Fundamental thermodynamic aspects of the CaO–Al2O3–SiO2 system

The most important metallurgical effects of ladle treatment of aluminium-killed steels with calcium, are associated with the modification of alumina inclusion. For the development of the deoxidation-control model for inclusions, the thermodynamic slag model, based on the Gibbs energy minimization and modelling approaches postulated from J. Hastie et al., was used to calculate component oxide activities in the system CaO–Al₂O₃ and part of systems 3CaO · Al₂O₃ – SiO₂, 12 CaO · 7Al₂O₃ – SiO₂ and CaO · Al₂O₃ – SiO₂ at 1600°C.     

Simultaneous Modification of Alumina and MgO·Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Inclusions by Calcium Treatment During Electroslag Remelting of Stainless Tool Steel

Calcium modification of both alumina and MgO·Al₂O₃ inclusions during protective gas electroslag remelting (P-ESR) of 8Cr17MoV stainless steel and its effect on nitrides and primary carbides were studied by analyzing the transient evolution of oxide and sulfide inclusions in the P-ESR process. The oxide inclusions that were not removed during P-ESR without calcium treatment were found to retain their original state until in as-cast ingot. Calcium treatment modified all MgO·Al₂O₃ and alumina inclusions that had not been removed in the P-ESR process to liquid/partially liquid CaO-Al₂O₃-(MgO) with uniformly distributed elements, in addition to a small proportion of partially modified inclusions of a CaO-MgO-Al₂O₃ core surrounded by a liquid CaO-Al₂O₃. The modification of low-MgO-containing MgO·Al₂O₃ inclusions involves the preferential reduction of MgO from the MgO·Al₂O₃ inclusion by calcium and the reaction of calcium with Al₂O₃ in the inclusion. It is the incomplete/complete reduction of MgO from the spinel by calcium that contributes to the modification of spinels. Alumina inclusions were liquefied by direct reaction with calcium. Calcium treatment during P-ESR refining also provided an effective approach to prevent the formation of nitrides and primary carbides in stainless steel through modifying their preferred nucleation sites (alumina and MgO·Al₂O₃ inclusions) to calcium aluminates, which made no contribution to improving the steel cleanliness.     

Calcium Modification of Spinel Inclusions in Aluminum-Killed Steel: Reaction Steps

Calcium treatment is a well-established way to modify solid alumina inclusions to liquid or partially liquid calcium aluminates. Spinels (Al₂O₃·xMgO) can also form in liquid steel after aluminum deoxidation. Like alumina, the spinels can be modified readily to liquid inclusions by a calcium treatment. The modification of spinels was studied by observing the transient evolution of inclusions, in laboratory and industrial heats. Spinel modification involves the preferential reduction of MgO from the spinel, with Mg dissolving in the steel, and it proceeds through transient calcium sulfide formation, just like in the case of alumina inclusions. Because magnesium dissolves in steel after the calcium treatment of spinels, the reoxidation of the melt will produce new spinels.     

Investigation on the removal efficiency of inclusions in Al-killed liquid steel in different refining processes

Industry trials were carried out to study the removal efficiency of inclusions in Al-killed liquid steel in the processes of BOF–LF–RH–CC and BOF–RH–CC. It was found that the removal efficiency of inclusions has a high dependence on inclusion types. Solid inclusions are more easily to be removed than liquid inclusions. The removal efficiency of solid Al₂O₃ inclusions is higher than that of solid CaO–Al₂O₃–MgO inclusions. As liquid CaO–Al₂O₃–MgO inclusions coexisted with solid CaO–Al₂O₃–MgO inclusions in the liquid steel, the low removal efficiency of inclusions in RH degassing process was found in BOF–LF–RH–CC process. However, high removal efficiency and ultra-low total oxygen (T.O) content were obtained in BOF–RH–CC process because the inclusions were mainly composed of solid Al₂O₃ although initial T.O content before RH degassing was relatively high. This is due to the fact that solid Al₂O₃ tends to form cluster-shaped inclusions which have both a higher contact angle and a lower work of adhesion with steel than calcium aluminate, resulting in easier removal by RH degassing. Therefore, it is proposed to weaken steel–slag reaction and calcium treatment before RH degassing to retain solid Al₂O₃ inclusions in the steel.     

