Reoxidation Of Liquid Steel Research Articles

Reoxidation of liquid steel and evolution of inclusions during protective atmosphere electroslag remelting of Ce-containing heat-resistant stainless steel

Reoxidation of liquid steel and evolution of inclusions during protective atmosphere electroslag remelting of Ce-containing heat-resistant stainless steel

Ensemble prediction of mean bubble size in a continuous casting mold using data driven modeling techniques

Gas injection into a slab continuous casting mold is a common practice as it enables long casting sequences by reducing SEN clogging. However, too much gas injection can generate lots of bubbles, and bubbles formed inside the mold are also responsible for the occurrence of various quality defects such as deterioration of steel cleanliness, reoxidation of liquid steel, and occurrence of sliver and blister defects, etc. Mean bubble size is a key parameter and controlling it can resolve/reduce these quality issues. Operating parameters such as gas flow rate and liquid flow rate are the major factors affecting the mean bubble size. Two-phase water modeling​ experiments were performed to generate bubbles in the mold, and the mean bubble diameter was captured using various gas and liquid flow rates. Clear images of bubbles were recorded using a high-speed high-resolution camera along with the application of shadowgraphy. Bubble images were processed using an image processing software, ImageJ to obtain bubble characteristics, and Sauter mean diameter was calculated for each operating condition. Advance machine learning techniques such as Multilinear Regression (MLR), Decision Tree (DT), Support Vector Machine (SVM), and Artificial Neural Network (ANN) were used on the experimental data to predict the combined effect of these operating parameters on the mean bubble diameter. All four ML techniques were compared considering the values of cross-validated adjusted R2, and a performance metric is presented to compare the suitability of ML techniques in this case.

Designing and Computational Modeling Study of Innovative Tundish Furnitures

Slag vortexing, Reoxidation of the liquid steel from tundish open eye with high superheat casting are main causes behind many customer complains as well as high rejection of metal in integrated steel plants. In present investigation a novel stopper rod with refractory water cooled sub-chamber has been proposed. This is capable to reduce 60% skull generation and will help to reduce supeheat of liquid steel within the tundish nearly 50%. A Novel Filter Shroud concept has been developed which will capable to generate 'No Tundish Eye' formation. This will avoid re-oxidation of liquid steel during transfer of liquid steel from ladle to tundish. Finally a modified multi reactor with argon purging technology has been proposed as a future scope of study. All design concepts are studied computationally by ANSYS FLUENT Software. All proposed works are patentable.

Evolution of Oxide–Sulfide Complex Inclusions and Its Correlation with Steel Cleanliness During Electroslag Rapid Remelting (ESRR) of Tool Steel

The evolution of CaO-Al2O3-MgO-(SiO2) inclusions associated with CaS during electroslag rapid remelting (ESRR) of the tool steel with ultra low oxygen (8 ppm) under protective atmosphere was investigated. Thermodynamic calculation was performed to evaluate the oxygen level of liquid steel and the inclusion evolution during ESRR with the help of FactSage software (CON2 database). The results demonstrated that the reoxidation of liquid steel took place during industrial ESRR, resulting in the oxygen pickup of approximately double content. The oxygen level in the steel was determined by [Al]-[O] equilibrium during ESRR, whereas FeO in the slag transferred oxygen into the liquid steel, as indicated by the significant oxygen pickup. The inclusions in the steel electrode are oxide–sulfide type of CaO-Al2O3-MgO partially wrapped with CaS. The original oxide inclusions that had not been removed in ESRR process were transformed to liquid CaO-Al2O3-SiO2-MgO inclusions through reacting with the dissolved oxygen supplied from the reoxidation of liquid steel and concerned elements in liquid steel during ESRR. These CaO-Al2O3-SiO2-MgO inclusions were invariably associated with patch-type or shell-type CaS, which hold different formation mechanisms. The inclusions ranging from 2 to 4 µm take up a dominant proportion (a little more than 50 pct of the total inclusions) in the remelted ingots, and those larger than 8 µm account for about 3 pct. Melting rates of ESRR not only exerted a negligible effect on the steel cleanliness and the removal efficiency of oxide inclusions, but also on the inclusion size distribution.

