Effect of the Slag Composition and a Protective Atmosphere on Chemical Reactions and Non‐Metallic Inclusions during Electro‐Slag Remelting of a Hot‐Work Tool Steel

The remelting behavior of the hot‐work tool steel X37CrMoV5‐1 is investigated with several experimental melts on a lab‐scale ESR‐plant. The investigated parameters comprise a variation of the slag compositions and the use of a protective nitrogen atmosphere. Variations of the slag composition include slags with different contents of CaF₂, CaO, and Al₂O₃ as well as a variation of the SiO₂‐content in the slag. The remelted ingots are forged and analyzed regarding their chemical composition. The distribution and composition of the non‐metallic inclusions (NMI) is studied by an automated SEM‐EDX method. Additionally, the chemical composition of the slag after remelting is analyzed. The results show clearly an equilibrium reaction between Si and Al in the steel with SiO₂ and Al₂O₃ in the slag as well as the effect of oxygen in open ESR operation. A protective atmosphere reduces the Si‐losses during remelting, but has no major effect on the number or composition of NMI compared to open remelting. The content of NMI, especially the larger ones, is reduced significantly in all remelting experiments. The majority of the NMI are MA‐spinel type except for the CaO‐free slag, where alumina inclusions prevail. In general, remelting leads to an almost complete removal of sulfides, a reduction of oxisulfides, and a slight increase of oxides.     

Distribution of Bi Between Slags and Liquid Copper

The distribution of Bi between liquid copper and calcium ferrite slag containing 24 wt pct CaO, iron silicate slag with 25 wt pct SiO₂, and calcium iron silicate slags was measured at 1573 K (1300 °C) under controlled CO-CO₂ atmosphere. The experimental results showed that bismuth distribution is affected by the oxygen partial pressure, and bismuth is likely to exist in slags in the 2+ oxidation state, i.e., as BiO. The distribution ratio between calcium ferrite slag and metal was found to be close to that of iron silicate slag. The Bi distribution ratio was found to decrease with increasing SiO₂ and Al₂O₃ content in slag. Increasing temperature was found to decrease the Bi distribution ratio between slag and metal. Using the measured equilibrium data on Bi content of the metal and slag and composition dependence of the activity of Bi in liquid copper, the activity and hence activity coefficient of BiO in the slag was calculated. The close value of activity coefficient of BiO in both slags at the same oxygen partial pressure indicates that the CaO-BiO and SiO₂-BiO interactions are likely to be at the same level, or the FeO _x -BiO interaction is the predominant interaction for BiO in the slag. Therefore at a constant FeO _x content in the slag, the CaO-BiO and SiO₂-BiO interactions doesn’t affect \( \gamma_{\text{BiO}} \) significantly.     

Phase Equilibria of “Cu2O”-“FeO”-CaO-MgO-Al2O3 Slags at PO2 of 10-8.5 atm in Equilibrium with Metallic Copper for a Copper Slag Cleaning Production

Limited data are available on phase equilibria of the multicomponent slag system at the oxygen partial pressures used in the copper smelting, converting, and slag-cleaning processes. Recently, experimental procedures have been developed and have been applied successfully to characterize several complex industrial slags. The experimental procedures involve high-temperature equilibration on a substrate and quenching followed by electron probe X-ray microanalysis. This technique has been used to construct the liquidus for the “Cu₂O”-“FeO”-SiO₂-based slags with 2 wt pct of CaO, 0.5 wt pct of MgO, and 4.0 wt pct of Al₂O₃ at controlled oxygen partial pressures in equilibrium with metallic copper. The selected ranges of compositions and temperatures are directly relevant to the copper slag-cleaning processes. The new experimental equilibrium results are presented in the form of ternary sections and as a liquidus temperature vs Fe/SiO₂ weight ratio diagram. The experimental results are compared with the FactSage thermodynamic model calculations.     

