Argon Stirring Research Articles

Evolution of Nonmetallic Inclusions with Varied Argon Stirring Condition during Vacuum Degassing Refining of a Bearing Steel

In the current study, industrial trials are performed to investigate the effect of argon stirring conditions on nonmetallic inclusions of a bearing steel during vacuum degassing (VD). Three argon stirring modes, including strong stirring, moderate stirring, and weak stirring during the last 10 min of the high‐vacuum period, are compared. The removal fraction of >1 μm inclusions and >5 μm inclusions is the highest with the moderate stirring mode, reaching 15.7% and 42.2%, respectively. Inclusions of MgO·Al2O3 are easier to remove than CaO–Al2O3 and entrained CaO–Al2O3–SiO2 ones as MgO·Al2O3 inclusions have a higher coagulation coefficient, a higher contact angle, and a lower adhesion work between the inclusion and the molten steel. Meanwhile, the removal efficiency of inclusions in the bearing steel is in the order of Al2O3 > MgO·Al2O3 > solid CaO–Al2O3 > liquid CaO–Al2O3 and entrained CaO–Al2O3–SiO2. Liquid CaO–Al2O3 inclusions with >5 μm size originate from slag entrainment and can be efficiently removed from the molten steel by the moderate stirring mode.

МАТЕМАТИЧНЕ МОДЕЛЮВАННЯ ВАКУУМНОЇ ДЕГАЗАЦІЇ СТАЛІ У КОВШІ З АРГОННОЮ ПРОДУВКОЮ

У статті подано математичну модель нестаціонарного процесу деазотації і дегідрогенізація розплаву сталі у вакууматорі камерного типу з аргонною продувкою. Дегазація сталі за допомогою вакууму — поширена серед металургійних підприємств технологія, яка дає можливість досягати надзвичайно низької концентрації водню та азоту у металевому розплаві, що необхідно для підвищення якості сталевих виробів. За відомою гіпотезою спочатку атоми газу знаходяться у розплаві у розчиненому стані. Бульбашки водню і азоту формуються з розчину на поверхні ковшової футерівки при умові достатньо низького феростатичного тиску металевого розплаву. Тиск, необхідний для появи бульбашки, визначається відповідно закону Сівертса. Значною мірою на дегазацію впливає і продувка аргоном, бульбашки якого збирають водень і азот на своєму шляху, спливаючи через розплав. Також важливим завданням є зменшення тривалості дегазації для зберігання температури розплаву на достатньо високому рівні. Проведення чисельних досліджень означеного вище процесу на математичній моделі зменшує витрати часових і фінансових ресурсів, тому побудова моделі є актуальним завданням. Опис плину розплаву і газів у ковші здійснюється на основі законів збереження маси та вектору кількості руху суцільного середовища, що виправдано через дрібний розмір бульбашок і їх велику кількість. З огляду на складність пошуку аналітичного розв’язку нелінійних диференціальних рівнянь у часткових похідних у тривимірній постановці, пропонується використовувати метод центральних різниць, який має достатню точність і широко використовується для подібних задач. Обчислювати математичну модель пропонується у комп’ютерній програмі на мові C#, яка має широкі можливості по програмуванню алгоритмів. Програмний додаток дозволить оцінити вплив інтенсивності аргонної продувки, а також глибини розплаву, на ступінь його дегазації, що може бути використано при впровадженні технологічних рекомендацій у виробництво сталі.

Physical Simulation of Molten Steel Homogenization and Slag Entrapment in Argon Blown Ladle

Argon stirring is one of the most widely used metallurgical methods in the secondary refining process as it is economical and easy, and also an important refining method in clean steel production. Aiming at the issue of poor homogeneity of composition and temperature of a bottom argon blowing ladle molten steel in a Chinese steel mill, a 1:5 water model for 110 t ladle was established, and the mixing time and interface slag entrainment under the different conditions of injection modes, flow rates and top slag thicknesses were investigated. The flow dynamics of argon plume in steel ladle was also discussed. The results show that, as the bottom blowing argon flow rate increases, the mixing time of ladle decreases; the depth of slag entrapment increases with the argon flow rate and slag thickness; the area of slag eyes decreases with the decrease of the argon flow rate and increase of slag thickness. The optimum argon flow rate is between 36–42 m3/h, and the double porous plugs injection mode should be adopted at this time.

