is less than 10 × 10−6, in addition to the [Al] and [Ti] dissolved in the Metals 2020, 10, 528 8 of 13 molten steel, other trace Al and Ti in the steel are stable in the form of Al2O3 and Ti3O5, and Al2O3·TiOx will not be stable in the molten steel. −6 6 WhenWhen the content the [O] of content [Mg] in thein steel molten increase steel increasess to 10 × to 10 1.85, the10 stable− , the region Al-containing of Al2O inclusions3 increases. in × −6 However,the molten Ti steel exists all as exist Ti2O as3 MgOinsteadAl of2O Ti3 spinel.3O5. As Asthethe dissolved content [O] of [Mg]in steel in theincreases molten above steel 15 increases × 10 , 6 · Alto2 10.15O3 may 10react− , thewith inclusions Ti or TiO inx by the reaction molten (3) steel or include(4) to form MgO Al inclusions2O3·TiOx. besides MgO Al2O3 spinel. × 6 · When the content of [Mg] in the molten steel increases to 16.39 10− , the Mg-containing inclusions in [Ti] (Al 2O3 )inclusion  (Al 2O3 TiO x )×inclusion [Al ] (4) the molten steel all exist as MgO. It can be seen from the phase diagram that Al2O3·TiOx may exist in steel in three situations: (1) when3.2.3. the Typical dissolved Al-Ti Inclusions[O] in the molten in Molten steel Steel is up after to 15 RH × 10 Refining−6. (2) When the local Ti concentration in the moltenFigure steel5 isshows up to the 0.49% predominance during the areatitanium diagram alloying of Fe-Al-Ti-O adjusting process. system with(3) When di fferent the local oxygen Al concentrationcontents at 1600 in the◦C. molten It can be steel seen is fromas low the as figure 0.0079%. that As Al 2theO3 dissolvedcan remain oxygen stable in in ultra the molten-low carbon steel. steelWhen isthe low, content the Al of-Ti [O] inclusions in steel is in less the than ultra 10-low10 carbon6, in addition steel liqui tod the cannot [Al] and exist [Ti] stably. dissolved When in the the × − compositionmolten steel, of other Al and trace Ti Al in and the Ti steel in the liquid steel is are uniform, stable in the the Al form-Ti ofinclusions Al O and will Ti reactO , and with Al theO AlTiO inx 2 3 3 5 2 3· thewill steel not becontinuously stable in the to molten form Al steel.2O3 inclusions.

Figure 5. Predominance area diagram of Fe-Al-Ti-O system with different oxygen contents at 1600 ◦C.

When the [O] content in steel increases to 10 10 6, the stable region of Al O increases. However, × − 2 3 Ti exists as Ti O instead of Ti O . As the dissolved [O] in steel increases above 15 10 6, Al O may 2 3 3 5 × − 2 3 react with Ti or TiOx by reaction (3) or (4) to form Al O TiOx. 2 3·

[Ti] + (Al O ) (Al O TiOx) + [Al] (4) 2 3 inclusion → 2 3· inclusion

It can be seen from the phase diagram that Al O TiOx may exist in steel in three situations: 2 3· (1) when the dissolved [O] in the molten steel is up to 15 10 6. (2) When the local Ti concentration in × − the molten steel is up to 0.49% during the titanium alloying adjusting process. (3) When the local Al concentration in the molten steel is as low as 0.0079%. As the dissolved oxygen in ultra-low carbon steel is low, the Al-Ti inclusions in the ultra-low carbon steel liquid cannot exist stably. When the composition of Al and Ti in the steel liquid is uniform, the Al-Ti inclusions will react with the Al in the steel continuously to form Al2O3 inclusions.

3.2.4. Typical Al-Mg Inclusions in Tundish Molten Steel The average [O] content in the six experimental heats steel dropped to 3.9 10 6 before Ti adding, × − but the [O] content increases to 10 10 6 after RH refining. In order to understand the transformation × − process of MgO Al O spinel, the predominance area diagram of inclusions in Fe-Al-Mg-O system · 2 3 with [O] = 10 ppm and [Ca] = 1 ppm at 1600 ◦C is shown in Figure6. Metals 2020, 10, x FOR PEER REVIEW 9 of 13

Figure 5. Predominance area diagram of Fe-Al-Ti-O system with different oxygen contents at 1600 °C .