Reduction of CaO and MgO Slag Components by Al in Liquid Fe

This study documents laboratory-scale observations of reactions between Fe-Al alloys (0.1 to 2 wt pct Al) with slags and refractories. Al in steels is known to reduce oxide components in slag and refractory. With continued development of Al-containing Advanced High-Strength Steel (AHSS) grade, the effects of higher Al must be examined because reduction of components such as CaO and MgO could lead to uncontrolled modification of non-metallic inclusions. This may lead to castability or in-service performance problems. In this work, Fe-Al alloys and CaO-MgO-Al₂O₃ slags were melted in an MgO crucible and samples were taken at various times up to 60 minutes. Inclusions from these samples were characterized using an automated scanning electron microscope equipped with energy dispersive x-ray analysis (SEM/EDS). Initially Al₂O₃ inclusions were modified to MgAl₂O₄, then MgO, then MgO + CaO-Al₂O₃-MgO liquid inclusions. Modification of the inclusions was faster at higher Al levels. Very little Ca modification was observed except at 2 wt pct Al level. The thermodynamic feasibility of inclusion modification and some of the mass transfer considerations that may have led to the differences in the Mg and Ca modification behavior were discussed.     

Effect of protected electroslag remelting on cleanliness of G20CrNi2Mo bearing steel

To meet the high cleanliness requirements of bearing steel used in high-speed railway trains, a new production process combining vacuum induction melting and electroslag remelting (ESR) was used to produce G20CrNi2Mo bearing steel. To investigate the effect of remelting on the cleanliness of the steel, two kinds of G20CrNi2Mo steels were prepared using an ESR furnace with and without high-purity argon protection. The results show that the G20CrNi2Mo electrodes smelted using a vacuum induction furnace have very high cleanliness 0.010[P%]–0.004[S%]–0.0012[O%]–0.0041[N%]). Unprotected ESR leads to an increased oxygen content, while protected ESR prevents any increase in oxygen content. Both protected and unprotected ESR results in significant desulphurisation, with desulphurisation rates reaching over 50%. The protected ESR process removes Al₂O₃–SiO₂–MnO inclusions, and the remaining inclusions in the steel can be divided into two categories, Al₂O₃ and Al₂O₃–MnS.     

Effects of FeO and CaO/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Ratio in Slag on the Cleanliness of Al-Killed Steel

The slag composition plays a critical role in the formation of inclusions and the cleanliness of steel. In this study, the effects of FeO content and the C/A (CaO/Al₂O₃) ratio in the slag on the formation of inclusions were investigated based on a 10-minute slag–steel reaction in a MgO crucible. The FeO content in the top slag was shown to have a significant effect on the formation of MgO·Al₂O₃ spinel inclusions, and critical content exists; when the initial FeO content in the slag was less than 2 pct, MgO·Al₂O₃ spinel inclusions formed, and the T.O (total oxygen) was 20 ppm; when the initial FeO content in the slag was more than 4 pct, only Al₂O₃ inclusions were observed and the T.O was 50 ppm. It was clarified that the main source of Mg for the MgO·Al₂O₃ spinel inclusion formation was the top slag rather than the MgO crucible. In addition, the cleanliness of the steel increased as the initial FeO content in the top slag decreased. As regards the effects of the C/A ratio, the MgO amount in the observed inclusions gradually increased, whereas the T.O content decreased gradually with the increasing C/A ratio. Slag with a composition close to the CaO-saturated region had the best effect on the inclusion absorption.     

Control of MgO·Al2O3 Spinel Inclusions during Protective Gas Electroslag Remelting of Die Steel

