Decarburization Rate Research Articles

Mathematical Model of RH Blow Argon Mode Affecting: Decarburization Rate in Ultra-Low Carbon Steel Refining

AbstractA mathematical model in present study has been established to investigate the effect of Rheinstahl Heraeus (RH) blow argon mode on the decarburization rate in ultra-low carbon steel refining based on the RH equipment of Shougang Qian'an Iron and Steel Co Ltd (SQISCO). The calculated results show that the increase of argon flowrate promotes the carbon elimination from argon gas bubble surface, molten steel free surface in vacuum as well as splash droplet surface, while reduces that from the interior of liquid steel. It has been found the critical turning point of flowrate ascension is at the 5th minute and the optimum blow argon flowrate in later stage is 2100 L/min in accordance with the 2 stages argon blow mode, which have been confirmed in the commercial production in SQISCO.

Decarburization of Hot-Rolled Non-Oriented Electrical Steels

ABSTRACTThe high temperature decarburization-oxidation behavior of hot rolled, non-oriented electrical steel strips is investigated during air-annealing treatments. Annealing temperature and time are varied from 700 – 1050 °C and 10 to 150 min, respectively. The experimental results show that uniform external oxidation affects strongly the rate at which carbon can be removed from this material. The thickness of the oxide layer formed after 150 minutes of annealing increases linearly with increasing temperature in the range 828 and 920 °C. The effect of temperature on the thickness of the oxide scale at temperatures outside this range is significantly smaller. These results indicate that the rate of oxidation in this material is strongly influenced by the microstructure of the steel during annealing. Decarburization rates are very slow during annealing at T ≤ 750 °C where the oxide layer is thin and porous. In contrast, fast and intense decarburization of the strips is observed as a result of annealing at temperatures between 800 and 850 °C. Finally, decarburization at T ≥ 875 °C becomes slower as the temperature is increased until at T ≥ 950 °C this process is practically inhibited. Measurements of C content as a function of time and temperature show that the observed decarburization kinetics follows Wagner’s model at 800 and 850 °C. However, at higher annealing temperatures decarburization is slower than that predicted by the model. This behavior is related to the increment of the oxide scale thickness and a transition from cracked to crack-free oxide structure which makes C diffusion through the oxide film very difficult.

The Influence of Critical Turning Point of Argon Flowrate Ascension on Decarburization Rate of RH Treatment for Ultra-Low Carbon Steel

Through the mathematical model of RH vacuum decarburization, the decarburization mechanism of argon gas bubble surface was studied. By the RH equipment of Shougang Qian’an Iron and Steel Co. Ltd for background, the calculated results of this model showed that the decarburization quantity of argon gas bubble surface was about 8% that of overall decarburization. In the initial conditions was constant, the ascension of argon flowrate will promote the decarburization quantity of argon gas bubble surface, free surface and splash droplet surface as well as reducing the decarburization quantity of internal liquid steel decarburization. Based on the commercial production data, it was found the best opportunity of ascension blow argon flowrate to acquire lower carbon content in relatively shorter time.

Influence of Electrode Argon-Hydrogen Co-Injection on Carbon Content in a Alternating Current Ladle Furnace

About 80 kg low carbon steel was refined in a multifunction ladle furnace (AC-LF) with alternating current supplying. The argon-hydrogen gas mixtures were injected into the electric arc zone through one hollow graphite electrode. The flow rate of the gas mixtures was 3 m3/h. Results of the tests and the calculation based on the plasma local equilibrium thermodynamics and metallurgical thermodynamics theories show that, when the argon-hydrogen gas mixtures are injected into the electric arc zone, the carburetion rate is 4.7×10-6 per minute for the steel heated by conventional AC-LF, which is 1.26 and 1.51 times of the heats blown 90%Ar-10%H2 and 80%Ar-20%H2, respectively. The carburetion rate is reduced by 20% at least when argon-hydrogen gas mixtures are blown into the furnace. When H2 content in the gas mixtures reached 20% from 0, the decarburization rate increased but the total reaction rate decreases by 1.59×10-6 per minute.

