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Three Co-based dual-phase high-entropy alloys (HEAs) were produced by different manufacturing conditions: arc-melting with Ar protection (Ar-HEA), vacuum induction melting in Al2O3 crucible (Cr-HEA) and vacuum induction melting with 0.5 at. pct Al (Al-HEA), which resulted in different levels of impurity elements and inclusion characteristics. The inclusions that precipitated in different HEA samples were investigated through an electrolytic extraction process and scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS) characterization. The results showed that Mn(S,Se) inclusions were presented in all three alloys. MnCr2O4 inclusions were presented only in Ar-HEA, pure Al2O3 inclusions were presented in Cr-HEA and Al-HEA, and Mn–Cr–Al–O inclusions were also found in Al-HEA. Thermodynamic calculation software FactSage and Thermo-calc were used to predict the inclusion formations of the HEAs, which showed a good agreement with the experimental findings. The stable inclusions can transform from MnCr2O4 to Mn(Cr,Al)2O4 and then to pure Al2O3 with the increase of Al content. The inclusions in Al-containing HEA are spinel or Al2O3 depending on the content levels of Al and O. It is proposed that the formation of spinel and Al2O3 inclusions can be avoided in liquid HEA when the O content is controlled to be very low, which can result in smaller-sized inclusions. Moreover, the calculated coagulation coefficient of spinel inclusions is close but lower than that of Al2O3 inclusions. The collision growth of inclusions was affected by a combination of physical parameters of HEA and inclusions as well as the inclusion size and amount. Graphical Abstract

Medium Mn steel (MMS) is a new category of the third-generation advanced high strength steel (3rd AHSS) which is developed in the recent 1-2 decades due to a unique trade-off of strength and ductility. Thus, this steel grade has a wide application potential in different fields of industry. The current work provides a fundamental study of the effect of hot-rolling on the inclusion deformation in MMS including a varied 7 to 9 mass pct Mn. Specifically, the deformation behavior of different types of inclusions (i.e., Mn(S,Se), liquid oxide (MnSiO3), MnAl2O4, and complex oxy-sulfide) was investigated. The results show that both MnSiO3 and Mn(S,Se) are soft inclusions which are able to be deformed during the hot-rolling process but MnAl2O4 does not. The aspect ratio of soft inclusions increases significantly from as-cast to hot-rolling conditions. When the maximum size of different inclusions is similar, Mn(S,Se) deforms more than MnSiO3 does. This is due to a joint influence of physical parameters including Young’s modulus, coefficient of thermal expansion (α), etc. However, when the maximum size of one type of inclusion (e.g., MnSiO3) is much larger than another one (e.g., Mn(S,Se)), this maximum size of soft inclusions plays a dominant role than other factors. In addition, the deformation behavior of dual-phase inclusion depends on the major phase, i.e., either oxide or sulfide. Last but not least, empirical correlations between the reduction ratio of the thickness of plate, grain size, and aspect ratio of oxide and sulfide inclusions after hot-rolling are provided quantitatively. This work aims to contribute to the ‘inclusion engineering’ concept in the manufacturing of new generation AHSS.

The laboratory-scale experiments between the CaF2–CaO–Al2O3–SiO2–B2O3 slag and 9CrMoCoB steel were carried out in alumina and magnesia crucibles at 1823 K to investigate the oxidation behavior of boron (B) during the electroslag remelting (ESR) process. The activities of SiO2 and B2O3 in the slag and the activities of Si and B in the molten steel were calculated by the ion and molecule coexistence theory (IMCT) and the Wagner formalism, respectively. The results showed that both SiO2 and B2O3 have a significant influence on the equilibrium B content. The calculated content of B was in good agreement with the experimental value when the SiO2+B2O3 content in the slag is more than 3.3 wt%. The temperature had little influence on the equilibrium B content when the SiO2, B2O3 and CaO content were in the ranges of 3–5 wt%, 0–1 wt%, and 20–30 wt%, respectively. However, the Si is more prone to oxidation than is B as the temperature increases, indicating that more SiO2 should be added in the slag to reduce the oxidation of Si. From the 80 tonnes industrial tests, the distribution of B and Si content along with the radial direction of the remelted ingot was almost uniform, which is in line with the calculated B content (approx. 30 ppm) under conditions of the Si content in the liquid steel and the (%B2O3)/(%SiO2) ratio are 0.07% and 0.05, respectively, at 1973 K.

The influence of slag composition of FeO/TiO2 (=F/T) ratio, Al2O3/(SiO2+Al2O3) (=A/(S+A)) ratio and oxygen partial pressure ( p O 2 = 10 − 10 t o 10 − 9 a t m ) on the thermodynamic behavior of vanadium oxide in molten ilmenite smelting slag, i.e., FetO-TiO2-MgO-Al2O3-SiO2 system, was investigated at 1550 °C. The distribution ratio of vanadium (= L V ) between molten iron and FetO-TiO2-MgO-Al2O3-SiO2 slag increases with increasing F/T ratio, while it decreases with increasing A/(S+A) ratio. In addition, l o g L V generally increases with increasing theoretical optical basicity of the slag (= Λ s l a g ). Vanadium oxide (VOx) is expected to behave as an acidic component in the present system. Vanadium in ilmenite smelting slags primarily consist of V3+, V4+, and V5+ ions, as determined by Gaussian deconvolution of X-ray photoelectron spectra. A linear relationship between l o g ( w t % V 4 + / w t % V 3 + ) and l o g p O 2 is confirmed. Both (wt% V3+) and (wt% V5+) ions increase with increasing F/T ratio, whereas (wt% V4+) ions show the opposite behavior with increasing F/T ratio, irrespective of A/(S+A) ratio. Meanwhile, (wt% V3+) increase while (wt% V4+) and (wt% V5+) decrease with increasing A/(S+A) ratio. The linear relationship between l o g ( w t % V 3 + / w t % V 5 + ) and l o g ( w t % V 4 + ) has a slope of 1.8, which is good agreement with the theoretical slope of 2.0, which indicates that multi-valent vanadium ion species, i.e., V3+, V4+, and V5+, are thermodynamically balanced based on redox equilibria.