The effect of calcium treatment and/or aluminum-based deoxidant addition on the oxygen control and modification of MgO·Al₂O₃ spinel inclusions during protective gas electroslag remelting (P-ESR) of H13 die steel with low oxygen content was experimentally studied. It is found that all the inclusions in the consumable electrode are MgO·Al₂O₃ spinels, besides a few MgO·Al₂O₃ spinels surrounded by an outer (Ti,V)N or MnS layer. After P-ESR refining combined with proper calcium treatment, all the original MgO·Al₂O₃ spinels in the electrode (except for the original MgO·Al₂O₃ spinels having been removed in the P-ESR process) were modified to mainly CaO-MgO-Al₂O₃ and some CaO-Al₂O₃ inclusions, both of which have a low melting point and homogeneous compositions. In the case of only Al-based deoxidant addition, all the oxide inclusions remaining in ESR ingots are MgO·Al₂O₃ spinels. The operation of Al-based deoxidant addition and/or calcium treatment during P-ESR of electrode steel containing low oxygen content is invalid to further reduce the oxygen content and oxide inclusions amount compared with remelting only under protective gas atmosphere. All the original sulfide inclusions were removed after the P-ESR process. Most of the inclusions in ESR ingots are about 2 μm in size. The mechanisms of non-metallic inclusions evolution and modification of MgO·Al₂O₃ spinels by calcium treatment during the P-ESR process were proposed.     

Electrode reactions in dc electroslag remelting of steel rod

Abstract

The non-metallic inclusion content increased significantly when a steel rod of Fe-Ni was remelted by dc electroslag remelting. The silicon content increased slightly. The manganese and sulphur contents did not change. The total aluminium content in the ingot was max. 0·7%, while that in the electrode was only 10 ppm. The aluminium cations Al³+ in the slag are reduced to metallic aluminium at the slag/electrode interface, while O² - anions are oxidised to dissolved O in the metal pool. This Al and O subsequently recombine to form alumina inclusions in the metal pool. The inclusion content was dependent on the alumina content in the slag. When a rod of plain carbon steel was remelted, however, the increase in nonmetallic inclusion content was as little as one-tenth of that for the remelted Fe-Ni rod. The non-metallic inclusion content was independent of the polarity of the electrode.     

Effect of Sulfur in Steel on Transient Evolution of Inclusions During Calcium Treatment

In the current study, the effect of S content in the molten steel on inclusions during calcium treatment was studied using an induction furnace. The calcium in steel decreased from 48 to 2 ppm, and the sulfur in steel changed a little with time. When sulfur content in steel was as low as 25 ppm during calcium treatment, inclusions shifted from CaO-Al₂O₃-CaS to Al₂O₃-CaO with about 35 pct CaO. When the sulfur increased over 90 ppm, more CaS-CaO formed just after the addition of calcium, and then the CaS content decreased from over 45 pct to lower than 15 pct and inclusions were mostly Al₂O₃-CaO-CaS and Al₂O₃-CaO with a high Al₂O₃ content. Thermodynamic calculation predicted the variation of the composition of inclusions, indicating good agreement with the measurement, while a certain deviation existed, especially for heats with 90 and 180 ppm sulfur. A reaction model was proposed for the formation of CaO and CaS, which considered the reaction between calcium vapor bubbles in the zone and the dissolved oxygen and sulfur in the molten steel, as described by a Langmuir-type adsorption isotherm with a reaction occurring on the remaining vacant sites. The variation of transient CaS inclusions was discussed based on the thermodynamic calculation and the morphology evolution of typical inclusions containing CaS.     

Constitutional Segregation of Al2O3 in Mold Slag and Its Impact on Steel Cleanliness During Continuous Casting

The alumina pickup in a range of mold fluxes used for continuous casting of aluminum (Al)-killed ultralow carbon, low carbon, and peritectic steel have been measured. The Al₂O₃ pickup in slag varies approximately from 7 to 12 pct and depends on the slag basicity. Significantly higher Al₂O₃ pickup reported in basic slags and polynomial relationship exists between them. The effect of chemical composition on microstructure evolution and Al₂O₃ partitioning during crystallization was identified in all three types of mold slags. The microsegregation of Al₂O₃ inclusions in the constituent phase of CaO-SiO₂-Al₂O₃ based slag film is presented. Constitutional segregation of Al₂O₃ inclusion in slag was found to affect the Al₂O₃ pickup phenomena during continuous casting. Segregation of alkalis like Na and K was also observed in an Si-rich interdendritic matrix, whereas F was retained in the dendrites of all the slags studied. The Al₂O₃ inclusion partitioning and interdendritic segregation in the mold slag is studied with metallographic evidence.     