Formation and Evolution Mechanism of the Inclusions of Al2O3·SiO2·CaO and Al2O3·SiO2·CaO·MgO in Seamless Steel Tube Steel

Herein, the formation and evolution mechanism of inclusions of Al₂O₃·SiO₂·CaO and Al₂O₃·SiO₂·CaO·MgO in seamless steel tube steel are investigated. In the long strip defects on the longitudinal cross section of the steel tube after the rolling bar piercing, the defect is mainly formed by Al₂O₃·SiO₂·CaO·MgO inclusions and Al₂O₃·SiO₂·CaO·inclusions dotted with·CaS inclusions after the rolling. The typical inclusions in the different steelmaking stages are mainly composed of CaS, Al₂O₃·(SiO₂), CaO·(SiO₂), MnS·(TiN), Al₂O₃·SiO₂·CaO·(CaS)·(MnS), Al₂O₃·SiO₂·CaO·MgO·(MnO), Al₂O₃·SiO₂·CaO·MgO·(CaS)·(MnS), etc. In the billet, the average sizes of Al₂O₃·SiO₂·CaO-based and Al₂O₃·SiO₂·CaO·MgO-based inclusions are much larger than those of the other types of inclusions. Part of SiO₂ in the deoxidized products SiO₂ can be reduced by [Al], resulting in the formation of the Al₂O₃·SiO₂ composite inclusions. The SiO₂ in Al₂O₃·SiO₂ inclusions can continuously be reduced by the dissolved [Ca] to form the Al₂O₃·SiO₂·CaO composite inclusions. The SiO₂ in the Al₂O₃·SiO₂·CaO inclusions can be reduced by the dissolved [Mg] to form the Al₂O₃·SiO₂·CaO·MgO composite inclusions. Another formation process of Al₂O₃·SiO₂·CaO·MgO inclusions is the entrapment of ladle slag in the vacuum degassing (VD) stage, due to the strong agitation of the rising Ar bubbles in the vacuum condition of the VD stage.     

Behavior of MgO· Al2O Based Inclusions in Alloy Steel During Refining Process

The transformation of MgO ? Al₂O₃ based inclusions in alloy steel during refining has been studied by industrial trials. Besides Factsage software is used to study the formation and modification of spinel inclusions in alloy steel using calcium treatment during refining process. The results show that the transformation sequence of inclusions is: MgO ? Al₂O₃→CaO-Al₂O₃-MgO complex inclusions→MgO ? Al₂O₃, and under present experimental condition, in order to avoid forming MgO ? Al₂O₃ inclusions the content of dissolved Ca in the molten steel has to reach 1×10^?6. Also the results show that when more calcium was added to molten steel, the content of Al₂O₃ and MgO will be lower. Besides, increasing the content of CaO in the inclusions will increase even if the content of SiO₂ changes little.     

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.     

Influence of calcium treatment on cleanness and fatigue life of 60Si2MnA spring steel

Contrasting experiments of Al killed 60Si2MnA spring steel were carried out between using and excluding calcium treatment under LF refining slags with low and high basicity ratios (R: CaO/SiO₂?=?3.4, 5.0), respectively. Results showed the high basicity refining slag had a certain effect on controlling inclusions and improving the cleanness of spring steel similarly to calcium treatment. The T.[O] (total oxygen) content of steel without calcium treatment got to below 15?ppm and the fatigue life was long, up to 7.8?×?10⁶?cycles. But in order to reduce the T.[O] below 10?ppm, as well as inclusion number and size in spring steel further, meanwhile, the appropriate calcium treatment should still be used. Besides, as the [Ca] content in the steel with calcium treatment increased, inclusions transformed from Al₂O₃–SiO₂–CaO–MgO to Al₂O₃–SiO₂–CaO–MgO–CaS completely, which reduced the formations of voids between inclusions and steel matrix, and voids decreased with the increase of CaO/Al₂O₃ value and CaS content of inclusions. Finally, the fatigue life of spring steel with high basicity slag and calcium treatment increased to 9.1?×?10⁶ cycles.     

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.     

Transformation of Alumina Inclusions by Calcium Treatment

The objectives of this study were to investigate reactions of calcium with Al₂O₃ by different model experiments both on the laboratory and on the industrial scale. Experiments with solid Al₂O₃ and CaO were performed between 1350 °C and 1600 °C. Reaction rate constants were determined based on scanning electron microscopy/energy dispersive spectroscopy (SEM/EDS) observations of reaction products and weight measurements of the Al₂O₃ reacted via dissolution of the CaO bearing phases from the specimens after the annealing period. The results showed that the formation of calcium aluminate phases proceeded rapidly at temperatures greater than 1405 °C when a liquid calcium aluminate was formed. In the lowest temperature range (1350 °C–1405 °C), when the formation of liquid phase ceased, the reaction rate was several orders of magnitude lower. Industrial trials including Ca-alloy injection into steel, sampling and SEM/EDS analyses, as well as an inclusion rating in the samples show the concept of rapid transformation of the alumina inclusions with Ca treatment.     