Fluorspar (CaF2) is commonly used to control the fluidity of slag in ladle-refining of steel. However, because it is desirable to reduce CaF2 consumption because of its environmental impacts, the industrial waste material such as white mud (WM) was investigated as a potential substitute for fluorspar. Steel sample (Fe-0.3C-0.9Mn-0.3Si-0.03Al-0.05S, mass pct) was melted in a high-frequency induction furnace, followed by additions of ladle slag (CaO-Al2O3-SiO2-5MgO-xCaF2, CaO/Al2O3=3, x = 0 to 10 mass pct) and fluxing agent (WM) at 1823 K (1550 °C). The desulfurization experiments were carried out by reducing CaF2 content in the ladle slag and increasing the addition of WM. Ladle slag with added WM showed an overall mass transfer coefficient of sulfur (k O) equivalent to or higher than that of conventional 10 mass pct CaF2-containing ladle slag. In a slag melting experiment based on DIN 51730 standard, the melting rate of mixed slag increased with the amount of WM added, which is considered to have a positive effect on the initial desulfurization rate. In addition, adding WM provided sulfide capacity of the slag equivalent to that of CaF2-containing slag. Consequently, the use of WM yielded slag having $$k_{{\text{O}}}$$ k O equivalent to or higher than that of conventional ladle slag with 10 pct CaF2, and thus, WM shows promise as a partial replacement for fluorspar.

The effect of slag composition under M’O (monoxide, M = Fe,Mg) saturation and fully liquid conditions on dephosphorization and slag foamability was evaluated in an electric arc furnace (EAF) steelmaking plant operation by considering thermodynamics and phase equilibria. It was confirmed that M’O saturation slag is more favorable for higher dephosphorization than fully liquid slag due to its high activity of CaO as a thermodynamic driving force. As a quantitative measure of slag foamability, the foaming index was clearly dependent on the temperature and slag composition. Moreover, foam stability was evaluated by applying the predominance stability diagram based on phase equilibria. Consequently, it can be suggested that an efficient direction for dephosphorization and slag foaming during the EAF process is to generate high CaO activity as well as small amounts of M’O monoxide and dicalcium silicate (Ca2SiO4) compounds.

Laboratory experiment and thermodynamic calculation for the Ti-containing 24 mass pct Cr ferritic stainless steel with a CaO-SiO2-Al2O3-MgO system slag were performed to investigate the effect of slag addition on the inclusion characteristics in molten steel. The morphology, composition, and size evolution of inclusions in steel samples were analyzed in three-dimensional by the electrolytic extraction method and in two-dimensional by the automatic analysis method. The results showed that the Ti content significantly decreased after the slag addition. However, the change of the Si content showed an opposite tendency. The decrease of the Ti content in steel was due to the reduction of SiO2 and Al2O3 in the slag by dissolved Ti in steel. An increase of the TiO2 content in the slag can decrease the Ti loss in steel based on the slag-steel kinetic analysis. The total O content in the steel melt decreased from 62 to 26 ppm, and the steel cleanliness was improved, since the number density of inclusions decreased after the slag refining. The results of a kinetic analysis showed that the rate-determining step of the oxidation of Ti in the steel and the reduction of SiO2 in the slag were the mass transfer on the slag side. In addition, high Ti2O3-containing inclusions were found to be transformed to Cr2O3-Ti2O3-Al2O3 and Cr2O3-Ti2O3-SiO2 system inclusions after the slag addition. The Al2O3 contents in inclusions increased while the Ti2O3 contents decreased with time. However, there were some amount of high melting point inclusions with high Al2O3 content, which were not what we expected. When plotted on logarcxithmic scales, the mole ratio $$X_{{{\text{Al}}_{2} {\text{O}}_{3} }} /(X_{{{\text{Ti}}_2 {\text{O}}_{3} }} \cdot X_{{{\text{Cr}}_{2} {\text{O}}_{3} }} )$$ X Al 2 O 3 / ( X Ti 2 O 3 · X Cr 2 O 3 ) values of the inclusions were expressed as a linear function of the $$a_{\text{Al}}^{2} /(a_{Ti}^{2} \cdot a_{\text{Cr}}^{2} \cdot a_{\text{O}}^{3} )$$ a Al 2 / ( a Ti 2 · a Cr 2 · a O 3 ) values of the steel melts with a slope of unity, which was theoretically expected.