Study on Mold Slag with High Al2O3 Content for High Aluminum Steel

The slag-steel equilibrium reaction between the newly developed mold slag ND-MSL and 20Mn23AlV steel has been studied at high temperatures in the laboratory. The crystal morphology, microanalysis, and phase analysis of the original and final ND-MSL slags were studied by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and X-ray diffraction (XRD). Results show that, in the final ND-MSL slag, the constitution of SiO₂ decreased by 0.7 wt pct and Al₂O₃ increased by 6.46 wt pct, while the melting temperature, viscosity, and crystallization rate increased by 62 K, 0.66 dPa s, and 15 pct, respectively. NaAlSi₃O₈ and CaAl₂Si₂O₈ were found to be precipitated in the final ND-MSL slag. Both the original and final ND-MSL slags have a small amount of LiF crystal and good glass form. The ND-MSL slag has little change in the composition and properties compared with the two currently used mold slags.     

Evaluation of Secondary Steelmaking Slags and Their Relation with Steel Cleanliness

Based on data provided from an industrial plant and FactSage commercial software use, a study of secondary refining slags and inclusion cleanliness was performed. Six heats of two slag series, namely, A and B, with average chemical composition (wt pct) of 43.00CaO-25.90SiO₂-12.96Al₂O₃-18.13MgO for series A and 49.98CaO-23.88SiO₂-10.11Al₂O₃-11.99MgO-4.03CaF₂ for series B, were used for the study. Both series used DIN 38MnS6 modified steel. The effective viscosity, solid fraction, composition of the liquid fraction, and slag saturation degree in MgO (calculated through thermodynamic software) were related to the experimental results obtained for the inclusion cleanliness. The B slags showed lower effective viscosity than the A slags, due to their high liquid fraction. Regarding the capacity of slags in the inclusion removal, slag B5 resulted in the lowest inclusion density and was considered as the best choice among the slags studied. The inclusion species formed using B slags are constituted especially of CaO-Al₂O₃-SiO₂ and MgO-Al₂O₃ and are Al₂O₃ rich. The presence of sulfide-type inclusions (AlMnS and CaS) were more pronounced among A slags.     

Effect of Physicochemical Properties of Slag and Flux on the Removal Rate of Oxide Inclusion from Molten Steel

The slag-metal reaction experiments were carried out using a high-frequency induction furnace to confirm the effect of slag composition on the removal rate of inclusions in molten steel through the CaO-based slags. The apparent rate constant of oxygen removal (k _O) was obtained as a function of slag composition. It increased with increasing basicity, and the content of MgO and CaF₂, whereas it decreased by increasing the content of Al₂O₃ in the slag. The removal rate of inclusions was strongly affected not only by the driving force of the chemical dissolution but also by the viscosity of the slags and fluxes.     

Interfacial Reaction Between CaO-SiO2-MgO-Al2O3 Flux and Fe-xMn-yAl (x = 10 and 20 mass pct, y = 1, 3, and 6 mass pct) Steel at 1873 K (1600 °C)