Three-phase numerical simulation of oxygen penetration and decarburisation in EAF using injection system

The flow field of the molten bath and the decarburisation rate in a 150 t electrical arc furnace (EAF) are discussed based on a three-phase numerical simulation, while the furnace profile of the EAF and the installation location of oxygen lance are fixed. As shown in the results, with increasing charge mass of the molten bath and lowering oxygen lance height, the penetration depth of oxygen jet and the velocity of uncovered steel created by the jet increase, the area of uncovered steel becomes small and the flowrate of steel flowing through the uncovered area increases; thus, the absolute decarburisation rate increases. Simultaneously, the optimum operating mode for this EAF is with a charge mass of 150 t and an oxygen lance relative height of 0·45 m, in which the low speed area on the bottom of the EAF is the minimum and the relative decarburisation rate is the maximum; this contributes to a reduction in the tap to tap time of EAF steelmaking.

Comprehensive Model of Oxygen Steelmaking Part 2: Application of Bloated Droplet Theory for Decarburization in Emulsion Zone

The development of a global model for oxygen steelmaking and its validation against industrial data was reported in Part 1 of this paper. This paper focused on the development of one sub-model on the decarburization reaction in the emulsion zone incorporating the bloated droplet theory. This paper also critically evaluated the current knowledge on the kinetics of decarburization reaction in the emulsion phase and discussed the repercussions of the new model for industrial practices. The decarburization model, in conjunction with the industrial data, indicates that the decarburization rates in the emulsion phase reaches up to approximately 60% of the overall decarburization rate during the main blow. It was found that the residence time of droplets as well as decarburization reaction rate via emulsified droplets was strong function of bloating behavior of metal droplets in the emulsion phase. The estimated residence times of the metal droplets in the emulsion were between 0.4 to 45 s throughout the blow. The influence of variations in droplet size and ejection angle on residence time and decarburization rates via emulsified droplets was also investigated. It was shown in this study that the decarburization rates in the emulsion were accelerated if droplet size was decreased or if the ejection angle was decreased.

Comprehensive Model of Oxygen Steelmaking Part 3: Decarburization in Impact Zone

The development of a global model for oxygen steelmaking and its validation against industrial data has been reported in Part 1 of this paper. Part 2 of this paper explained the model development of decarburization reaction of emulsified droplets and discussed the effects of bloating behaviour of metal droplets on the overall kinetics of the process. Part 3 of this paper focussed on the development of one sub-model on the decarburization reaction in the impact zone and critically evaluates the important process variables affecting the decarburization kinetics. Decarburization rates in the impact zone were calculated using the semi-empirical relationships developed from experimental results. Based on previous experimental studies, both diffusion through the gas phase and surface control were considered in the rate calculations. The model was validated against experimental data from Belton and Sain, and against plant data from Cicutti et al. The model developed was consistent with the experimental and plant data, and provided a reasonable basis for predicting the decarburization of iron in the impact region of oxygen steelmaking. The model predicted that the decarburization rate is sensitive to the partial pressure of carbon dioxide and oxygen in the impact zone. As the partial pressure of oxygen decreased from 26.4 to 16.2 kPa, the model predicted that that the reaction rate of decarburization via oxygen decreased from 202 to 134 kg/min. The model predicted that approx. 40% of decarburization takes place in the impact zone during the main blow.

Effect of Mixed Gas Top Blowing on Decarburization and Manganese Evaporation of Steel Melt in Low Carbon Content Range

Experimental and theoretical studies have been carried out to clarify the effects of mixed gas top blowing on decarburization and manganese evaporation of steel melt in low carbon content range.20 kg scale of induction furnace was used for decarburization experiment. Oxygen utilization efficiency for decarburization increased with an increase in inert gas flow rate and a decrease in oxygen flow rate of mixed gas top blowing. Decarburization rate below carbon content of 0.03 mass% depended on CO partial pressure caused by mixed gas of oxygen and inert gas. To explain these results, an index, IM for decarburization was introduced. The IM is found to be inversely proportional to the oxygen utilization efficiency for decarburization in the low carbon content range, and there is a good correlation between the IM and oxygen content in steel melt.Unexplained manganese content, which seems to depend on its evaporation, decreased with an increase in inert gas ratio of top blowing to oxygen. Manganese evaporation rate was introduced according to Hertz–Knudsen–Langmuir's equation which showed an increase with increasing temperature. Assuming that manganese evaporation occurs at the fire spot caused by top blowing during the final stage of refining, the temperature of the fire spot was estimated to reach 2890K for oxygen top blowing, and decrease to 2475K for mixed gas blowing of (Ar+N2)/O2=2/1.