The effect of industrial waste flux addition on dephosphorization of molten steel and refractory corrosion(-protection) by electric arc furnace (EAF) slag using direct reduced iron (DRI) at 1823 K (1550 °C) was investigated. The activity of CaO and FeO is the thermodynamic driving force for dephosphorization, but it decreased with the use of DRI due to increased SiO2 content in the slag. Industrial waste fluxes were added to the slag to minimize refractory corrosion and improve dephosphorization efficiency. Red mud (RM) and white mud (WM), which are generated in the process of manufacturing aluminum hydroxide from bauxite using the Bayer process, were used as fluxing materials. The RM and WM mainly consist of Fe2O3 and CaO, respectively, and these positively affect the dephosphorization. Dephosphorization efficiency increased as industrial waste fluxes addition increased (3 to 7 mass pct). At the same time, the effect of industrial waste fluxes on the refractory corrosion-protection behavior was confirmed by considering the change in thickness of the magnesiowustite (MW) layer that formed at the interface between the ‘FeO-rich’ EAF slag and the magnesia refractory. Experimental results confirmed that 3 to 4 mass pct WM addition in the slag can compensate for the dephosphorization efficiency while minimizing the refractory corrosion in the EAF process using 20 pct DRI as an alternative iron source.

The effect of oxygen on interfacial phenomena between slag and copper melt was investigated at 1400 °C. The removal of impurities (iron and silicon) in the copper melt during oxygen blowing was confirmed. Also, it was revealed that composition change of the slag occurs due to the oxygen potential in the slag depending on the oxygen injection method. In addition, over-blowing of oxygen after removing impurities is a factor that significantly changes the surface tension of molten copper and slag. Over blown oxygen increases the instability of the interface by reducing interfacial tension between slag and liquid copper. Due to lowered interfacial tension and increased instability, copper droplets can be mixed into slag phase which induces copper loss. Oxygen blowing and/or bubbling is involved in chemical and weak physical reactions in the gas-slag-metal multiphase system, which should be carefully controlled because it is significant factor affecting the recovery rate of the metal. Overall reactions and slag/metal interfacial phenomena in impurity removal regime and over-blowing regime for (a) bottom blowing and (b) top blowing conditions.

Chromium is normally added to liquid alloy in the form of different grades of ferrochromium (FeCr) alloys for the requirement of different alloy grades, such as stainless steels, high Cr cast iron, etc.. In this work, inclusions in two commercially produced alloys, i.e., high-carbon ferrochromium (HCFeCr) and low-carbon ferrochromium (LCFeCr) alloys, were investigated. The FeCr alloy/liquid iron interactions at an early stage were investigated by inserting solid alloy piece into contact with the liquid iron for a predetermined time using the liquid-metal-suction method. After quenching these samples, a diffusion zone between the alloys and the liquid Fe was studied based on the microstructural characterizations. It was observed that Cr-O-(Fe) inclusions were formed in the diffusion zone, FeO x inclusions were formed in the bulk Fe, and an “inclusion-free” zone was detected between them. Moreover, it was found that the HCFeCr was slowly dissolved, but LCFeCr alloy was rapidly melted during the experiment. The dissolution and melting behaviors of these two FeCr alloys were compared and the mechanism of the early-stage dissolution process of FeCr alloys in the liquid Fe was proposed.

Nb is an important microalloying element in steelmaking. Its interaction with liquid Fe during an early stage of the alloying process has a considerable influence on the Nb recovery. In the present ...

The specific distribution characteristics of inclusions along with the sliver defect were analyzed in detail to explain the formation mechanism of the sliver defect on the automobile exposed panel surface. A quantitative electrolysis method was used to compare and evaluate the three-dimensional morphology, size, composition, quantity, and distribution of inclusions in the defect and non-defect zone of automobile exposed panel. The Al2O3 inclusions were observed to be aggregated or chain-like shape along with the sliver defect of about 3–10 µm. The aggregation sections of the Al2O3 inclusions are distributed discretely along the rolling direction, with a spacing of 3–7 mm, a length of 6–7 mm, and a width of about 3 mm. The inclusion area part is 0.04%–0.16% with an average value of 0.08%, the inclusion number density is 40 mm−2 and the inclusion average spacing is 25.13 µm. The inclusion spacing is approximately 40–160 µm, with an average value of 68.76 µm in chain-like inclusion parts. The average area fraction and number density of inclusions in the non-defect region were reduced to about 0.002% and 1–2 mm−2, respectively, with the inclusion spacing of 400 µm and the size of Al2O3 being 1–3 µm.

The effect of CaF2 addition on dephosphorization reaction between molten steel and electric arc furnace (EAF) slag using direct reduced iron (DRI) at 1823 K (1550 °C) was investigated. Basicity, the thermodynamic parameter affecting the removal capacity of phosphorus (i.e., phosphate capacity), decreased using DRI due to an increased SiO2 concentration in the slag. To minimize slag volume and maximize dephosphorization efficiency, the CaF2 was added to the slag. As CaF2 content increased (0 to 10 mass pct), dephosphorization efficiency also increased. Thermodynamic analysis in conjunction with slag structural studies using Raman spectroscopy were conducted to explain the complicated phenomena. The results exhibited that even 5 to 6 mass pct CaF2 can improve the dephosphorization efficiency when DRI is used as raw material in an EAF process.

The formation of multiphase inclusions was investigated in resulphurised steel. Samples were taken at different stages of the steelmaking process, from primary deoxidation through calcium modificat ...

