Ruhrstahl Heraeus Research Articles

Effects of gas injection with multihole orifices in upleg snorkel on bubble behaviour and decarburisation rate during RH refining

Physical model experiments have been conducted to investigate the effects of multihole orifices in the upleg snorkel on bubble behaviour and decarburisation rate during Ruhrstahl–Heraeus refining. Bubble behaviour was recorded by a high speed video camera and a volumetric mass transfer coefficient kA/V was used to evaluate the decarburisation rate. Measurement of the volumetric mass transfer coefficient kA/V was made by means of the CO2 desorption from a NaOH solution. Pictures of bubble behaviour show that gas bubbles are generated separately from the multihole orifice in the low region of gas flowrate, but they tend to coalesce at high gas flowrate. It was demonstrated that kA/V increases with increasing gas flowrate and decreasing orifice hole diameter. In the low region of gas flowrate, kA/V for multihole orifices is larger than that for one-hole orifices. In this case, kA/V increases as the number of holes increases, and the hole distance of orifice has little effect on kA/V. However, in the high region of gas flowrate, kA/V for multihole orifices is lower than that for one-hole orifices, and kA/V decreases with the increase in orifice interhole distance. It was found that there exists a critical gas flowrate, above which kA/V for multihole orifices is lower than that for one-hole orifices. This critical value increases with increasing hole distance and hole number. Some explanations were also made in terms of bubble behaviour.

Final Temperature Prediction Model of Molten Steel in RH-TOP Refining Process for IF Steel Production

In order to precisely control the final temperature of molten steel in RH (Ruhrstahl Heraeus)-TOP blowing refining, the final temperature prediction models of molten steel in RH-TOP blowing refining process for Interstitial Free (IF) steel production were established under the condition of oxygen blowing and non-oxygen blowing respectively. The results show that the beginning molten steel temperature of refining and the amount of added scrap were influential factors, the baking temperature in vacuum chamber was a factor that had small influence. When the model was operated, the hitting probability was above 95 % (under the condition of both oxygen blowing and non-oxygen blowing) of prediction deviation of ±10 °C. The accuracy is analyzed.

Investigation of slag-refractory interactions for the Ruhrstahl Heraeus (RH) vacuum degassing process in steelmaking

Abstract In order to determine the effect of slag composition during the RH process on refractory wear, magnesia–carbon and magnesia–chromite refractories were immersed for 10 min at 1600 °C in a ladle slag, two FeO-rich slags (20 and 40 wt% FeO) and two CaO–Al2O3 slags. Corrosion of magnesia–carbon refractory by the ladle and CaO–Al2O3 slags was limited as the refractory carbon phase efficiently prevented slag infiltration. Severe degradation was observed in contact with FeO-rich slags. FeO oxidized the carbon phase with formation of Fe droplets at the hot face. Regarding magnesia–chromite refractory, the corrosion mechanism consisted of severe slag infiltration, high temperature inactivation of the secondary chromite and primary chromite dissolution in the infiltrating slag. The FeO-rich slags seem to have generated more severe conditions as the infiltrating slag pushed apart the periclase grains, leading to severe refractory erosion. The degradation mechanisms are discussed by combining experimental results and thermodynamic calculations.

A Statistical Model for Predicting the Liquid Steel Temperature in Ladle and Tundish by Bootstrap Filter

A statistical model for predicting the liquid steel temperature in the ladle and in the tundish is developed. Given a large data set in a steelmaking process, the proposed model predicts the temperature in a seconds with a good accuracy. The data are divided into four phases at the mediation of five temperature measurements: before tapping from the converter (CV), after throwing ferroalloys into the ladle, before and after the Ruhrstahl-Heraeus (RH) processing, and after casting into the tundish in the continuous casting (CC) machine. Based on the general state space modeling, the bootstrap filter predicts the temperature phase by phase. The particle approximation technique enables to compute general-shaped probability distributions. The proposed model gives a prediction not as a point but as a probability distribution, or a predictive distribution. It evaluates both uncertainty of the prediction and ununiformity of the temperature. It is applicable to sensitivity analysis, process scheduling and temperature control.

Novel concept of cleansing IF molten steel with dispersed in situ phase induced by composite ball explosive reaction in RH ladles

A novel technology for the removal of fine inclusions has been put forward using a dispersed in situ phase induced by an explosive reaction of a composite ball of slag forming materials. A composite ball with this function has been designed and prepared using a laboratory model batch type balling disc (at 12 rev min−1) and the composite ball has been fed at the end of Ruhrstahl–Heraeus refining. The results indicate that feeding composite ball in Ruhrstahl-Heraus ladle is a novel technology and the inclusion in the interstitial free molten steel can be removed effectively. Compared with conventional inclusion removal technology, the number of the oxide inclusions can be decreased to a lower level and the inclusion size becomes finer. Using this novel technology, the total oxygen in the as cast slab can approach to 5 ppm and the steel production cost for per ton can be reduced by 5–12 RMB.