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.

Effects of Reoxidation of Liquid Steel and Slag Composition on the Chemistry Evolution of Inclusions During Electroslag Remelting

Electroslag remelting (ESR) is increasingly used to produce some varieties of special steels and alloys, mainly because of its ability to provide extreme cleanliness and an excellent solidification structure simultaneously. In the present study, the combined effects of varying SiO2 contents in slag and reoxidation of liquid steel on the chemistry evolution of inclusions and the alloying element content in steel during ESR were investigated. The inclusions in the steel before ESR refining were found to be oxysulfides of patch-type (Ca,Mn)S adhering to a CaO-Al2O3-SiO2-MgO inclusion. The oxide inclusions in both the liquid metal pool and remelted ingots are CaO-Al2O3-MgO and MgAl2O4 together with CaO-Al2O3-SiO2-MgO inclusions (slightly less than 30 pct of the total inclusions), which were confirmed to originate from the reduction of SiO2 from the original oxide inclusions by dissolved Al in liquid steel during ESR. CaO-Al2O3-MgO and MgAl2O4 are newly formed inclusions resulting from the reactions taking place inside liquid steel in the liquid metal pool caused by reoxidation of liquid steel during ESR. Increasing the SiO2 content in slag not only considerably reduced aluminum pickup in parallel with silicon loss during ESR, but also suppressed the decrease in SiO2 content in oxide inclusions. (Ca,Mn)S inclusions were fully removed before liquid metal droplets collected in the liquid metal pool.

Physical Modeling of Slag ‘Eye’ in an Inert Gas-Shrouded Tundish Using Dimensional Analysis

The formation of an exposed eye in the gas-stirred metallurgical vessels such as ladle or tundish is a common observation. Although gas stirring results in proper homogenization of melt composition and temperature, the resulting exposed eye leads to higher heat losses, re-oxidation of liquid steel, and formation of inclusions. Most of the previous research related to slag eye were carried out explicitly for ladles. In the present work, a large number of experiments were performed to measure the slag eye area in full scale and one-third scale water models of an inert gas-shrouded tundish under various operating conditions. Based on the polynomial regression of experimental data, and the method of dimensional analysis, correlations for diameter of gas bubbles and plume velocity were developed. Subsequently, these results were used to obtain correlations for the slag eye area, and critical gas flow rate in an inert gas-shrouded tundish in terms of the operational parameters viz., gas flow rate, thickness of the slag and melt baths, along with the physical properties of the liquids viz., kinematic viscosity and density. It was observed that the dimensionless slag eye area can be expressed in terms of dimensionless numbers such as the density ratio, Froude number, and Reynolds number.

Inclusion Variations of Hot Working Die Steel H13 in Refining Process

Inclusion variations of die steel H13, including changes of species, morphologies, compositions, amounts and sizes, in the production of EAF→LF→VD→ingot casting→electro-slag refining (ESR) procedure, were investigated by systematic sampling, and analyzed with scanning electron microscope (SEM), energy dispersive spectrum (EDS), and metallographic microscope. The variation mechanism was studied by comprehensive analysis of total oxygen, nitrogen, and acid soluble aluminum as well as chemical test of refining slag. Based on the investigations, technical measures for cleanness improvement were discussed. The results show that oxide inclusions in H13 steel change from irregular Al2O3→near globular CaO-MgO-Al2O3 and CaO-Al2O3-SiO2 complex inclusions→finer CaO-Al2O3-SiO2 inclusions with higher CaO content→CaO-Al2O3-SiO2 inclusions with higher Al2O3 content and irregular MgO-Al2O3 inclusions→fine irregular MgO-Al2O3-CaS inclusions in various steps of the production; the variations are related with changes of acid soluble aluminum content, reactions between slag and steel, re-oxidation of liquid steel during casting, and refining of ESR. It is also found that A12O3 inclusions are modified by refining slag in LF and VD refining; and ESR plays a good role in inclusion removal, especially in controlling the large linear VC-CrC-MoC inclusions distributed in grain boundaries. It is suggested that casting protection should be improved, and the basicity of refining slag and acid soluble aluminum content in steel should be raised.