3.2.4. Typical Al-Mg Inclusions in Tundish Molten Steel

The average [O] content in the six experimental heats steel dropped to 3.9 × 10−6 before Ti adding, but the [O] content increases to 10 × 10−6 after RH refining. In order to understand the transformationMetals 2020, 10, 528 process of MgO·Al2O3 spinel, the predominance area diagram of inclusions9 of in 13 Fe-Al-Mg-O system with [O] = 10 ppm and [Ca] = 1 ppm at 1600 °C is shown in Figure 6.

Figure 6. Phase diagram of inclusions in Fe-Al-Mg-O system with [O] = 10 ppm and [Ca] = 1 ppm Figure 6. Phase diagram of inclusions in Fe-Al-Mg-O system with [O] = 10 ppm and [Ca] = 1 ppm at at 1600 ◦C. 1600 °C .

As can be seen from Figure6, in the Fe-Al-Mg-O system, when the content of [O] and [Al] s in As can be seen 6from Figure 6, in the Fe-Al-Mg-O system, when the content of [O] and [Al]s in the steel is 10 10− and 0.033%, respectively, MgO and MgO Al2O3 and Al2O3 inclusions will be the steel is 10 ×× 10−6 and 0.033%, respectively, MgO and MgO·Al· 2O3 and Al2O3 inclusions will6 be precipitated separately with the difference in Mg content. If [Mg] content is less than 0.48 10− in precipitated separately with the difference in Mg content. If [Mg] content is less than 0.48 × 10×−6 in the the steel, Al2O3, CA2, and CA6 inclusions will be precipitated in the molten steel. If [Mg] content steel, Al2O3, CA2, and6 CA6 inclusions will be precipitated in the molten steel. If [Mg] content increases to 0.48 10− , that is, the critical line of the Al2O3 and MgO Al2O3 phases, MgO Al2O3 spinel increases to 0.48 ×× 10−6, that is, the critical line of the Al2O3 and MgO·Al· 2O3 phases, MgO·Al· 2O3 spinel begins to precipitate in the molten steel. At this time, [Mg] and Al2O3 inclusions in the molten steel begins to precipitate in the molten steel. At this time, [Mg] and Al2O3 inclusions in the molten steel react to form MgO Al2O3 spinel at the boundary between the Al2O3 phase and the MgO Al2O3 phase. react to form MgO·Al· 2O3 spinel at the boundary6 between the Al2O3 phase and the MgO·Al· 2O3 phase. As the Mg content increase to 0.99 10− , Al2O3 will be transformed to MgO Al2O3, and the CA6 As the Mg content increase to 0.99 × 10× −6, Al2O3 will be transformed6 to MgO·Al2O·3, and the CA6 phase phase will disappear. When [Mg] content reaches to 3.87 10− , Al2O3 will be completely transformed will disappear. When [Mg] content reaches to 3.87 × 10−×6, Al2O3 will be completely transformed into into MgO Al2O3 spinel, and the CA2 phase will disappear. When [Mg] content continues to increase MgO·Al2O·3 spinel,6 and the CA2 phase will disappear. When [Mg] content continues to increase to to 10.73 10− , MgO begins to precipitate in the molten steel. When the content Mg increases to 10.73 × 10×−6,6 MgO begins to precipitate in the molten steel. When the content Mg increases to 13.66 × 13.66 10− , the spinel is completely transformed to MgO. With the increase of Mg content, the change 10−6, the× spinel is completely transformed to MgO. With the increase of Mg content, the change path path of inclusions is that Al2O3 change to MgO Al2O3, and finally change to MgO. of inclusions is that Al2O3 change to MgO·Al2O3·, and finally change to MgO. 3.3. Change and Control of Inclusions during Refining and Holding Time 3.3. Change and Control of Inclusions during Refining and Holding Time After the RH deoxidization is completed, the histogram of the change in the number of inclusions in theAfter molten the steel RH at deoxidization different times is shown completed, in Figure the7 . histog The number,ram of size, the and change chemical in the composition number of inclusionsinclusions werein the analyzed molten steel automatically at different by times using is ASPEX shown SEM. in Figure As can 7. be The seen number from Figure, size, 7and, the chemical number compositionof inclusions of reaches inclusions a maximum were analyzed after 4 min automatically of aluminum by deoxidization,using ASPEX whichSEM. As indicates can be thatseen within from Figure4 min, the 7, the oxygen number in aluminum of inclusions and steel reache reactss a maximumquickly to form after Al 4 2minO3 inclusions. of aluminum When deoxidization, aluminum is whichadded indicates for 4–8 min, that the within removal 4 min rate, the of oxygen inclusions in aluminum floating up and isgreater steel react thans quickly the rate to of form generation, Al2O3 inclusions.and the total When number aluminum of inclusions is added in molten for 4 steel–8 min decreases,, the removal especially rate the of number inclusions of inclusions floating largerup is greaterthan 10 thanµm. the Therefore, rate of ingeneration, order to ensure and the that total the number inclusions of inclusions generated in by molten deoxidization steel decreases, are fully especiallyfloated and the removed, number the of pure inclusions circulation larger time than after 10 alloying μm. Therefore, should be in greater order than to ensure8 min. that the inclusionsAfter generated RH refining, by the deoxidization change of the are number fully floated of inclusions and removed, in the molten the pure steel circulation with holding time time after is alloyingshown in should Figure be8. Itgreater can be than seen 8 thatmin. the number of inclusions in the molten steel decreases with the extension of the holding time within 20 min, especially the number of large particle inclusions above 2 2 10 µm decreases to 1 mm , and Al O TiOx also decreases to 1 mm . − 2 3· − Metals 2020, 10, x FOR PEER REVIEW 10 of 13 Metals 2020, 10, x FOR PEER REVIEW 10 of 13 After RH refining, the change of the number of inclusions in the molten steel with holding time is shownAfter in RH Figure refining, 8. It thecan change be seen of that the the number number of inclusionsof inclusions in the in themolten molten steel steel with decreases holding timewith isthe shown extension in Figure of the 8. holding It can be time seen within that the 20 number min, especially of inclusions the number in the molten of large steel particle decreases inclusions with Metals 2020, 10, 528 −2 −2 10 of 13 theabove extension 10 μm decreasesof the holding to 1 mm time, andwithin Al 220O3 ·TiOminx, alsoespecially decreases the numberto 1 mm of. large particle inclusions 50 above 10 μm decreases to 1 mm−2, and Al2O3·TiOx also decreases to 1 mm−2. Al-Ti-O 50 ＞10μm Al-Ti-O 5-10μm 40 ＞10μm 4μm