A Kinetic Model for the Ruhrstahl Heraeus (RH) Degassing Process

A kinetic model (effective equilibrium reaction zone model) was developed to simulate the decarburization reaction in the Ruhrstahl Heraeus (RH) degassing process. The model assumes that the chemical reactions reach equilibrium in the designated effective reaction volumes near the reaction interfaces. After the RH degassing process was divided into various reaction zones, the effective reaction volumes of each reaction zone were expressed as a function of the process conditions based on the physical descriptions of the reaction mechanisms. The influence of the chemical reaction between the RH slag and the RH steel to the decarburization phenomena was considered for the first time. The calculated C and O profiles by the present model are in good agreement with the industrial operation data for various steel compositions and process conditions. RH slag can serve as an oxygen reservoir to supply O during the RH decarburization process, which induces the observed deviation of the C and O contents from their ideal stoichiometric trajectory. The present model provides an efficient tool to understand the RH degassing process.

Decarburisation rates in RH–KTB degasser of CST steel plant through physical modelling study

A physical model based on similarity principles was constructed to simulate the RH (Ruhrstahl–Heraeus) degasser of CST (Companhia Siderúrgica Tubarão, Vitória, Brazil) to evaluate the influence of metal circulation rate, which essentially defines the degree of exposure of the metal to the vacuum in the chamber, on the decarburisation rate. The circulation rate in addition influences the removal of dissolved gases such as nitrogen, and hence it is essential to optimise this parameter to achieve maximum refining efficiency with minimum refining time, thus improving the productivity of the secondary refining process. In the present work, simulation experiments investigating the kinetics of decarburisation have been carried out using the CO2 adsorption–desorption process in sodium hydroxide solution as the circulating fluid. The effect of bottom gas flowrate and snorkel diameter in this case was also evaluated. Bottom gas injection practice, under the upleg, improves the circulation rate. Increasing the circulation rate by bottom injection leads to an increased degassing rate. However, degasification efficiency does not remain at the same level. The circulation and degasification rates can be improved by an increase in diameter of the downleg snorkel. The relative gain in degasification seems to be higher at the higher flowrate range. The results have been translated to a prototype, to optimise the process.

Mathematical modelling of molten steel flow in a whole degasser during RH refining process

A three-dimensional mathematical model for molten steel flow in a whole degasser during the RH (Ruhrstahl–Heraeus) refining process is proposed. The model has been developed considering the physical characteristics of the process, particularly the behaviour of gas–liquid two phase flow in the up snorkel and the momentum exchange between the two phases. The fluid flow fields and gas holdups of liquid phases, among other parameters, in a 90 t RH degasser and a water model unit of one-fifth linear scale have been computed using this mathematical model. The results show that the flow pattern of molten steel in a whole RH degasser can be well represented by the mathematical model. Apart from the area close to the free surface and the zone between the two snorkels in the ladle, the molten steel in an RH degasser, especially in the vacuum vessel, is reasonably fully mixed during the refining process. However, there is a boundary layer between the descending liquid stream from the down snorkel and the surrounding liquid, which is typical liquid–liquid two phase flow, and the molten steel in the ladle is not perfectly mixed. The blown lifting gas ascends mostly near the up snorkel wall, which is more obvious under the conditions of an actual RH degasser, and the flow pattern of bubbles and molten steel in the up snorkel is closer to annular flow. Calculated circulation rates for the water model unit at various lifting gas rates are in good agreement with values determined by means of water modelling experiments.

Study on flow and mixing characteristics of molten steel in RH and RH-KTB refining processes

The flow and mixing characteristics of molten steel during the vacuum circulation refining, including RH(Ruhrstahl-Heraeus) and RH-KTB(Ruhrstahl-Heraeus-Kawasaki top blowing) processes, were investigated on a 1/5 linear scale water model of a 90 t multifunction RH degasser. The circulation rate was directly and more accurately determined, using a new method by which the more reliable results can be obtained. The fluid flow pattern and flow field in the ladle were demonstrated, observed and analyzed. The mixing time of liquid in the ladle was measured using electrical conductivity method. The residence time distribution in the RH model was obtained by tracer response technique. The influence of the main technological and geometric factors, including the gas top blowing (KTB) operation, was examined. The results indicated that the circulation rate of molten steel in the RH degasser can be fairly precisely calculated by the formula: Q lp=0.0333Q g 0.26 D u 0.69 D d 0.80 (t/min), where Q g - the lifting gas flow rate (NL/min); D u and D d - the inner diameters of the up and down-snorkels (cm), respectively. The maximum value of circulation rate of molten steel in the case of the 30 cm diameters either of the up-and down-snorkels for the RH degasser (the “saturated” rate) is approximately 31 t/min. The corresponding gas flow rate is 900 NL/min. Blowing gas into the vacuum chamber through the top lance like KTB operation does not markedly influence the circulatory flow and mixing characteristics of the RH process under the conditions of the present work. There exist a major loop and a large number of small vortices and eddies in the ladle during the RH refining process. A liquid-liquid two-phase flow is formed between the descending stream from the down-snorkel and the liquid around the stream. All of these flow situation and pattern will strongly influence and determine the mixing and mass transfer in the ladle during the refining. The correlation between the mixing time and the stirring energy density is τ m∞e −0.50 for the RH degasser. The mixing time rapidly shortens with an increase in the lifting gas flowrate. At a same gas flow rate, the mixing times with the up-and down-snorkel diameters either of 6 and 7 cm are essentially same. The 30 cm diameters either of the up-and down-snorkels for the RH degasser would be reasonable. The concentration-time curve showed that three circulation cycles are at least needed for complete mixing of the liquid steel in the RH degasser.