Study on Inclusions of Low Carbon Steel Slabs Produced in Different Casting Stages

Through studying the inclusions of low carbon steel slabs produced in different casting stages, the results are dawn as follow. The reoxidation of liquid steel is very serious in casting starting,but the casting ending is not. Most of large-inclusions containe K or Na are found in the first and final slabs, which may relate to serious slag entrapment in the mould in the casting starting and ending. Many MgO-Al2O3 and Al2O3-CaO-SiO2-MgO inclusions suggest the serious entrapment of the tundish in the casting ending.

Study on Cleanliness of Ultra-Low Carbon Steel First Slab

By means of Oxygen and Nitrogen Analyzer, Metallographic Examination, SLIME, SEM, EDS, etc, the cleanliness of ultra-low carbon steel first slab produced by LD-RH-CC, and the comparative analysis with the cleanliness of normal slab is studied. The results show that T[O] and [N] are obviously decreasing with the increasing of casting length. The micro-inclusions and large-inclusions are generally decreasing with the increasing of casting length. The micro-inclusions of the first slab are mainly from deoxidization products and reoxidation of liquid steel. And the sources of large-sized inclusions mainly are reoxidation of liquid steel, slag entrapment in mould or tundish and stuffing sand. The cleanliness of ultra-low carbon steel first slab is closed to those normal slabs at the length of 3.5m.

Influence of Modified Casting Practice on Steel Cleanliness

Consistently high level of cleanliness needs to be ensured, particularly in grades of steel used for critical applications. In addition, preventing the formation of alumina rich non-deformable inclusions is important in high carbon steels for the deep drawing applications. In the present work, influence of billet caster up-gradation on the overall cleanliness of high carbon steel has been investigated, mechanical characteristics of oxide inclusions evaluated and assessment of the existing deoxidation practice have been done. In general, there has been improvement in the overall level of steel cleanliness due to increasing tundish capacity, submerge pouring of liquid steel and application of in-mold EMS. These measures have effectively controlled the inclusions originating from reoxidation of liquid steel. However, the characteristics of the oxide inclusions generated during the deoxidation process in ladle remained same even after the caster up-gradation. The conventional deoxidation practice was found to generate mostly inclusions of variable compositions and alumina rich non-deformable inclusions, which makes the steel less suitable for very fine wire drawing applications. Appropriate measures have been proposed for further improvement in the inclusion characteristics while processing those grades of steels during the secondary steelmaking.

Reoxidation of Al-Ti Containing Steels by CaO-Al2O3-MgO-SiO2 Slag

Reoxidation of liquid steel containing Al and Ti by 14%CaO-35%Al2O3-10%MgO-41%SiO2 (in mass%) slag was investigated at 1823 K with initial Al content of 820 mass ppm and Ti content varied from 100 to 500 mass ppm. It was observed that Al and Ti in the steel were simultaneously oxidized by SiO2 in the slag, and the soluble oxygen was supersaturated during the course, particularly with respect to Al. Based on the experimental results, a new mechanism of the reoxidation reaction was proposed, which involves chemical reactions both at the metal/slag interface and in the bulk metal. Self-dissociation of SiO2 into Si and O at the slag/metal interface was found to play an important role in both supersaturation of oxygen, and subsequent formation of complex oxide inclusions. Formation of inclusions having a two-layer structure where an Al2O3 core was enclosed by complex Al-Ti-O oxide was explained in relation with supersaturation of oxygen in the steel.

Reoxidation during ladle treatment

Cleanliness of steel is mainly determined by the presence of oxide inclusions. The reoxidation of liquid steel before, during and after ladle treatment at the Sidmar steel plant was quantified for three steel grades, as it is a potential source of inclusions. A mass balance calculation was performed to compare the expected and measured steel composition. The total reoxidation was calculated from the reactions of the dissolved elements. A dissimilarity in behaviour of the steel grades was found. The resultant compositions of the slags were calculated and compared with measured compositions. Some differences between computed slag compositions and average measured values are explained. Finally, the relation between the reoxidation and internal cleanliness at the end of ladle treatment was examined.