Fluorspar (CaF2) has been widely used to control the physicochemical properties of slag in steelmaking processes. However, it is very important to reduce the CaF2 consumption because it causes various environmental problems. In the current study, therefore, the effect of industrial waste materials such as red mud (RM) and ferromanganese slag (FMS) on the desulfurization behavior of molten steel was investigated to find potential substitutes for fluorspar. Compared to conventional 10 pct CaF2-containing ladle slag, when excess industrial wastes were added (i.e., more than 10 mass pct), the composition of the slag was dramatically changed (SiO2 increases, while FetO and MnO decrease). Therefore, the sulfide capacity of the slag decreased, and the viscosity of the slag increased. Thus, the desulfurization efficiency was also reduced. However, when the RM or FMS were added not more than 10 mass pct to lean-CaF2 (3 pct) slag, the sulfide capacity of the slag was equivalent to 10 pct CaF2-containing ladle slag; thus, the desulfurization efficiency was also equivalent with lower melting point and higher fraction of liquid phase. Consequently, it was confirmed that a small amount (less than 10 mass pct) of industrial wastes can be added as a desulfurization fluxing agent as a partial substitute for fluorspar.

In this study, the effects of nitrogen addition on the phase formation and mechanical properties in compositionally complex alloy (CCA) of N-CoCrNi are investigated. Recrystallized N-CoCrNi exhibits Cr2N precipitate formation on the microstructure. The Cr2N formation is found to inhibit grain growth during recrystallization, causing fine grain sizes in N-CoCrNi. The microhardness and yield strength are also significantly improved in N-CoCrNi compared to CoCrNi. This occurs owing to several strengthening mechanisms by nitrogen addition to N-CoCrNi via solid-solution, grain boundary, and Cr2N precipitation hardening. It is observed that nitrogen addition is a cost-effective method to provide a potential strengthening element in CCAs, particularly in a CoCrNi matrix.

We developed a simulation model to predict inclusion transformation during the solidification of steel by linking the Ohnaka model with the FactSage macro simulation. We named this model the “Steel Solidification - Inclusion (SSI) model.” Specifically, the phase transformation of trapped inclusions in solidified steel during the solidification process was also considered in the SSI model. A newly developed SSI model was linked with the Refractory-Slag-Metal-Inclusion (ReSMI) multiphase reaction model, which was already reported by the present authors, to predict the inclusion evolution procedure during the refining and casting processes. The changed steel composition after the ReSMI multiphase reaction was utilized to calculate the microsegregation of solutes in the steel using the developed SSI model. From this combinatorial simulation, the inclusion evolution procedure is successfully predicted during the refining and casting processes.

It is an important issue to prevent molten steel in tundish from being reoxidized during the continuous casting process for improving the cleanliness of steel products. Rice husk ash (RHA) is an excellent insulation powder to prevent heat loss of the molten steel in the tundish. The addition of RHA increases the silica activity in the molten slag layer, which can cause reoxidation of molten steel. Therefore, the effect of calcium aluminate-based flux (CA-flux) on reoxidation behaviour was investigated with different ratios of CA-flux to RHA + CA-flux (=RCA). The silicon and oxygen pick-up, which is caused by the self-dissociation of silica at the slag/metal interface was suppressed with an increasing RCA, and hence the number of inclusions formed by reoxidation in the molten steel was reduced at high RCA. The evolution of inclusions with different RCA was investigated by comparing experimental results with predictions from a refractory–slag–metal-inclusion (ReSMI) multiphase reaction simulation. Finally, the inclusion formation behaviour in tundish was investigated using a ReSMI simulation with different RCA and including the temperature drop of the steel melts in the ladle during casting.

In order to understand sulfur solubility in the MnO–SiO2–Al2O3–Ce2O3 slag or inclusion system (mol MnO/mol SiO2 = M/S = 2.2 or 0.9, Al2O3 = 14.2[± 1.8] mol%, Ce2O3 = 0–5.6 mol%), the effect of Ce3+ ions on the structure of the Mn–aluminosilicate system with different M/S molar ratios has been studied by micro-Raman spectroscopy, solid-state nuclear magnetic resonance (NMR) spectroscopy and X-ray photoelectron spectroscopy (XPS). Because constant oxygen partial pressure was maintained at p(O2) = 2.8 × 10−7 atm, oxidation state of Mn and Ce would be in Mn2+ and Ce3+ in the MnO–SiO2–Al2O3–Ce2O3 system, which were also confirmed by XPS analysis. Although it was difficult to measure the 27Al solid-state NMR spectra of the quenched MnO–SiO2–Al2O3 system due to the paramagnetic effect, 27Al NMR spectra did show structural changes in aluminate when cerium was added to the Mn–aluminosilicate melts. Addition of Ce2O3 in the high M/S(= 2.2) system caused both an explosive enhancement in the intensity of Raman bands at 600 cm−1 and a peak shift in 27Al NMR spectra, indicating the transition from the [AlO4]–:0.5Mn2+ tetrahedron to a [AlO6]3–:Ce3+ octahedron complex due to a strong attraction between aluminate and the cerium ion. XPS spectra show that the transition of the aluminate structure upon introduction of Ce3+ ions consumes free oxygen in Mn–aluminosilicate melts in high M/S(= 2.2) system. However, the same structural changes were not observed when Ce2O3 was added to lower M/S(= 0.9) system, because Ce3+ ions primarily interact with the pre-existing [AlO6]-octahedron in low M/S system.