Steel refining possibilities in LF

This article presents the main possibilities for steel refining in Ladle Furnace (LF). These, are presented: steelmaking stages, steel refining through argon bottom stirring, online control of the bottom stirring, bottom stirring diagram during LF treatment of a heat, porous plug influence over the argon stirring, bottom stirring porous plug, analysis of porous plugs disposal on ladle bottom surface, bottom stirring simulation with ANSYS, bottom stirring simulation with Autodesk CFD.

Hydrodynamic effects created by argon stirring liquid steel in a one-strand tundish

ABSTRACTContinuous casting of steel ensures the flexible production of different grades of slabs intended for the Rolling Mill departments. Therefore, it seems justifiable to search new solutions for tundish metallurgy. This study puts forward a new gas-permeable barrier’s design solution with two circular porous plugs, whose purpose is to punctually agitate the liquid steel in tundish. Two argon injection system locations were tested. The results for the hydrodynamic pattern of liquid steel and bubbles’ behaviour in the tundish were obtained from laboratory experiments with water glass model and computer simulations. Within the framework of the investigation, three argon flow rates, i.e. 5, 10 and 15 NL min−1, were considered. The designed gas-permeable barriers effectively stimulate active flow in the tundish for isothermal and non-isothermal conditions, while reducing the stagnant flow share and the transient zone extent.

Effect of Stirring Practice on Sulphur and Nitrogen Refining as well as Inclusion Removal in Ladle Treatment

Steel cleanliness as a function of stirring practice during vacuum degassing treatment have been investigated in industrial studies at the steel plants of SSAB Special Steels in Oxelosund and Uddeholm AB in Hagfors. The cleanliness was examined with regards to the contents of sulphur, nitrogen and large inclusions (>10µm).The stirring practice during the vacuum degassing treatment has been studied with respect to time, namely by shortening the vacuum degassing treatment time from 24 to 15 minutes. Furthermore, the effect of the stirring intensity was studied by measuring the open eye zone using camera recordings. The focus has been to study the effect of a shortened vacuum degassing time as well as a controlled stirring intensity on the steel cleanliness. Moreover, study the effect of subsequent induction stirring, after vacuum degassing, on the amount of inclusions. By taking steel and slag samples taken before and after the vacuum degassing treatment, the different stirring practices could be investigated. It should be noted that all heats reached the desired composition regardless of the treatment time and stirring practice.The results from the trials at SSAB Special Steels showed that the stirring practice during vacuum degassing have an effect on the total number of inclusions. The strong argon stirring during vacuum degassing with visible open eyes resulted in an average increase of 400% of the total amount of inclusions >10µm. However, no difference between a vacuum degassing time of 24 or 15 minutes was seen with respect to the increased amount of inclusions, denitrogenization or desulphurization. During the subsequent induction stirring, which was softer with no open eyes, the effect of the stirring practice was unclear due to overlapping confidence intervals. An average decrease of 65% of the total amount of inclusions >10 µm was seen for the heats with non-overlapping confidence intervals. Based on these results it can be suggested that the use of a soft induction stirring after the vacuum degassing treatment as a possible process change to reach lower amounts of large inclusions.The stirring intensity during the vacuum degassing treatment was measured as the average ladle eye size during the trials at Uddeholm AB. This, due to that stronger stirring results in larger ladle eye zone. The results show that the amount of smaller inclusions ( 22.4 μm). Moreover, the denitrogenization and desulphurization was not affected by the changed stirring intensity during vacuum degassing. It was also found that the average ladle eye size did not correlate to the logged average argon flow.