This study investigated the interfacial reaction kinetics and related phenomena between CaO-SiO₂-MgO-Al₂O₃ flux and Fe-xMn-yAl (x = 10 and 20 mass pct, y = 1, 3, and 6 mass pct) steel, which simulates transformation-induced plasticity (TRIP) and twinning-induced plasticity (TWIP) steels at 1873 K (1600 °C). It also examines the effect of changes in the composition of the steel and slag phases on the interfacial reaction rate and the reaction mechanisms. The content of Al and Si in the 1 mass pct Al-containing steel was found to change rapidly within the first 15 minutes of the reaction, but then it remained relatively constant. The content of Al and Si in the 3 to 6 mass pct Al-containing steels, in contrast, changed continuously throughout the entire reaction time. In addition, the content of Mn in the 1 mass pct Al-containing steels initially decreased with increasing time, but the content did not change in the 3 to 6 mass pct Al-containing steels. Furthermore, the mass transfer coefficient of Al, k _Al, in the 1 mass pct Al-containing systems was significantly higher than that in other systems; i.e., the k _Al can be arranged such that 1 mass pct Al systems >> 3 mass pct Al systems ≥ 6 mass pct Al systems. The compositions of the final slags were close to the saturation lines of the [Mg,Mn]Al₂O₄ and MgAl₂O₄ spinels when the slags reacted with 1 mass pct Al and 3 to 6 mass pct Al-containing steels, respectively. These results, which show the effect of Al content on the reaction phenomena, can be explained by the significant increase in the apparent viscosity of the slags that reacted with the 3 to 6 mass pct Al-containing steels. This reaction was likely caused by the precipitation of solid compounds such as MgAl₂O₄ spinel and CaAl₄O₇ grossite at locally alumina-enriched areas in the slag phase. This analysis is in good accordance with the combination of Higbie’s surface renewal model and the Eyring equation.     

Assessment of Oxygen Control and Its Effect on Inclusion Characteristics during Electroslag Remelting of Die Steel

Deoxidation during electroslag remelting of S136 die steel was experimentally studied. The characteristics of inclusions in the electrode and ESR ingots were determined by image analyzer and SEM‐EDS. The results show that the oxygen content can be reduced from 89 ppm in the electrode to the lowest (12 ppm) in the ingot only when protective Ar gas remelting in combination with specially designed slag deoxidation treatment were employed simultaneously. The proportion of the oxygen combined as oxide inclusions increases with decreasing the total oxygen content in ESR ingot. The original inclusions in the electrode are mainly large (Mn,Cr)S and the large inclusions in the form of Al₂O₃ core surrounded by an outer sulfide layer, besides a few pure Al₂O₃ inclusions. After ESR process, while only pure Al₂O₃ inclusions with the size of about 1 µm were observed in ESR ingots. The large inclusions in the electrode were removed during ESR process. With higher oxygen content in the ingot, the contents of inclusions and large inclusions would be relatively higher. The results from industrial experiments have confirmed the availability of the present oxygen control technique. The mechanisms of oxygen behavior and control as well as inclusion evolution during ESR process were proposed based on experimental results along with thermodynamic analysis.     

Inclusion Characterization in Aluminum-Deoxidized Special Steel with Certain Sulfur Content Under Combined Influences of Slag Refining,Calcium Treatment,and Reoxidation

Inclusions in Al-killed steel with [S] of about 0.0060 to 0.0070 mass pct were characterized and discussed, evaluating the combined effects of basic slag refining and Ca treatment in ladle, together with reoxidation of liquid steel in casting tundish. Inclusions were changed from Al₂O₃ to MgO-Al₂O₃ spinel and then to MgO-Al₂O₃-CaO during basic slag refining. After Ca treatment, many (MgO-Al₂O₃)?+?CaS inclusions were formed, featuring the coexistence of MgO-Al₂O₃ and CaS to form a dual-phased structure. In the following Ar blowing, the number density of (MgO-Al₂O₃)?+?CaS inclusions and pure CaS increased obviously, which implied that [Ca] preferentially reacted with [S] rather than [O] in steel. Reoxidation in casting tundish caused the pickup of oxygen in steel, and the rise of total oxygen (T.O) was 0.0002 mass pct; even 55t steel has been poured. As a result, the content of CaO in inclusions increased and MgO-Al₂O₃-CaO inclusions were formed again. Thermodynamic calculations revealed that the driving force was strong for the formation of CaS-based inclusions. Higher carbon content in steel would help to reduce oxygen content while enhancing the activity of [S] in steel, which further stabilized the existence of CaS-based inclusions. Therefore, inclusions were mostly the solid (MgO-Al₂O₃)?+?CaS dual-phase ones, without the formation of liquid calcium aluminates. Contents of CaS and CaO in inclusions were affected by the [mass pct S]/[mass pct O] ratio, which was calculated as about 4.58 K and 5.34 K at 1873 K and 1823 K, respectively. This finding implied that lower oxygen was not favorable to prevent the solid inclusions in the calcium treatment of high carbon special steel.     