Chromia-Assisted Decarburization of W-Rich Ni-Based Alloys in Impure Helium at 1273 K (1000 °C)

The microstructure and surface stability of two experimental W-rich Ni-based alloys have been studied at 1273 K (1000 °C) in an impure-He environment containing only CO and CO2 as impurities. The alloy Ni-2.3Al-12Cr-12W contained 0.08 wt pct carbon in solution, whereas the second alloy Ni-2.3Al-3Mo-12Cr-12Co-12W contained M6C carbides at the same carbon level. Both alloys, which were preoxidized with ~2.3 μm Cr2O3 layer, were decarburized completely within 50 hours of exposure to the helium gas mixture at 1273 K (1000 °C) via the following chromia-assisted decarburization reaction: Cr2O3 (s) + 3Calloy (s) → 2Cr (s) + 3CO (g). Microstructural observations, bulk carbon analysis, and microprobe measurements confirmed that the carbon in solid solution reacted with the surface chromium oxide resulting in the simultaneous loss of chromia and carbon. The Cr produced by the decomposition of the Cr2O3 diffused back into the alloy, whereas CO gas was released and detected by a gas chromatograph. Once the alloy carbon content was reduced to negligible levels, subsequent exposure led to the uninterrupted growth of Cr2O3 layer in both alloys. In the preoxidized alloys, chromia-assisted decarburization rates were slower for an alloy containing carbides compared with the alloy with carbon in solid solution only. The formation of Cr2O3 is shown to be the rate-limiting step in the chromia-assisted decarburization reaction. Exposure of as-fabricated alloys to the impure-He environment led to the formation of a thin layer of Al2O3 (<1 μm) between the substrate and surface Cr2O3 oxide that inhibited this decarburization process by acting as a diffusion barrier.

The Reaction Behavior of Direct Reduced Iron (DRI) in Steelmaking Slags: Effect of DRI Carbon and Preheating Temperature

An experimental study was conducted to quantify the rate of direct reduced iron (DRI) decarburization in a steelmaking slag using the constant volume pressure increase technique. Experiments were conducted by dropping DRI pellets into molten slag at temperatures from 1773 K to 1873 K (1500 °C to 1600 °C). Subsequent experiments were carried out in which the DRI pellets were preheated while the slag temperature remained constant. The effect of the initial carbon content and the preheating temperature of the DRI on the reaction rate was investigated. The decarburization of DRI seems to comprise two stages, a reaction between the FeO and DRI followed by decarburization through the iron oxide of slag. Carbon has a significant effect on the kinetics of both stages, whereas the preheating temperature mainly influences the rate of decarburization between FeO and carbon inside the pellet.

Refining high-silicon hot metal in an oxygen converter. New process to decrease slopping and fume emissions

The recent decrease in Blast Furnace production as well as changes in raw materials and process conditions has produced an increase in the silicon and phosphorus contents in hot metal. This situation increases the probability of slopping and fume emissions during converter blowing because of the large volume of highly viscous slag. To carry out the silicon oxidation and dephosphorization steps, a double slag practice has to be used. The first slag is rich in silicon and phosphorus and has a low basicity. The first slag is removed. Then the second slag is formed; this low-viscosity slag is subject to the strong agitation created by a high decarburization rate. The second slag is characterized by higher CaO and phosphorus contents. These conditions yield a better dephosphorization at the end of the blow. In order to implement the double slag practice, the converter control system was modified. The changes included the design of the lance blowing pattern, the flux additions, the static control, and the logic of the gas collection and cleaning system. For a silicon content larger than 0.60% in the hot metal, the results show a significant reduction in the fume emissions and in slopping, a lower flux consumption, and open up the possibility of working with iron ore with higher phosphorus content.

Improving manufacturing of ULC steel grades by revamping of RH degasser in steelmaking shop No. 2 of ILVA, Taranto Works

In recent years, as the demand for steels of high added value, such as the ultralow carbon (ULC) steels, the demands on the Ruhrstahl–Heraeus-oxygen blowing (RH-OB) process, have increased. In order to meet these strong demands, the RH-OB equipment at Taranto Works has been revamped, particularly by means of the enlargement of the snorkel diameter. This has led not only to increase in the decarburisation rate due to the improvement of the metal recirculation, but also to an increase in productivity, improved process control and savings in electrical power and process gas consumption.