Chemical characteristics of inclusions formed at various stages during the ladle treatment of steel

Abstract

Industrial data were analysed to shed some light on the formation and growth of non-metallic inclusions during the ladle treatment of a particular grade of tool steel, Orvar Supreme (Fe-0·39C-1·0Si-0·4Mn- 5·2Cr-1·0Mo-0·9V). Seven types of inclusions were detected in samples taken along the processing evolution of the steel. The types of inclusions present were found to vary with the various stages of that evolution. While additions of aluminium to the steel bath were found to affect the composition of the inclusions, only a small number of pure alumina inclusions, agglomerated as clusters, were observed during the initial stages of deoxidation. Ladle glaze was found to be the major source of the inclusions. Most of those left in the steel before tapping were found to be of very small size and to contain high concentrations of Al₂O₃ and CaO and relatively minor ones of MgO and FeO.     

Study on refining slags targeting high cleanliness and lower melting temperature inclusions in Al killed steel

Abstract

Three high basicity slags (A, B and C) were used in laboratory to refine Al killed steel to target high oxide cleanliness and low melting temperature inclusions. Inclusions were of CaO–MgO–Al₂O₃–SiO₂ system after 90 min reaction, parts of which were MgO based. Total oxygen were in the range of 0·0007–0·0010 and 0·0005–0·0010% respectively when slag A (CaO/SiO₂, 6–8; Al₂O₃, ～40%) and slag B (CaO/SiO₂, 6–8; Al₂O₃, ～30%) were applied, with inclusions all in spherical shape and mainly <5 μm. Inclusion composition concentrated in or around the lower melting point region (<1500°C) under slag A, while it became more scattered under slag B. Total oxygen varied between 0·0008 and 0·0011% under slag C (CaO/SiO₂, 3–4; Al₂O₃, about 20–25%). Many of the inclusions were in larger size, irregular morphology and located far away from the lower melting point region. Formation of MgO based inclusions closely related to solubility behaviour of MgO in the slag.     

Experimental Study to Improve the Castability of Aluminum Killed Cold Heading Steel

Experimental study on the LF refining of aluminum killed cold heading steel shows that calcium content in the molten steel increased to about 0.0010% at the end of refining, and the aluminum deoxized products were transformed from Al₂O₃ to the complex inclusions CaO–MgO–Al₂O₃ with lower melting point by the high basicity, high Al₂O₃, and strong deoxidizing slag. The inclusions are in liquid state and can be easily floated up during LF refining and continuous casting. The total oxygen content of the steel falls to about 0.0020%. The experimental technology uses only 50 m calcium wire to the 80‐t heat or even without calcium treatment. As compared to the traditional technology with higher amount of calcium for treatment, which forms CaS and CaO–MgO–Al₂O₃ inclusions with high melting point, the experimental technology improves the castability and reduces the manufacturing cost.     

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.     

Dissolution rate of Al2O3 into molten CaO–Al2O3–CaF2 flux

Abstract

Dissolution of Al₂O₃ into molten CaO–Al₂O₃–CaF₂, a base system of mould flux for continuous casting of high Al steel, has been investigated by employing a rotating cylinder method. The dissolution rate of an alumina rod into molten CaO–Al₂O₃–CaF₂ flux increased with increase in rotating speed and temperature. The apparent activation energy for mass transport of flux C was calculated to be 255·6 kJ mol^?1. The rate controlling step during the dissolution process of the alumina rod into molten CaO–Al₂O₃–CaF₂ flux was found to be the diffusion of the solute in the flux boundary layer. The dissolution rate of alumina into molten CaO–Al₂O₃–CaF₂ flux increased with increasing CaO/Al₂O₃, and it may be caused by the increase in thermodynamic driving force and the decrease in the viscosity of the flux. When the Al₂O₃ rod was immersed into molten flux, an intermediate compound of CaO.2Al₂O₃ formed firstly and then dissolved into molten flux.     