Argon Blowing Technology Using in the LF Refining

The process of LF Refining is introduced in this paper. Because the LF ladle furnace has the advantages are widely used and developed in many companies. The argon is blown into the ladle, so the purpose of refining is achieved. The argon stirring can accelerate the chemical reaction between slag and steel. In the circulation process, large inclusion and deoxidation product in the lower region transfer into the slag. The material transfer between steel and slag can be accelerated. At the same time, argon stirring can make molten steel composition and temperature uniformity. Thus, the ladle of blowing argon can be roughly divided into three important flow region has been introduced. Two-phase region, the top horizontal flow zone and recirculation zone exists in the ladle.

The Introduction of the LF Refining Function and Process

The process of LF Refining is introduced in this paper.The LF method is a ladle without vacuum equipment can arc refining method.Because the LF ladle furnace has the advantages of simple equipment, low investment cost, flexible operation and excellent refined effect, are widely used and developed in many countries.The argon is blown from the bottom of the ladle, and molten steel is heated by the graphite electrode, so the purpose of refiningis achieved. The argon stirring can accelerate the chemical reaction between slag and steel.LF refining function includes reductive atmosphere, submerged arc heating, argon stirring and white slag refining.

Analysis of Micro Inclusions in Q195 Steel

Aimed at the Q195 steel production process in a steel mill, using tracer analysis make system analysis to the process of production and microscopic inclusions content. In the steady casting average T[O] is 117.22ppm, [N] is 22.6ppm, micro inclusions average content is 11.96/mm2, while the average T[O] and [N] in transient casting billet is 161.93ppm and 30.49ppm, micro inclusions averaged content is 16.068/mm2. The total oxygen content in steel on the high side, should strengthen deoxidization. Argon stirring strength is too large, inclusions in casting slab containing tracer element, illustrate the ladle slag, tundish slag, mould protection slag are forming pollution on billet, should reduce the influence of transient casting on the steel quality.

Optimisation of the Deoxidation Degree and Modification on the Cast Steel Properties Improvement at Low Temperature

Abstract Industrial conditions of obtaining thick-walled and shaped castings intended for operations at temperatures: minus 40÷60°C are presented in the paper. The selection of a cast steel grade is based on known studies [1,10,14], however due to castings wall thickness (70-240mm), the way of preparing liquid metal (deep deoxidation and modification, argon stirring in a ladle) and conditions of filling mould cavities (gating system assuring quiet filling, directional solidification and avoiding a formation of inessential heat centres), were developed to assure the required impact strength. Maintaining these parameters as well as the selection of heat treatments for the produced massive castings allowed to achieve the impact strength over 50 J/cm2 at minus 40°. This value was obtained for walls of various thickness

Design on Optimization of Argon Bottom Blowing of Molten Steel ladle

The cause of controlling argon blowing on steel-making process is analyzed. The strategy of controlling argon stirring energy is proposed, and the scheme controlling flow rate and fixing the pressure is achieved. The argon blowing of molten steel ladle is controlled automatically and properly, which is based on fuzzy control theory and Pulse Code Modulation. The intelligent control system has worked well in several steel plants.

Emulsification of Top Slags during Argon Stirring through a Porous Plug in the Ladle of Si Deoxidised Carbon Steels

Intensive experiments with 170-t heats of carbon steels in the LD steel plant of Saarstahl AG were performed to study the emulsification of an acid top slag during different stirring conditions in this research project. During ladle treatment samples were taken from steel for the analysis of the composition of the inclusions together with the top slag. Within these investigations gas flow rates and stirring times were systematically varied in order to study their influence on the entrapment of top slag in the steel melt. At the same time a model was developed for the evaluation of the performance of the porous plug with regard to gas flow rate and gas pressure or finally for blockades and leakages. According to the experiments of this project the following results were established. Small top slag particles are discovered nearly in all steel samples together with endogenous SiO2-Al2O3 deoxidation products. But emulsified phases or parts of the top slag generating larger inclusions with a size of 30 – 60 μm show the low melting eutectic composition. On the other hand, this emulsification process leads simultaneously to CaO depletion and SiO2 accumulation of the top slag particularly when a longer stirring period is applied as shown during these experimental trials. The whole process requires low melting top slags and low melting emulsified inclusions in combination with a low viscosity level. To meet the emulsification requirements the gas flow is characterised by high gas flow rates in the order of 30 – 40 STP m3/h and high pressures with 8 – 12 bar indicating a blockade of the porous plug and the existence of a gas jet.