Prediction of gas‐liquid two‐phase mixture flow in a vertical pipe with a sudden expansion in diameter

The principle of gas‐lift pumps is applied to vacuum‐decarburization with the RH (Ruhrstahl Heraeus) process to circulate molten steel. Gas‐lift pumps are also applicable to the transportation of molten iron/steel between different refining processes. This paper treats theoretical analysis of steady‐state flow characteristics of gas‐liquid two‐phase mixtures rising in a vertical pipe with an abrupt expansion of its diameter. The system of governing equations is based upon a one‐dimensional multi‐fluid model. Flow pattern transitions are taken into consideration. A new numerical procedure to predict the flow characteristics at the sudden expansion has been proposed. Experiments have also been performed for several conditions to confirm the applicability as well as the validity of the present numerical model. It has been found that the predictions agree reasonably well with the experimental data. Next, the effect of the sudden expansion of pipe diameter on the pump performance was investigated numerically. As a result, it has been confirmed that the sudden expansion of pipe diameter contributes to improve the pump efficiency.

767. Special applications of vacuum metallurgy: D Hartmann,Elektrowarme, 25, Oct 1967, 368–376 ( in German)

To realize stable production in the steel industry, it is important to control molten steel temperature in a continuous casting process. The present work aims to provide a general framework of gray-box modeling and to develop a gray-box model that predicts and controls molten steel temperature in a tundish (TD temp) with high accuracy. Since the adopted first-principle model (physical model) cannot accurately describe uncertainties such as degradation of ladles, their overall heat transfer coefficient, which is a parameter in the first-principle model, is optimized for each past batch separately, then the parameter is modeled as a function of process variables through a statistical modeling method, random forests. Such a model is termed as a serial gray-box model. Prediction errors of the first-principle model or the serial gray-box model can be compensated by using another statistical model; this approach derives a parallel gray-box model or a combined gray-box model. In addition, the developed gray-box models are used to determine the optimal molten steel temperature in the Ruhrstahl–Heraeus degassing process from the target TD temp, since the continuous casting process has no manipulated variable to directly control TD temp. The proposed modeling and control strategy is validated through its application to real operation data at a steel work. The results show that the combined gray-box model achieves the best performance in prediction and control of TD temp and satisfies the requirement for its industrial application.

Impulsive coronal and the prebreakdown mechanism of long sparks

In the Ruhrstahl-Heraeus (RH) refining process, liquid steel flow pattern in a ladle is controlled by the fluid flow behavior in the vacuum chamber. Potassium chloride solution and NaOH solution saturated with CO2 were respectively used as a tracer to investigate the liquid and gas flow behaviors in the vacuum chamber. Principal component and comparative analysis were made to show the factors controlling mixing and circulation flow rate. The liquid level and bubble behavior in the vacuum chamber greatly affect fluid flow in RH process. Experiments were performed to investigate the effects of liquid steel level, gas flow rate, bubble residence time, and gas injection mode on mixing, decarburization, and void fraction. The results indicate that the mixing process can be divided into three regions: the flow rate-affected zone, the concentration gradient-affected zone, and their combination. The liquid steel level in the vacuum chamber of 300 mm is a critical point in the decarburization transition. For liquid level lower than 300 mm, liquid steel circulation controls decarburization, while for liquid level higher than 300 mm, bubble behavior is the main controlling factor. During the RH process, it is recommended to use the concentrated bubble injection mode for low gas flow rates and the uniform bubble injection mode for high gas flow rates.

744. Instrument for observation of magnetization vector position in thin magnetic films: C. J. Bader and D. M. Ellis, Rev. Sci. Instrum., 33 (12), Dec. 1962, 1429–1435

In the Ruhrstahl-Heraeus (RH) refining process, liquid steel flow pattern in a ladle is controlled by the fluid flow behavior in the vacuum chamber. Potassium chloride solution and NaOH solution saturated with CO2 were respectively used as a tracer to investigate the liquid and gas flow behaviors in the vacuum chamber. Principal component and comparative analysis were made to show the factors controlling mixing and circulation flow rate. The liquid level and bubble behavior in the vacuum chamber greatly affect fluid flow in RH process. Experiments were performed to investigate the effects of liquid steel level, gas flow rate, bubble residence time, and gas injection mode on mixing, decarburization, and void fraction. The results indicate that the mixing process can be divided into three regions: the flow rate-affected zone, the concentration gradient-affected zone, and their combination. The liquid steel level in the vacuum chamber of 300 mm is a critical point in the decarburization transition. For liquid level lower than 300 mm, liquid steel circulation controls decarburization, while for liquid level higher than 300 mm, bubble behavior is the main controlling factor. During the RH process, it is recommended to use the concentrated bubble injection mode for low gas flow rates and the uniform bubble injection mode for high gas flow rates.