The composition of slags of an electric arc furnace (EAF) in a commercial melt shop was systematically analyzed with a focus on slag foaming. Basic behavior of FeO in an EAF slag was confirmed using fundamental thermodynamics. Monoxide ([Mg,Fe,Mn]O = M’O) and spinel ([Mg,Fe]Al2O4) phases in EAF slag were confirmed by X-ray diffraction analysis, and these results were interpreted in the context of equilibrium cooling calculation using FactSage™ software. Furthermore, the distribution of MgO with respect to the M’O-saturation limit at different basicity ratios (=CaO/SiO2=C/S) and temperatures was evaluated. In particular, the relationship between MgO and FeO for C/S ratios ranging from 1.3 to 1.6 was considered with reference to phase equilibria. Foam height was affected by slag viscosity (η) and gas generation according to changes in C/S ratio and FeO content. Foaming index (Σ) decreased with the increasing C/S ratio and FeO content. The measured foam heights were relatively higher than the calculated values, and the result indicates that the effect of M’O on slag foaming in commercial process is remarkable. Therefore, slag chemistry should be optimized based on thermodynamic considerations and thermophysical properties to achieve good foaming characteristics.

The carbothermic smelting reduction process of ilmenite ore at high temperature was investigated by thermodynamic calculations in conjunction with smelting experiments. Based on thermodynamic calculations, conducting the smelting process at a higher temperature was recommended to achieve a larger amount of FeO reduction, i.e., higher Ti-enrichment, as less precipitate and thus large amounts of a liquid slag were predicted. However, even though the reduction of FeO in ilmenite ore at the initial stage seemed to be faster as the temperature increased, no significant difference in the TiO2 or FeO concentration was observed after the reaction was complete, regardless of the temperature. This was caused by the precipitation of pseudobrookite due to the local depletion of FeO during reaction at higher temperatures, by which further reduction reaction was prohibited. The apparent rate constant increased with increasing temperature and the activation energy of the reduction process was estimated to be 144 kJ/mol, from which it was concluded that the reduction reaction of FeO in ilmenite slag by carbonaceous reductant was generally controlled through the mass transfer in the slag phase. Additionally, the formation of TiC also occurred in the iron bath. At 1923 K (1650 °C), approx. 20 pct more TiC was generated as compared to TiC formation at 1823 K (1550 °C), which also prevented further reduction of Fe at higher temperatures.

The current study was motivated to reveal the evolution of MnO-SiO2-Al2O3 inclusions during protective atmosphere electroslag remelting (P-ESR), and the effect of slag composition on the inclusion evolution. The oxide inclusions in the consumable steel electrode were ternary (7.1-26.4 mass pct) MnO-(53.3-82.8 mass pct) SiO2-(8.3-23.1 mass pct) Al2O3 without exception, which were fully removed during the P-ESR in two ways: a portion of these inclusions were dissociated in its individual chemical species into liquid steel, whereas the others were removed by absorbing them into molten slag before liquid metal droplets collected in liquid metal pool during P-ESR. The oxide inclusions both in liquid metal pool and in remelted ingots are mostly MgAl2O4 (containing about 3 mass pct MgO) and Al2O3 inclusions, irrespective of the SiO2 contents (1.9 to 9.0 mass pct) in the slag. These inclusions readily formed in the liquid metal pool as a result of the reaction between alloying elements and the dissolved oxygen in liquid steel that dissociated from MnO-SiO2-Al2O3 inclusions.

We investigated the effect of Al-C composite pellets (ACCP) on the reduction behavior of FeO in electric arc furnace (EAF) slag and the iron recovery at different nAl/(nAl + nC) at 1823 K (1550 °C). A carbothermic reaction was the dominant process at nAl/(nAl + nC) 0.6; these observations are based on the final content of FeO and Al2O3 in the molten slag. The aluminum and carbon present in the ACCP competitively affected the reaction stoichiometry (i.e., the material balance) between the production of CO + Al2O3 and the consumption of FeO. Iron recovery increased up to a yield of approx. 90 pct as the nAl/(nAl + nC) ratio increased. Because Al in the ACCP readily reacts to reduce FeO in the molten slag, iron recovery is proportional to the nAl/(nAl + nC) ratio. The precipitation of solid compounds in the slag phase, such as monoxide ([Mg,Fe]·O) and spinel (MgO·Al2O3), occurred during FeO reduction; this was experimentally confirmed as well as by thermochemical computation. Furthermore, we proposed a schematic reaction mechanism in the present study.

In the present study, the mechanism of improving HAZ toughness of steel plate with Mg deoxidation after the simulated welding with the heat input of 400 kJ/cm was investigated through in situ observation, characterization with SEM-EDS and TEM-EDS, and thermodynamic calculation. It was found that intragranular acicular ferrite (IAF) and polygonal ferrite (PF) contributed to the improvements of HAZ toughness in steels with Mg deoxidation. With the increase of Mg content in steel, the oxide in micron size inclusion was firstly changed to MgO-Ti2O3, then to MgO with the further increase of Mg content in steel. The formation of nanoscale TiN particles was promoted more obviously with the higher Mg content in the steel. The growth rates of austenite grains at the high-temperature stage (1400~1250 &deg, C) during the HAZ thermal cycle of steels with conventional Al deoxidation and Mg deoxidation containing 0.0027 and 0.0099 wt% Mg were 10.55, 0.89, 0.01 &mu, m/s, respectively. It was indicated that nanoscale TiN particles formed in steel with Mg deoxidation were effective to inhibit the growth of austenite grain. The excellent HAZ toughness of steel plates after welding with a heat input of 400 kJ/cm could be obtained by control of the Mg content in steel to selectively promote the formation of IAF or retard the growth of austenite grain.