Rational argon stirring for a 150-t ladle furnace

Based on the principle of similarity, water modeling experiments were carried out for a 150-t ladle furnace. The rational parameters of argon stirring were determined as the optimized positions of nozzles, top area of the porous brick, and gas flow rate. The following results are obtained: 1) the optimized positions of two nozzles are at 0.333 R ( R refers to the radius of the ladle at bottom) with an angle of 135°; 2) the top diameter of the porous brick should be 130 mm; 3) the flow rate of gas should be 25.0-30.6 m 3/h. The plant trial shows that the improved process is effective in enhancing the cleanliness of round billets. The total oxygen content, microinclusions, and macroinclusions in round billets are reduced by 12.5%, 8.2%, and 20%, respectively.

Preliminary Investigation of Mathematical Modeling of Stainless Steelmaking in an AOD Converter: Application of the Model and Results

The changes in the contents of C, Cr, Si, and Mn in molten steel and the bath temperature during the refining of 304‐grade stainless steel, including both the oxidation (decarburization) and reduction processes, in a side and top combined blowing AOD converter of 120 t capacity have been predicted. The calculations were performed using the mathematical model proposed and presented in Part I of the present work [1] and were based on the designed operational mode of the AOD converter. The model predictions were compared to the referenced values given by the technological design. The results demonstrate that the predictions by the model are in good agreement with the reference values. Not only the competition of oxidation among the elements dissolved in the steel during the oxidative refining process and the corresponding distribution ratios of oxygen, but also the competition of reduction among the oxides during the argon stirring and reductive refining process and the relevant supplied oxygen ratios of the oxides, can all be characterized more comprehensively and determined more reasonably by using the Gibbs free energies of the oxidation and reduction reactions. Corresponding to the top, side, and side and top combined (overall) refining processes of 304‐grade stainless steel in a 120 t AOD converter, the carbon concentrations at the critical rates, i.e. the critical carbon concentrations, after which the decarburization changes to be controlled by the mass transfer of carbon in molten steel, are 1.20, 0.37 and 0.53 mass%, respectively, under the given designed operational mode. The model can offer some useful information for determining the technology of the side and top combined blowing AOD refining process of stainless steel.

Study of Vacuum Carbon-Deoxidization of High Carbon Chromium Bearing Steel

Abstract This paper describes the negative influence of high residual aluminum which is used as a deoxidant in the steel and raises the newest control requirement for high carbon steel on residual aluminum. By means of thermodynamic calculation and production practices, the total oxygen (Σ[O]) in the steel can be 10 ppm through carbon-oxygen reaction under the conditions of high vacuum, proper top slag composition, and argon stirring during ladle refining/vacuum degassing (LF/VD) processes of high carbon chromium bearing steel. Meanwhile, the size of D-type inclusion in the steel is obviously reduced. Thus, the purity of the bearing steel is improved. Since the consumption of aluminum used for deoxidation is reduced, the unit cost of bearing steel is obviously reduced.