-2 5-10μm 40 3μm 4μm

mm 2μm -2

， 30 3μm 1μm

mm 2μm ， 30 1μm

20

20

Number of inclusions of Number 10

Number of inclusions of Number 10

0 deoxidation 2min 4min 5min 6min 0 Figuredeoxidation 7. Change 2min of inclusions4min after 5minRH deoxidization6min . FFigureigure 7. 7. ChangeChange of of inclusions inclusions after after RH RH deoxidization deoxidization..

16

＞10μm 16 14 5-10μm ＞10μm 14 4μm 12 5-10μm

-2 3μm 4μm 12 2μm

mm

-2 10 3μm ， 1μm 2μm mm 10

， 8 1μm

8 6

6 4

Number of inclusions of Number 4 2

Number of inclusions of Number

2 0 Al2O3 Al-O-Ti Al2O3 Al-O-Ti Al2O3 Al-O-Ti 0 Holding time 10min 15min 20min Al2O3 Al-O-Ti Al2O3 Al-O-Ti Al2O3 Al-O-Ti FigureHolding 8. timeChange 10min of inclusions15min during holding20min time. Figure 8. Change of inclusions during holding time. Inclusions float in theFigure molten 8. Change steel in o thef inclusions way of during Stokes holding to the surfacetime. of the ladle. If the time requiredInclusions to reach float the in top the of themolten molten steel steel in the is less way than of Stokes the average to the residence surface of time the of ladle. the molten If the steel,time requiredthe inclusionsInclusions to reach can float the be in floatedtop the of molten tothe the molten topsteel of insteel the the molten is way less of steelthan Stokes andthe toaverage removed the surface residence from of the the moltentime ladle. of steel theIf the molten [17 time]. requiredsteel,The the toinclusions floating reach velocitythe can top be of offloat the inclusionsed molten to the insteeltop the ofis still theless molten than the steel steel average and can removed be residence calculated from time usingthe of molten the the molten Stokes steel steel,[settlement17]. the inclusions Formula c (5).an be floated to the top of the molten steel and removed from the molten steel 2 [17]. The floating velocity of inclusionsV ins = thegd still(ρ1 moltenρ2)/(18 steelη) can be calculated using the Stokes (5) settlement formula (5). − The floating velocity of inclusions in the still molten steel can be calculated using the2 Stokes where Vs is the floating velocity of the inclusions, cm/s; g is the acceleration of gravity, m/s ; d is the settlement formula (5). 2 3 diameter of the inclusions, mm; ρ1 is theV densitys  gd ( of1 the 2 molten)/(18) steel, ρ1 = 7.0 g/cm ; ρ2 is the density(5) of the inclusions, ρ = 3.5 g/cm3; η is molten steel2 viscosity, η = 0.05 g/cm s at 1600 C. 2 Vs  gd (1  2 )/(18) ◦ (5) · 2 whereIt V cans is bethe seen floating from velocity the above of formulathe inclusions, that the cm/s; floating g is speedthe acceleration of inclusions of gravity, is proportional m/s ; d tois the 3 wdiametersquarehere V ofs isof the thethe diameter floatinginclusions, of velocity the mm; inclusions, ρof1 isthe the inclusions, so density large inclusions of cm/s; the molten g is are the easy steel, acceleration to ρ float.1 = 7.0 Substituting g/cm of gravity,; ρ2 is them/s datadensity2; d is gives: the of 3 diameterthe inclusions, of the ρinclusions,2 = 3.5 g/cm mm;; η is ρ 1molten is the density steel viscosity, of the molten η = 0.05 steel, g/cm·s ρ1 = at 7.0 1600 g/cm °C.3; ρ2 is the density of 2 It can be seen from the3 above formulaV thats = 0.00381 the floatingd speed of inclusions is proportional to the(6) the inclusions, ρ2 = 3.5 g/cm ; η is molten steel viscosity,× η = 0.05 g/cm·s at 1600 °C. of the diameter of the inclusions, so large inclusions are easy to float. Substituting the data ItThe can depth be seen of moltenfrom the steel above in thisformula ladle that is about the floating 3.5 m, andspeed the of floating inclusions time is ofproportional inclusions canto the be gives: squarecalculated of the by diameter Equation of (7), the and inclusions, the result isso shownlarge inclusions in Figure9 .are easy to float. Substituting the data gives:

Metals 2020, 10, x FOR PEER REVIEW 11 of 13

2 Vs  0.0 0 3 8 1d (6)

The depth of molten steel in this ladle is about 3.5 m, and the floating time of inclusions can be calculated by Equation (7), and the result is shown in Figure 9.

2 Metals 2020, 10, 528 t  350/(0.00381d ) 11( of7) 13

100

80

60

40 Floating time, min

20

0 4 6 8 10 12 14 16 18 20 Inclusion size, μm FigureFigure 9. 9. TheThe relationship relationship between between the the particle particle size size and and the the time time of of inclusion inclusion floating floating up up..

It can be seen from Figure 9 that inclusionst = 350/(0.00381 with particled2) size less than 6 μm need about 42 min (7) to float up. When smaller inclusions aggregate into inclusions× with a particle size greater than 9 μm, they canIt can float be up seen and from remove Figure within9 that about inclusions 19 min, with so particlethe holding size lesstime than should 6 µ bem 19 need–42 aboutmin. 42 min to float up. When smaller inclusions aggregate into inclusions with a particle size greater than 9 µm, 3.4.they Mechanical can floatup Properties and remove of IF Steel within about 19 min, so the holding time should be 19–42 min.

3.4. MechanicalThe results Properties of the DC06 of IF IF Steel steel mechanical properties test and requirements (GB/T 5213-2019) are shown in Table 3. According to Table 3, the yield strength Rp0.2, tensile strength Rm, elongation The results of the DC06 IF steel mechanical properties test and requirements (GB/T 5213-2019) are after fracture A80, tensile strain hardening index N90, and plastic strain ratio R90 of DC06 IF steel are shown in Table3. According to Table3, the yield strength R , tensile strength R , elongation after 138.7 MPa, 304.7 MPa, 45.3%, 0.24, and 2.91, respectively.p0.2 Compared with standardm values of fracture A , tensile strain hardening index N , and plastic strain ratio R of DC06 IF steel are 138.7 MPa, GB/T5213-802019, the mechanical properties of90 DC06 IF steel meet the requirements90 . 304.7 MPa, 45.3%, 0.24, and 2.91, respectively. Compared with standard values of GB/T5213-2019, the mechanical properties ofTable DC06 3. IF Mechanical steel meet propert the requirements.ies of DC06 IF steel.