We investigated the effect of direct reduced iron (DRI) addition on dephosphorization of molten steel by electric arc furnace (EAF) slag at 1823 K (1550 °C). Various phenomena such as CO gas evolution and slagmaking by gangue oxides in DRI were experimentally observed at each reaction step. Thermodynamic behaviors of phosphorus, oxygen, and carbon were strongly dependent on DRI content. Basicity, which is the thermodynamic driving force of dephosphorization, decreased with the increasing DRI content because SiO2 concentration in the slag was proportional to DRI addition. The excess free energy of P2O5 increased with the increasing SiO2 content in slag. A higher DRI content made dephosphorization difficult by decreasing the basicity and stability of P2O5 in the slag. Therefore, when using DRI in EAF process, it is very important to control the basicity of slag.

The characteristics of non-metallic inclusions (NMI) that precipitated in an equiatomic CoCrFeMnNi high-entropy alloy (HEA) were investigated in order to understand their effect on the mechanical properties of the HEA. As the existence of NMI could degrade the mechanical properties, improved information concerning NMI could hold key importance in controlling the promising applications of HEA. An equiatomic HEA composed of CoCrFeMnNi was manufactured using vacuum induction melting (VIM) method. A thermodynamic computation program (FactSage™7.0) was used to investigate the solidification process of the HEA at both equilibrium and non-equilibrium states. Furthermore, the computational program also predicted the type of inclusions that would precipitate. Through an electrolytic extraction process and scanning electron microscopy - energy dispersive spectroscopy (SEM-EDS) observations, the actual compositions of the precipitated inclusions were observed and classified as (a) Mn-Cr-Al oxide, (b) Mn(S,Se), and (c) mixed type; a Mn-Cr-Al oxide core with a Mn(S,Se) shell. Mn-Cr-Al oxide, in a brittle spinel-structured phase with high melting temperature, was also observed in dimples on the fracture surface. The relationship between the tensile properties of HEA and the characteristics of NMI were discussed by comparing two CoCrFeMnNi specimens with the same structure and composition. Overall, the present results indicate that the tensile properties of the HEA were significantly degraded as the area fraction (AF) and number density (ND) of NMI increased.

The effect of nitrogen content on the formation of an equiaxed solidification structure of Fe-16Cr steel was investigated. Moreover, two different kinds of refractory materials, i.e., alumina and magnesia, were employed to control the type of oxide inclusion. The characteristics of TiN(-oxide) inclusions were quantitatively analyzed in both molten steel and solidified samples. When the melting was carried out in the alumina refractory, the grain size continuously decreased with increasing nitrogen content. However, a minimum grain size was observed at a specific nitrogen content (approx. 150 ppm) when the steel was melted in the magnesia refractory. Most of the single TiN particles had a cuboidal shape and fine irregularly shaped particles were located along the grain boundary due to the microsegregation of Ti at the grain boundary during solidification. The type of TiN-oxide hybrid inclusion was strongly affected by the refractory material where Al2O3-TiN and MgAl2O4-TiN hybrid-type inclusions were obtained in the alumina and magnesia refractory experiments, respectively. The formation of oxide inclusions was well predicted by thermochemical computations and it was commonly found that oxide particles were initially formed, followed by the nucleation and growth of TiN. When the nitrogen content increased, the number density of TiN linearly increased in the alumina refractory experiments. However, the number of TiN exhibits a maximum at about [N] = 150 ppm, at which a minimum grain size was obtained in the magnesia refractory experiments. Therefore, the larger the number density of TiN, the smaller the primary grain size after solidification. The number density of TiN in the steel melted in the magnesia refractory was greater than that in the steel melted in the alumina refractory at given Ti and N contents, which was due to the lower planar lattice disregistry of MgAl2O4-TiN interface rather than that of Al2O3-TiN interface. When ∆TTiN (= difference between the TiN precipitation temperature and the liquidus of the steel) was 20 K to 40 K, the number density of effective TiN was maximized and thus, the grain size was minimized after solidification. Finally, although most of the TiN particles were smaller than 1 μm in the molten steel samples irrespective of the nitrogen content, TiN particles larger than 10 μm were observed in the solidified samples when the nitrogen content was greater than 150 ppm. The growth of TiN particles during melting and solidification was well predicted by the combinatorial simulation of the ‘Ostwald ripening model’ based on the Lifshitz–Slyozov–Wagner theory in conjunction with the ‘Diffusion controlled model’ using Ohnaka’s microsegregation equation.

The use of industrial waste [mill scale (MS), red mud (RM), Al-dross (AD)] as fluxing agents in the sulfurization of Fe-Ni-Cu-Co alloy to matte (Fe-Ni-Cu-Co-S) by carbothermic reduction of CaSO4 was investigated at 1673 K (1400 °C). The sulfurization efficiency (SE) was 76 (± 2) pct at RM or AD single fluxing. However, SE drastically increased to approximately 89 pct at a ‘5AD + 5MS’ combination, which was equivalent to reagent-grade chemical ‘5Al2O3 + 5Fe2O3’ fluxing (SE = 88 pct). The present results can be used to improve the cost-effective recovery of rare metals (Ni and Co) from deep sea manganese nodules.