EFFECT OF PLUG LOCATION ON DESULFURIZATION CHARACTERISTICS OF SLAG DURING LADLE FURNACE OPERATION

Three hundred ton steel ladles at the steel melting shop (SMS-II) at Bokaro Steel Limited (BSL) are equipped with two porous plugs located at adjacent quadrants of the ladle bottom. High stirring rates are used to affect a high production rate and quick deep desulfurization. Wide spout eye formation and the resultant reoxidation of metal has been observed at high purging rates because of this typical plug location. The degree of reoxidation indexed with increase in wt% Al2O3 in slag was found to be proportional with the stirring rate. The equilibrium sulfide capacity (Cs) and alumina activity of slag (aAl2O3) were related with the sulfur partition (Ls) achieved in actual practice. It was found that while actual Ls vary directly with Cs, an inverse relationship exists between Ls and aAl2O3. Based on mathematical modelling, the installation of porous plugs in all three quadrants of the ladle bottom has been recommended to promote increased slag metal interaction at lower argon stirring rates and thereby control the reoxidation of steel.Les poches de coulée d’acier de 300 tonnes à l’atelier de fonte d’acier (SMS-II) de Bokaro Steel Limited (BSL) sont équipées de deux tampons poreux situés dans des quadrants adjacents au fond de la poche. On utilise des taux élevés d’agitation pour obtenir un taux élevé de production et une désulfuration rapide et profonde. Aux taux élevés de purge, on a observé la formation d’un gros oeil de bec de coulée et la réoxydation du metal qui en résultait à cause de ce positionnement typique du tampon. On a trouvé que le degré de réoxydation indexé par l’augmentation du % en poids d’Al2O3 dans la scorie était proportionnel au taux d’agitation. La capacité d’équilibre du sulfure (Cs) et l’activité de l’alumine de la scorie (aAl2O3 ) étaient reliées par le partage du soufre (Ls) obtenu en pratique. On a trouvé qu’une relation inverse existait entre Ls et aAl2O3 , bien que le Ls actuel variait directement en fonction du Cs. Basé sur la modélisation mathématique, on a recommandé l’installation de tampons poreux dans les trois quadrants au fond de la poche de coulée afin de favoriser l’interaction accrue de la scorie de métal à des taux plus bas d’agitation à l’argon et ainsi de contrôler la réoxydation de l’acier.

Temperature Prediction Model for Controlling Casting Superheat Temperature

A temperature prediction model has been developed for controlling the casting superheat temperature. For ease of implementation, the model is intentionally made simpler having a combination of a one dimensional heat transfer model and a simple regression model. The model is based on the fact that the BOF temperature of the liquid steel along with the bath cooling behaviours controls the aimed casting superheat temperature. Starting with the steel liquidus temperature and calculating the required steel temperature backwards throughout the process line gives the targeted BOF tap temperature. The on-line picking up of actual data helps the model to predict for the next stage more accurately in forward direction. Based on the predicted steel temperature, plant operators can take any necessary corrective action like additional ladle heating and extra/reduced argon stirring to ensure the aim final ladle/tundish temperatures at the casting are achieved.

Height of the spout of a gas plume discharging from a metal melt

In the present investigation, the height of the spout of a gas plume discharging from the surface of a metal melt has been measured in a laboratory model involving mercury and in real steel ladles. The spout geometry strongly fluctuates with time. This has to be taken into account in the measuring method and data evaluation. Long-term averages of the spout profile and momentary maximum height values have been determined, and it has been found that their nondimensional values are of similar size in the model and in the steel ladle. An engineering formula is presented for the estimation of maximum spout height in argon stirring.

Spout eyes formed by an emerging gas plume at the surface of a slag-covered metal melt

This article deals with the spout eyes developing, at the surface of a metal melt, in the ladle during argon stirring. Cold model experiments involving a mercury bath with an oil layer as slag and industrial experiments on a 350 t steel ladle have been carried out. The eye geometry as measured with a video technique is highly dynamic. The time average of the free surface area and the time fraction of complete coverage have been determined and are represented with dimensionless correlations.

Ladle deoxidation, desulphurisation and inclusions in steel –Part 2: Observations in practice

Practical observations on the application of oxygen sensors, argon stirring, mould additions for rimstabilized steel. Discussion of ladle desulphurisation. Deformability of oxide inclusions, control of blowhole formation. Suppression of the precipitation of MnS by absorbing the dissolved sulphur with minute calcium aluminate particles entrained in liquid steel containing less than 0.002% S, and by rare earth treatments before casting steel containing 0.002 to 0.006% S. Aspects of steelmaking and casting clean steel, particularly high‐strength, low‐alloy steels.