Yield Table 3. MechanicalTensile propertiesElongation of DC06 IF steel.Tensile Strain Plastic Standard and Strength Strength Rm, after Fracture Hardening Strain ExperimentStandard and Yield Strength Tensile Strength Elongation after Tensile Strain Plastic Strain p0.2 80 90 90 Experiment RRp0.2,, MPa RmMPa, MPa FractureA ,A %80,% HardeningIndex Index N N 90 RatioRatioR 90R GB/TGB/ T5213 5213-2019-2019 ≤170170 260 260–330–330 ≥4242 ≥0.220.22 ≥2.12.1 ≤ ≥ ≥ ≥ EExperimentalxperimental data 138.7 304.7 45.3 0.24 2.91 138.7 304.7 45.3 0.24 2.91 data 4. Conclusions 4. Conclusions 1. The IF steel inclusions in the BOF process are large sphere-like SiO2-CaO-FeO-MgO-MnO 1 Themulti-phase IF steel compositeinclusions inclusionsin the BOF below process 50 µm are and large cluster sphere spherical-like FeO-MnOSiO2-CaO- inclusionsFeO-MgO- belowMnO multi5 µm.-phase With thecomposite addition inclusions of Al-slag modifierbelow 50 and μm Al and deoxidizer, cluster spherical a large amount FeO-MnO of cluster-like inclusions or belowcoral-like 5 μm. Al OWithinclusions the addition are formed. of Al Moreover,-slag modifier MgO andAl O Alspinel deoxidizer, inclusions a large reduce amount gradually. of 2 3 · 2 3 clusterAl O -TiOlikex orinclusions coral-like begin Al2O to3 forminclusions after Tiare addition, formed. and Moreover, Al O -type MgO·Al inclusions2O3 spinel increase inclusions slightly. 2 3· 2 3 reduceDuring gradually. the holding Al process,2O3·TiOx the inclusions inclusions beg arein removedto form andafter the Ti number addition, of Aland2O 3Alinclusions2O3-type inclusionsand Al O increaseTiOx inclusions slightly. in During the steel the is greatlyholding reduced. process, Inthe addition, inclusions Al OareTiO removedx inclusions and the are 2 3· 2 3· numberlarger in of size Al2 comparedO3 inclusions to Al and2O 3Alinclusions.2O3·TiOx inclusions After the in molten the steel steel is remains greatly reduced. unstirred In in addition, the ladle Alduring2O3·TiO thex inclusions holding process, are larger most in of size the compared large-particle to Al Al22OO3 3inclusions.inclusions After have beenthe molt removeden steel by floating. In the tundish, the inclusions of Al O TiOx in the molten steel disappear, and only 2 3· Al2O3 inclusions remain in the end. It is key to extend the holding time properly after RH to ensure the removal of Al2O3 inclusion. 2. The predominance area diagram of Fe-Al-Mg-O system show that, when the Mg concentration in the molten steel before RH is (0.54–45.79) 10 6, the MgAl O spinel inclusions will exist in the × − 2 4 molten steel. The type of inclusions is mainly affected by the [Mg] content, and it is difficult to Metals 2020, 10, 528 12 of 13

suppress MgO Al O spinel formation by controlling the oxygen in the steel. When Ca content · 2 3 in steel is 1 10 6, Ca can modify part of the MgO Al O spinel inclusions during RH refining, × − · 2 3 and the amount of Mg required to form spinel is reduced to (0.48–16.39) 10 6. With the increase × − of Mg content, the change path of inclusions in IF steel is that Al O change to MgO Al O , 2 3 · 2 3 and finally change to MgO.

3. Al O TiOx will not be stable in the IF molten steel, except for three situations: (1) when the 2 3· dissolved [O] in the molten steel is up to 15 10 6. (2) When the local Ti concentration in the × − molten steel is up to 0.49% during the titanium alloying adjusting process. (3) When the local Al concentration in the molten steel is as low as 0.0079%. 4. In order to ensure the removal of 6–10 µm inclusions, the holding time is suitable for 19–42min.

5. The yield strength Rp0.2, tensile strength Rm, elongation after fracture A80, tensile strain hardening index N90, and plastic strain ratio R90 of DC06 IF steel are 138.7 MPa, 304.7 MPa, 45.3%, 0.24, and 2.91, respectively. The mechanical properties of DC06 IF steel meet the requirements.

Author Contributions: R.C.: project administration, conceptualization, investigation, methodology, data curation, writing—original preparation. D.C.: Investigation. Q.F.: writing—review and editing. R.L.: project administration. J.L.: investigation, project administration. J.Z.: formal analysis. W.D.: visualization, drawing. H.Z.: funding acquisition, methodology, writing—review and editing. H.N.: conceptualization, supervision. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the financial support provided by the Open Youth Fund of State Key Laboratory of Refractories and Metallurgy, Wuhan University of Science and Technology (Grant No. 2018QN03) and and the National Natural Science Foundation of China (51774217). Conflicts of Interest: The authors declare no conflicts of interest.

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