The influence of CaF2 content on the formation behavior of spinel inclusions in the Ni–Cr–V-alloyed special steel was presented. The spinel was not formed at CaF2 content lower than 10 wt%. However, it was formed at early stage when the CaF2 content was greater than 30 wt%, followed by a modification to the aluminosilicate-type inclusions. Because the slag was saturated by MgO, the activity of MgO was unity irrespective of CaF2 content in the slag. Thus, Mg was transferred from slag to metal phase. Mg transferred to molten steel reacted with Al2O3-rich inclusions to form MgO·Al2O3 spinel. However, the spinel inclusion was modified to aluminosilicate-type inclusions by the reaction with Si and Ca transferred from slag to molten steel about 2–3 h later.

The effects of reducing agent, CaF2 content, and reaction temperature upon the silicothermic reduction of MnO in the BaO-MnO-MgO-CaF2 (-SiO2) slags were investigated. Mn recovery was proportional to Si activity in the molten alloy. Moreover, 90 pct yield of Mn recovery was obtained under 5 mass pct CaF2 content and 1873 K (1600 °C) reaction temperature. Increasing CaF2 content above 5 pct yielded little or no further increase in Mn recovery, because it was accompanied by increased slag viscosity owing to the precipitation of high melting point compounds such as Ba2SiO4.

The [S] content in resulfurized steel is controlled in the range of 200 to 800 ppm to ensure good machinability and workability. It is well known that “MgAl2O4(spinel)+CaS” complex inclusions are formed in molten steel during the ladle refining process, and these cause nozzle clogging during continuous casting. Thus, in the present study, the “Refractory-Slag-Metal-Inclusions (ReSMI)” multiphase reaction model was employed in conjunction with experiments to investigate the influence of slag composition and [S] content in the steel on the formation of oxide-sulfide complex inclusions. The critical [S] and [Al] contents necessary for the precipitation of CaS in the CaO-Al2O3-MgO-SiO2 (CAMS) oxide inclusions were predicted from the composition of the liquid inclusions, as observed by scanning electron microscopy–electron dispersive spectrometry (SEM-EDS) and calculated using the ReSMI multiphase reaction model. The critical [S] content increases with increasing content of SiO2 in the slag at a given [Al] content. Formation mechanisms for spinel+CaS and spinel+MnS complex inclusions were also proposed.

In this study, the electromagnetic cold crucible (EMCC) technique coupled with a Ca treatment method was used to test its potential for recycling Ti scraps. Deoxidation of molten Ti scraps was studied at Ca concentrations ranging from 0 to 0.5 wt%. With the addition of Ca, the contents of O, C, and H in the Ti scraps decreased, whereas the N content increased. Ca-based compounds were only observed on the top surface of the Ti ingots. The grain size and hardness of the fabricated Ti ingots before and after Ca addition were similar among the specimens. Noticeably, the ultimate tensile strength and the strain at fracture of the fabricated Ti ingots significantly increased from 641 ± 23 to 774 ± 8 MPa and from 5.8 ± 0.8% to 19.5 ± 5.1%, respectively, with the addition of Ca.

High-temperature experiments and Refractory-Slag-Metal-Inclusion (ReSMI) multiphase reaction simulations were carried out to determine the effect of the ladle slag composition on the formation behavior of non-metallic inclusions in molten steel. Immediately after the slag-metal reaction, magnesium migrated to the molten steel and a MgAl2O4 spinel inclusion was formed due to a reaction between magnesium and alumina inclusions. However, the spinel inclusion changed entirely into a liquid oxide inclusion via the transfer of calcium from slag to metal in the final stage of the reaction. Calcium transfer from slag to metal was more enhanced for lower SiO2 content in the slag. Consequently, the spinel inclusion was modified to form a liquid CaO-Al2O3-MgO-SiO2 inclusion, which is harmless under steelmaking conditions. The modification reaction was more efficient as the SiO2 content in the slag decreases.

The equilibrium reaction between Ni alloys and CaO-Al2O3-CaF2-TiO2 system electroslag remelting (ESR) slags was investigated in the temperature range of 1773 K to 1873 K (1500 °C to 1600 °C) at p(O2) = 10−16 atm in order to obtain the optimized composition of the slags for producing Ni alloys with various Al and Ti ratios. In addition, the temperature dependence of the reaction equilibria between the ESR slags and Ni alloys was also evaluated. The stable ionic species of titanium in the ESR slag under the present experimental conditions was experimentally confirmed to be mainly Ti4+ (i.e., TiO2) by X-ray photoelectron spectroscopy analysis of the quenched samples. The activity-composition relationship of TiO2 and Al2O3 in the ESR slag was determined as a function of the Al/Ti ratio of the alloys and the CaF2 content of the slags in conjunction with the activity ratio of Al to Ti in the alloys calculated from the FactSageTM 7.0 software. The temperature dependence of the activity-composition relationship of TiO2 and Al2O3 in the slag showed good linear correlations, and the equilibrium content ratio of TiO2 to Al2O3 at a fixed activity ratio increased with increasing temperature, which was expected based on the standard enthalpy change of the reaction. Thus, higher amounts of TiO2 should be added at higher operation temperatures in the ESR process. A 120 kg scale pilot ESR test (2000 A and 16 V) was performed to produce a commercial grade Ni-based superalloy based on the activity-composition relationship of the slag components obtained in the present study. Consequently, the contents of Al and Ti in the solidified ESR ingot were nearly the same as that of the original electrode throughout the entire length (280 mm) after the ESR process.