Study on Properties of Alumina‐Based Mould Fluxes for High‐Al Steel Slab Casting

During the continuous casting of high‐Al steel, the dynamic reduction of silica‐based mould fluxes by the aluminium in the steel leads to changes in their composition and physical properties. The alumina‐based mould flux has been suggested as an alternative to alleviate this reduction problem. However, until now, the smooth running of high‐Al steel continuous casting has been impeded by the lack of systematic investigation of properties of this slag. In this paper, the effects of typical components on the properties of alumina‐based mould fluxes are discussed. The experimental results show that: (a) an increase in F^? can reduce the viscosity while increasing the melting and break temperatures; (b) with increasing Li₂O, the viscosity, melting temperature, and break temperature first decrease and then increase; (c) with the addition of BaO, the viscosity, melting temperature, and break temperature remain at a low level, while a further increase in BaO causes a decrease in viscosity, an increase in melting temperature, and the stabilization of the break temperature; (d) BaO is favorable to stabilize the properties of mould fluxes for the dissolution of additional Al₂O₃; (e) the crystalline phases of the mould fluxes mainly contain 12CaO · 7Al₂O₃ and 11CaO · 7Al₂O₃ · CaF₂, and 12CaO · 7Al₂O₃ has great potential as a substitute for cuspidine.     

Characteristics analysis of inclusion of 60Si2Mn–Cr spring steel via experiments and thermodynamic calculations

A plant trial of the production of 60Si2Mn–Cr spring steel using silicon–manganese combined with aluminium to deoxidise was performed, and the characteristics of inclusions during ladle furnace refining, calcium treatment and in billets were investigated by scanning electron microscope–energy dispersive spectroscopy and thermodynamic calculations. The formation mechanisms of oxide and CaS inclusions are discussed. The experimental observation and thermodynamic analysis showed that calcium treatment cannot entirely modify large-size MgO·Al₂O₃ spinel inclusions into homogeneous CaO–MgO–Al₂O₃ inclusions, but formed a liquid xCaO·yAl₂O₃ layer on its surface. When the Al content was 0.05 mass%, [Mg], [Ca] and [O] in molten steel could be controlled at 2.7～5 ppm, 2.5～8 ppm and 4.1～5.2 ppm, respectively, to achieve inclusions in the low melting point region. A large amount of CaS was generated in the present process due to a higher sulphur concentration in the molten steel and an excessive amount of Ca–Si wire. To avoid/reduce its formation, the sulphur concentration should be controlled to below 70 ppm.     

Model for diffusion coefficient estimation of calcium ions in CaO–Al2O3–SiO2 slags

Abstract

As a common component in metallurgical slag, CaO plays an important role in desulphurisation, dephosphorisation and absorption of non-metallic inclusions. In order to better understand the mechanism of the slag/metal reactions, the diffusion dynamics of calcium ions in CaO–Al₂O₃–SiO₂ slags were studied. It was found that there was almost a linear relation between the logarithms of pre-exponential factor and diffusion activation energy. By combining the relation between the diffusion activation energy and the optical basicity corrected in the CaO–Al₂O₃–SiO₂ slags, a simple model used to estimate the diffusion coefficient of calcium ion is proposed. The estimated values of the CaO–SiO₂ and CaO–Al₂O₃–SiO₂ systems agree well with the experiment data, with a mean deviation of 35·3 and 22·5% respectively.     

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.     

Formation and Modification of MgO·Al2O3-Based Inclusions in Alloy Steels

The current study performed thermadynamic calculation, laboratory experiments, and industrial trials for the formation and modification of MgO-Al₂O₃ spinel inclusions in alloy steels. The stability Mg-Al-O diagram was obtained using the thermodymanic study. The resulting MgO-Al₂O₃-CaO inclusions from MgO-Al₂O₃ spinel inclusions after the calcium treatment were spherical, and?>?5???m MgO-Al₂O₃-CaO inclusions have a two-layer structure: an outside CaO-Al₂O₃ layer and a MgO-Al₂O₃ core. The modification of?>?5???m MgO·Al₂O₃ spinel inclusions by calcium treatment includes two steps: (1) reducing MgO in the inclusion into the dissolved magnesium by the dissolved calcium in the steel and (2) generating a liquid xCaO·yAl₂O₃ layer at the outside of the spinel inclusion. For <2???m MgO·Al₂O₃ spinel inclusions, they can possibly be modified into a xCaO·yAl₂O₃ inclusion by reducing all MgO component in the spinel inclusions with the added calcium.