Decarburization rate of RH refining for ultra low carbon steel

The decarburization behaviors of ultra low carbon steel in a 210-t RH vacuum degasser were investigated under practical operating conditions. According to the apparent decarburization rate constant (KC) calculated by the carbon content in the samples taken from the hot melt in a ladle at an interval of 1–2 min, it is observed that the total decarburization reaction period in RH can be divided into the quick decarburization period and the stagnant decarburization period, which is quite different from the traditional one with three stages. In this study, the average apparent decarburization rate constant during the quick decarburization period is 0.306 min−1, and that of the stagnant period is 0.072 min−1. Increasing the initial carbon content and enhancing the exhausting capacity can increase the apparent decarburization rate constant in the quick decarburization period. The decarburization reaction comes into the stagnant decarburization period when the carbon content in molten steel is less than 14×10−6 after 10 min of decarburization.

Computational Fluid Dynamics Study of New Vacuum Degassing Process

Recently, the demand of the steel having superior chemical and physical properties has increased for which the content of carbon must be in ultra low range. There are many processes which can produce low carbon steel such as tank degasser and RH (Rheinstahl-Heraeus) processes. It has been claimed that using a new process, called REDA (Revolutionary Degassing Activator), one can achieve the carbon content below 10ppm in less time. REDA process, in terms of installment cost, is in between the tank degasser and RH processes. As such, REDA process has not been studied thoroughly. Fluid flow phenomena affect the decarburization rate the most besides the chemical reaction rate. Therefore, momentum balance equations along with k-? turbulent model have been solved for gas and liquid phases in two-dimension (2D) for REDA process. The fluid flow phenomena have been studied in details for this process by varying gas flow rate, depth of immersed snorkel in the steel, diameter of the snorkel and change in vacuum pressure. It is found that the design of the snorkel affects the melt circulation of the bath significantly.

Experimental Research on Reducing the Dust of BOF in CO2 and O2 Mixed Blowing Steelmaking Process

The thermodynamic theory and experimental research on reducing the dust generation in CO2 and O2 Mixed Blowing (COMB) steelmaking process have been undertaken. In labo-scale experiment, data show that the amounts of dust and iron loss keep on decreasing as CO2 blowing rate increases. Further, dust sampled from 30 t BOF conventional process in Fujian Sanming Iron and Steel Plant has been analyzed, and the dust generation rate in the conventional process reduced from 666 g/s at the 1.5 min point to 215 g/s at the end of blowing. Introducing the technology of the stepped CO2 blow, the average generation rate of dust is reduced by 12.5% in this experiment compared with the conventional one, while the average generation rate of T-Fe in dust is reduced by 12.7%. This improvement has been made keeping the same decarburization rates. Therefore the stepped CO2 blowing rate has remarkable effect on dust generation. This research has proved that COMB steelmaking process is effective on a decrease in dust generation along with improved iron yield.

Evaluation of the Effects of Gas Injection in the Vaccum Chamber of a RH Degasser on Melt Circulation and Decarburization Rates

Steels with low concentrations of nitrogen, hydrogen and carbon have in large demand lately. The RH process is a secondary refining process that can simultaneously attain significant levels of removal of these interstitial elements from liquid steel.In the RH process, the melt circulation rate plays a very important role in determining productivity of the equipment, since it affects the decarburization rate.In the present work, a physical model of a RH degasser in a 1 : 5 scale of an industrial reactor from USIMINAS (Minas Gerais, Brazil). This model has been built and used in the study of the circulation rate and of the kinetics of decarburization. The effects of the gas flow rate and of the configurations of the nozzles used in the injection of the gas have been analyzed. The effect of gas injection in the vacuum chamber was also investigated.The decarburization reaction of liquid steel was simulated using a reaction involving CO2 and caustic solutions. The concentration of CO2 in the solution was evaluated using pH measurements.The experimental results indicated that the kinetic of decarburization is controlled by mass transfer in the liquid phase. It was also observed that, in certain configurations, the injection of gas in the vacuum chamber can increase the decarburization rate.