Rice husk ash (RHA) has been widely used as an insulation powder in steel casting tundish. Its effect on the reoxidation of molten steel in tundish as well as on the corrosion of magnesia refractory was investigated. The reoxidation of the steel, indicated by an oxygen pickup, was progressed by increasing the ratio of RHA to the sum of RHA and carryover ladle slag (R ratio) greater than about 0.2. The increase of the silica activity in the slag layer promoted the self-dissociation of SiO2 from the slag layer into the molten steel, resulting in the silicon and oxygen pickup as the R ratio increased. The total number of reoxidation inclusions dramatically increased and the relative fraction of Al2O3-rich inclusions increased by increasing the R ratio. Hence, the reoxidation of molten steel in tundish might become more serious due to the formation of alumina-rich inclusions as the casting sequence increases. MgO in the refractory directly dissolved into the molten slag layer without forming any intermediate compound layer (e.g., spinel), which is a completely different situation from the general slag-refractory interfacial reaction. A flow was possibly induced by the bursting of gas bubbles at the ash-slag (-refractory) interface, since the silica in the RHA powder continuously dissolved into the molten slag pool. Thus, the RHA insulation powder has a negative effect on the corrosion of MgO refractory.

Desulfurization behavior was investigated based on a wide slag composition and working temperature range. Moreover, the rate-controlling step (RCS) for desulfurization with regard to the ladle-refining conditions and the transition of the RCS by changing the slag composition was systematically discussed. The desulfurization ratio reached an equilibrium value within approximately 15 minutes irrespective of the CaO/Al2O3 (=C/A = 1.3 to 1.9) and CaO/SiO2 (=C/S = 3.8 to 6.3) ratios. However, the desulfurization behavior of less basic slags (C/A = 1.1 or C/S = 1.9) exhibited a relatively sluggish [S]-decreasing rate as a function of time. The equilibrium S partition ratio increased with an increase in slag basicity (C/A and C/S ratio), not only due to an increase in sulfide capacity but also due to a decrease in oxygen activity in the molten steel. There was a good correlation between the calculated and measured S partition ratios at various slag compositions. However, the measured S partition ratio increased by adding 5 pct CaF2, followed by a constant value. Multiphase slag exhibited a relatively slow desulfurization rate compared to that of fully liquid slag, possibly due to a decrease in the effective liquid slag volume, interfacial reaction area, and a relatively slow slag initial melting rate due to a high melting point. The activation energy of the desulfurization process was estimated to be 58.7 kJ/mol, from which it was proposed that the desulfurization reaction of molten steel via CaO-Al2O3-SiO2-MgO-CaF2 ladle slag was generally controlled by the mass transfer of sulfur in the metal phase. However, there was a transitional period associated with the rate-controlling mechanism due to a change in the physicochemical properties of the slag. For slag with a viscosity greater than about 1.1 dPa·s and an equilibrium S partition ratio lower than about 400, the overall mass-transfer coefficient was affected by the slag properties. Hence, it was theoretically and experimentally confirmed that the RCS of the desulfurization process under secondary refining conditions was strongly dependent on thermodynamic driving forces as well as the viscosity of the slag.

The interface between TiN and MgAl 2 O 4 compounds was investigated using TEM analysis in order to understand the formation mechanism of TiN on the surface of MgAl 2 O 4 compound through the orientation relationship of the TiN-MgAl 2 O 4 interface. The epitaxial growth of TiN on the surface of MgAl 2 O 4 compound is feasible due to the low planar lattice disregistry between materials as well as due to the same crystal system. TiN was confirmed to nucleate on the surface of MgAl 2 O 4 compound due to an epitaxial orientation relationship between the TiN and MgAl 2 O 4 phases.

The mechanism of iron dissolution and the effect of initial Fe content in a Zn bath on the dissolution rate of iron were investigated using a finger rotating method (FRM). When the initial iron content, [Fe]°, in the zinc bath was less than the solubility limit, the iron content in the zinc bath showed a rapid increase, whereas a moderate increase was observed when [Fe]° was close to the solubility limit. Based on Eisenberg’s kinetic model, the mass transfer coefficient of iron in the present experimental condition was calculated to be k M = 1.2 × 10−5 m/s, which was similar to the results derived by Giorgi et al. under industrial practice conditions. A dissolution of iron occurred even when the initial iron content in the zinc bath was greater than the solubility limit, which was explained by the interfacial thermodynamics in conjunction with the morphology of the surface coating layer. By analyzing the diffraction patterns using TEM, the outermost dendritic-structured coating layer was confirmed as FeZn13 (ζ). In order to satisfy the local equilibrium based on the Gibbs–Thomson equation, iron in the dendrite-structured phase spontaneously dissolved into the zinc bath, resulting in the enrichment of iron in front of the dendrite tip. Through the diffusion boundary layer in front of the dendritic-structured layer, dissolved Fe atoms diffused out and reacted with Zn and small amounts of Al, resulting in the formation of dross particles such as FeZn10Al x (δ). It was experimentally confirmed that the smaller the difference between the initial iron content in the zinc bath and the iron solubility limit at a given temperature, the lower the number of formed dross particles.