Influence of refining slag composition on cleanness and fatigue life of 60Si2MnA spring steel

The influence of basicity and Al₂O₃ content of LF refining slag on T.[O] (total oxygen) as well as type, number and size of non-metallic inclusions in Al killed 60Si2MnA spring steel was investigated. The results showed that with the increase of slag basicity R(CaO/SiO₂) or the decrease of Al₂O₃ content in slag, the T.[O], number and size of non-metallic inclusions decreased significantly. On the one hand, as the slag basicity increased, inclusions in steel were transformed from Al₂O₃–SiO₂–CaO–MgO quaternary system to Al₂O₃–SiO₂–CaO–MgO–CaS quinary system, which made the formation of voids between inclusions and steel matrix to decrease. Furthermore, thermodynamic calculations showed that CaS could only form in steel (R?≥?3.4). Al₂O₃–SiO₂–CaO–MgO came close to the compositions of the low melting point area, while Al₂O₃–SiO₂–CaO–MgO–CaS deviated from this. On the other hand, as the Al₂O₃ content in slag increased, Al₂O₃–SiO₂–CaO–MgO–CaS came close to the compositions of the low melting point area. In conclusion, the cleanness and fatigue life of 60Si2MnA spring steel had been improved by the increase of slag basicity or the decrease of Al₂O₃ content in slag.     

Effect of physical properties of slag on sulphur removal mechanism during ESR process

In this study, the effect of slag's physical properties on the desulphurization process in electroslag remelting (ESR) was investigated. Three different steel grades were used as consumable electrodes in ESR. Each steel was remelted under three slag compositions with different physical and chemical properties. The results obtained showed the efficiency of ESR in removal of up to 68 % of the sulphur contained in the metal. This pronounced effect of ESR on desulphurization is a result of slag/metal and gas/slag reactions. Although the greatest portion of sulphur is removed by gas/slag reaction which is enhanced by using a low viscosity slag, the metal/slag reaction represents the rate controlling step over all the desulphurization process. Slag/metal reaction occurring at the metal droplets/slag interface is the most effective and enhancing by relatively high slag viscosity and high CaO content in the slag. Slag composition (mass contents in %) of 70 CaF₂, 15 CaO and 15 Al₂O₃ has a good combination of chemical reactivity and physical properties for attaining the highest degree of desulphurization among the three investigated slags.     

Effect on cleanliness of molten steel with different refining slag systems for low alloy ship plate

To study the effect of refining slag on the compositions of molten steel and inclusions, the reaction between ship plate steel and different slag systems (slag A-CaO/Al₂O₃: 1.0, SiO₂: 5 mass-% and slag B-CaO/Al₂O₃: 1.5, SiO₂: 9 mass-%) was investigated by laboratory-scale high-temperature equilibrium experiments. Results showed that the desulphurisation capacity of the two slags was very similar, and the average sulphur content for both was 0.002?mass-% in steel, but the deoxidation capacity of slag B was slightly higher than slag A. The amount of inclusions <5?μm was more in steel balanced with slag B than in that balanced with slag A. To generate inclusions smaller than 5?μm, and spherical liquid CaO–MgO–Al₂O₃–SiO₂ inclusions, and to decrease the T[O] (total oxygen) content in steel, refining slag composition should be: CaO/Al₂O₃ ～ 1.5, and SiO₂ ～ 9?mass-%. Slag system optimisation can reduce or even eliminate calcium treatment during production. T[O] content in the slab could be controlled to 15–21?ppm, with typical micro-inclusions of ≤10?μm, and CaO–Al₂O₃–MgO and CaO–Al₂O₃–CaS systems in molten steel.