Kinetics and Mechanism of Decarburization and Melting of Direct-Reduced Iron Pellets in Slag

An experimental study was conducted to understand the decarburization and melting behavior of direct-reduced iron (DRI) pellets in SiO2-Al2O3-CaO-MgO-FeO slags with various FeO concentrations (10 to 25 wt pct) and basicities (Bs), ranging from 1.5 to 2.5. The behavior of the pellet in slag was observed using the X-ray fluoroscopy technique; the rate of decarburization was simultaneously measured with a constant volume pressure increase technique. The study shows that the decarburization of DRI in slag at 1600 °C takes place in two stages. The first stage is the reaction between the FeO and the carbon inside the pellet, which is controlled by heat transfer from the slag to the pellet; the second stage involves the decarburization reaction between the FeO in the slag and the remaining carbon in the DRI. The kinetics of this stage is determined by the mass transfer of FeO in the slag and is strongly dependent on the FeO concentration. Depending on the physicochemical properties of the slag and the rate of gas evolution, the DRI may sink through, float inside the slag, or remain on top before complete decarburization.

Dynamic Modeling of the Main Blow in Basic Oxygen Steelmaking Using Measured Step Responses

In the control and optimization of basic oxygen steelmaking, it is important to have an understanding of the influence of control variables on the process. However, important process variables such as the composition of the steel and slag cannot be measured continuously. The decarburization rate and the accumulation rate of oxygen, which can be derived from the generally measured waste gas flow and composition, are an indication of changes in steel and slag composition. The influence of the control variables on the decarburization rate and the accumulation rate of oxygen can best be determined in the main blow period. In this article, the measured step responses of the decarburization rate and the accumulation rate of oxygen to step changes in the oxygen blowing rate, lance height, and the addition rate of iron ore during the main blow are presented. These measured step responses are subsequently used to develop a dynamic model for the main blow. The model consists of an iron oxide and a carbon balance and an additional equation describing the influence of the lance height and the oxygen blowing rate on the decarburization rate. With this simple dynamic model, the measured step responses can be explained satisfactorily.

Mass Transfer Coefficients during Steel Decarburization in a RH Degasser

The RH process is a secondary refining process that can simultaneously attain significant levels of removal of interstitial elements, such as carbon, nitrogen and hydrogen, from liquid steel. In the RH process, the decarburization rate plays a very important role in determining the productivity of the equipment. The kinetics of this reaction is controlled by mass transfer in the liquid phase. In the present work, a physical model of a RH degasser has been built and used in the study of the kinetics of decarburization. The effects of the gas flow rate and of the configurations of the nozzles used in the injection of the gas have been analyzed. The decarburization reaction of liquid steel was simulated using a reaction involving CO2 and caustic solutions. The concentration of CO2 in the solution was evaluated using pH measurements. Based on the experimental results, it was possible to estimate the reaction rate constant. A volumetric mass transfer coefficient was then calculated based on these rate constants and on the circulation rate of the liquid. The logarithm of the mass transfer coefficient showed a linear relationship with the logarithm of the gas flow rate. The slope of the line was found to vary according to the relevance of the reaction at the free surface in the vacuum chamber. A linear relationship between the volumetric mass transfer coefficient and the nozzle Reynolds number was also observed. The slopes of the lines changed according to the relative importance of the two reaction sites, gas-liquid interface in the upleg snorkel and in the vacuum. At higher Reynolds number, the reaction in the vacuum chamber tends to be more significant.

Dynamic Coupling of Computational Fluid Dynamics and Thermodynamics Software: Applied on a Top Blown Converter

A novel modeling approach is presented where a computational fluid dynamics software is coupled to thermodynamic databases to obtain dynamic simulations of metallurgical process phenomena. The modeling approach has been used on a fundamental model of a top-blown converter. Reactions between gas–steel, gas–slag, steel–slag and gas–steel–slag have been considered. The results show that the mass transport in the surface area is totally controlled by convection. Also, that a large amount of CO produced during the decarburization might slow down the rate of decarburization in droplets ejected from the bath. For the present simulation conditions reflecting laboratory experiments, it was also seen that the amount of slag (FeO and/or SiO2) created is close to zero, i.e. only gas (CO+CO2) is created as the oxygen jet hits the steel bath. It was also illustrated how an extrapolation of the decarburization rate, sampled from a few seconds of simulation, could be done to get a rough estimate of the carbon content at a later stage in the process as long as the carbon content is relatively high. The overall conclusion is that it is possible to make a dynamic coupling of the Thermo-Calc databases and a CFD software to make dynamic simulations of metallurgical processes such as a top-blown converter.